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pb-8 lg:pb-0 lg:max-w-custom-xs z-10 hidden lg:block"><nav role="navigation" style="--bg-opacity:.2" class="w-full pt-6 scrolling-touch lg:h-auto grow pe-0 lg:pe-5 lg:pb-16 md:pt-4 lg:pt-4 scrolling-gpu"><ul><h3 class="mb-1 text-sm font-bold ms-5 text-tertiary dark:text-tertiary-dark">GET STARTED</h3><li><a title="Quick Start" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn"><div>Quick Start </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R8im6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Tutorial: Tic-Tac-Toe" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/tutorial-tic-tac-toe"><div>Tutorial: Tic-Tac-Toe </div></a></li><li><a title="Thinking in React" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/thinking-in-react"><div>Thinking in React </div></a></li></ul></div></div></li><li><a title="Installation" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/installation"><div>Installation </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R8qm6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Creating a React App" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/creating-a-react-app"><div>Creating a React App </div></a></li><li><a title="Build a React App from Scratch" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/build-a-react-app-from-scratch"><div>Build a React App from Scratch </div></a></li><li><a title="Add React to an Existing Project" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/add-react-to-an-existing-project"><div>Add React to an Existing Project </div></a></li></ul></div></div></li><li><a title="Setup" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/setup"><div>Setup </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R92m6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Editor Setup" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/editor-setup"><div>Editor Setup </div></a></li><li><a title="Using TypeScript" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/typescript"><div>Using TypeScript </div></a></li><li><a title="React Developer Tools" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/react-developer-tools"><div>React Developer Tools </div></a></li><li><a title="React Compiler" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/react-compiler"><div>React Compiler </div></a></li></ul></div></div></li><li role="separator" class="mt-4 mb-2 ms-5 border-b border-border dark:border-border-dark"></li><h3 class="mb-1 text-sm font-bold ms-5 text-tertiary dark:text-tertiary-dark mt-2">LEARN REACT</h3><li><a title="Describing the UI" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/describing-the-ui"><div>Describing the UI </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R9im6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Your First Component" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/your-first-component"><div>Your First Component </div></a></li><li><a title="Importing and Exporting Components" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/importing-and-exporting-components"><div>Importing and Exporting Components </div></a></li><li><a title="Writing Markup with JSX" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/writing-markup-with-jsx"><div>Writing Markup with JSX </div></a></li><li><a title="JavaScript in JSX with Curly Braces" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/javascript-in-jsx-with-curly-braces"><div>JavaScript in JSX with Curly Braces </div></a></li><li><a title="Passing Props to a Component" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/passing-props-to-a-component"><div>Passing Props to a Component </div></a></li><li><a title="Conditional Rendering" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/conditional-rendering"><div>Conditional Rendering </div></a></li><li><a title="Rendering Lists" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/rendering-lists"><div>Rendering Lists </div></a></li><li><a title="Keeping Components Pure" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/keeping-components-pure"><div>Keeping Components Pure </div></a></li><li><a title="Your UI as a Tree" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/understanding-your-ui-as-a-tree"><div>Your UI as a Tree </div></a></li></ul></div></div></li><li><a title="Adding Interactivity" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/adding-interactivity"><div>Adding Interactivity </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:R9qm6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Responding to Events" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/responding-to-events"><div>Responding to Events </div></a></li><li><a title="State: A Component's Memory" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/state-a-components-memory"><div>State: A Component's Memory </div></a></li><li><a title="Render and Commit" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/render-and-commit"><div>Render and Commit </div></a></li><li><a title="State as a Snapshot" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/state-as-a-snapshot"><div>State as a Snapshot </div></a></li><li><a title="Queueing a Series of State Updates" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/queueing-a-series-of-state-updates"><div>Queueing a Series of State Updates </div></a></li><li><a title="Updating Objects in State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/updating-objects-in-state"><div>Updating Objects in State </div></a></li><li><a title="Updating Arrays in State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/updating-arrays-in-state"><div>Updating Arrays in State </div></a></li></ul></div></div></li><li><a title="Managing State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/managing-state"><div>Managing State </div><span class="pe-1 text-tertiary dark:text-tertiary-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition -rotate-90 rtl:rotate-90" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-50" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:Ra2m6:" aria-hidden="true" role="region" style="box-sizing:border-box;display:none;height:0px;overflow:hidden"><ul><li><a title="Reacting to Input with State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/reacting-to-input-with-state"><div>Reacting to Input with State </div></a></li><li><a title="Choosing the State Structure" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/choosing-the-state-structure"><div>Choosing the State Structure </div></a></li><li><a title="Sharing State Between Components" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/sharing-state-between-components"><div>Sharing State Between Components </div></a></li><li><a title="Preserving and Resetting State" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/preserving-and-resetting-state"><div>Preserving and Resetting State </div></a></li><li><a title="Extracting State Logic into a Reducer" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/extracting-state-logic-into-a-reducer"><div>Extracting State Logic into a Reducer </div></a></li><li><a title="Passing Data Deeply with Context" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/passing-data-deeply-with-context"><div>Passing Data Deeply with Context </div></a></li><li><a title="Scaling Up with Reducer and Context" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/scaling-up-with-reducer-and-context"><div>Scaling Up with Reducer and Context </div></a></li></ul></div></div></li><li><a title="Escape Hatches" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between ps-5 text-base font-bold text-primary dark:text-primary-dark" href="/learn/escape-hatches"><div>Escape Hatches </div><span class="pe-1 text-link dark:text-link-dark"><svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20" class="duration-100 ease-in transition rotate-0" style="min-width:20px;min-height:20px"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span></a><div class="opacity-100" style="transition:opacity 250ms ease-in-out"><div id="react-collapsed-panel-:Raam6:" aria-hidden="false" role="region" style="box-sizing:border-box"><ul><li><a title="Referencing Values with Refs" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/referencing-values-with-refs"><div>Referencing Values with Refs </div></a></li><li><a title="Manipulating the DOM with Refs" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/manipulating-the-dom-with-refs"><div>Manipulating the DOM with Refs </div></a></li><li><a title="Synchronizing with Effects" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/synchronizing-with-effects"><div>Synchronizing with Effects </div></a></li><li><a title="You Might Not Need an Effect" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/you-might-not-need-an-effect"><div>You Might Not Need an Effect </div></a></li><li><a title="Lifecycle of Reactive Effects" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/lifecycle-of-reactive-effects"><div>Lifecycle of Reactive Effects </div></a></li><li><a title="Separating Events from Effects" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/separating-events-from-effects"><div>Separating Events from Effects </div></a></li><li><a title="Removing Effect Dependencies" target="" aria-current="page" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-link dark:text-link-dark bg-highlight dark:bg-highlight-dark border-blue-40 hover:bg-highlight hover:text-link dark:hover:bg-highlight-dark dark:hover:text-link-dark" href="/learn/removing-effect-dependencies"><div>Removing Effect Dependencies </div></a></li><li><a title="Reusing Logic with Custom Hooks" target="" class="p-2 pe-2 w-full rounded-none lg:rounded-e-2xl text-start hover:bg-gray-5 dark:hover:bg-gray-80 relative flex items-center justify-between text-sm ps-6 ps-5 text-base text-secondary dark:text-secondary-dark" href="/learn/reusing-logic-with-custom-hooks"><div>Reusing Logic with Custom Hooks </div></a></li></ul></div></div></li></ul><div class="h-20"></div></nav><div class="fixed bottom-0 hidden lg:block"><div class="max-w-custom-xs w-80 lg:w-auto py-3 shadow-lg rounded-lg m-4 bg-wash dark:bg-gray-95 px-4 flex"><p class="w-full text-lg font-bold text-primary dark:text-primary-dark me-4">Is this page useful?</p><button aria-label="Yes" class="px-3 rounded-lg bg-secondary-button dark:bg-secondary-button-dark text-primary dark:text-primary-dark me-2"><svg width="16" height="18" viewBox="0 0 16 18" fill="none" xmlns="http://www.w3.org/2000/svg"><path fill-rule="evenodd" clip-rule="evenodd" d="M9.36603 0.384603C9.36605 0.384617 9.36601 0.384588 9.36603 0.384603L9.45902 0.453415C9.99732 0.851783 10.3873 1.42386 10.5654 2.07648C10.7435 2.72909 10.6993 3.42385 10.44 4.04763L9.27065 6.86008H12.6316C13.5249 6.86008 14.3817 7.22121 15.0134 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class="min-w-0 isolate"><article class="font-normal break-words text-primary dark:text-primary-dark"><div class="ps-0"><div class=""><div class="px-5 sm:px-12 pt-3.5"><div class="max-w-4xl ms-0 2xl:mx-auto"><div class="flex flex-wrap"><div class="flex mb-3 mt-0.5 items-center"><a class="text-link dark:text-link-dark text-sm tracking-wide font-bold uppercase me-1 hover:underline" href="/learn">Learn React</a><span class="inline-block me-1 text-link dark:text-link-dark text-lg rtl:rotate-180"><svg width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg"><path d="M6.86612 13.6161C6.37796 14.1043 6.37796 14.8957 6.86612 15.3839C7.35427 15.872 8.14572 15.872 8.63388 15.3839L13.1339 10.8839C13.622 10.3957 13.622 9.60428 13.1339 9.11612L8.63388 4.61612C8.14572 4.12797 7.35427 4.12797 6.86612 4.61612C6.37796 5.10428 6.37796 5.89573 6.86612 6.38388L10.4822 10L6.86612 13.6161Z" fill="currentColor"></path></svg></span></div><div class="flex mb-3 mt-0.5 items-center"><a class="text-link dark:text-link-dark text-sm tracking-wide font-bold uppercase me-1 hover:underline" href="/learn/escape-hatches">Escape Hatches</a><span class="inline-block me-1 text-link dark:text-link-dark text-lg rtl:rotate-180"><svg width="20" height="20" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg"><path d="M6.86612 13.6161C6.37796 14.1043 6.37796 14.8957 6.86612 15.3839C7.35427 15.872 8.14572 15.872 8.63388 15.3839L13.1339 10.8839C13.622 10.3957 13.622 9.60428 13.1339 9.11612L8.63388 4.61612C8.14572 4.12797 7.35427 4.12797 6.86612 4.61612C6.37796 5.10428 6.37796 5.89573 6.86612 6.38388L10.4822 10L6.86612 13.6161Z" fill="currentColor"></path></svg></span></div></div><h1 class="mdx-heading mt-0 text-primary dark:text-primary-dark -mx-.5 break-words text-5xl font-display font-bold leading-tight">Removing Effect Dependencies<a href="#undefined" aria-label="Link for this heading" title="Link for this heading" class="mdx-header-anchor hidden"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h1></div></div></div><div class="px-5 sm:px-12"><div class="max-w-7xl mx-auto"><div class="max-w-4xl ms-0 2xl:mx-auto"><div class="font-display text-xl text-primary dark:text-primary-dark leading-relaxed"><p class="whitespace-pre-wrap my-4">When you write an Effect, the linter will verify that you’ve included every reactive value (like props and state) that the Effect reads in the list of your Effect’s dependencies. This ensures that your Effect remains synchronized with the latest props and state of your component. Unnecessary dependencies may cause your Effect to run too often, or even create an infinite loop. Follow this guide to review and remove unnecessary dependencies from your Effects.</p></div> <div class="p-6 xl:p-8 pb-4 xl:pb-6 bg-card dark:bg-card-dark rounded-2xl shadow-inner-border dark:shadow-inner-border-dark text-base text-secondary dark:text-secondary-dark my-8"><h3 class="mdx-heading text-primary dark:text-primary-dark mt-0 mb-3 leading-tight text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">You will learn</h3><ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1">How to fix infinite Effect dependency loops</li> <li class="leading-relaxed mb-1">What to do when you want to remove a dependency</li> <li class="leading-relaxed mb-1">How to read a value from your Effect without “reacting” to it</li> <li class="leading-relaxed mb-1">How and why to avoid object and function dependencies</li> <li class="leading-relaxed mb-1">Why suppressing the dependency linter is dangerous, and what to do instead</li> </ul></div> <h2 id="dependencies-should-match-the-code" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Dependencies should match the code <a href="#dependencies-should-match-the-code" aria-label="Link for Dependencies should match the code " title="Link for Dependencies should match the code " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2> <p class="whitespace-pre-wrap my-4">When you write an Effect, you first specify how to <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#the-lifecycle-of-an-effect">start and stop</a> whatever you want your Effect to be doing:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Then, if you leave the Effect dependencies empty (<code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code>), the linter will suggest the correct dependencies:</p> </div><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border 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22 3.5 22H20.5C20.8978 22 21.2794 21.842 21.5607 21.5607C21.842 21.2794 22 20.8978 22 20.5V13.5C22 13.3674 21.9473 13.2402 21.8536 13.1464C21.7598 13.0527 21.6326 13 21.5 13Z" fill="currentColor"></path></svg><span class="hidden md:block">Fork</span></a></div></div><div class=" sp-layout"><div class=" sp-editor sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// <-- Fix the mistake here!</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 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them in according to what the linter says:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4"><a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#effects-react-to-reactive-values">Effects “react” to reactive values.</a> Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is a reactive value (it can change due to a re-render), the linter verifies that you’ve specified it as a dependency. If <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> receives a different value, React will re-synchronize your Effect. This ensures that the chat stays connected to the selected room and “reacts” to the dropdown:</p> </div><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 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<span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute 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C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><div class="max-w-4xl ms-0 2xl:mx-auto"> <h3 id="to-remove-a-dependency-prove-that-its-not-a-dependency" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">To remove a dependency, prove that it’s not a dependency <a href="#to-remove-a-dependency-prove-that-its-not-a-dependency" aria-label="Link for To remove a dependency, prove that it’s not a dependency " title="Link for To remove a dependency, prove that it’s not a dependency " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">Notice that you can’t “choose” the dependencies of your Effect. Every <span data-step="2" class="code-step bg-opacity-10 dark:bg-opacity-20 relative rounded px-[6px] py-[1.5px] border-b-[2px] border-opacity-60 bg-yellow-40 border-yellow-40 text-yellow-60 dark:text-yellow-30">reactive value</span> used by your Effect’s code must be declared in your dependency list. The dependency list is determined by the surrounding code:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="code-step bg-opacity-10 dark:bg-opacity-20 relative rounded px-1 py-[1.5px] border-b-[2px] border-opacity-60 bg-yellow-40 border-yellow-40 text-yellow-60 dark:text-yellow-30">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// This is a reactive value</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="code-step bg-opacity-10 dark:bg-opacity-20 relative rounded px-1 py-[1.5px] border-b-[2px] border-opacity-60 bg-yellow-40 border-yellow-40 text-yellow-60 dark:text-yellow-30">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// This Effect reads that reactive value</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="code-step bg-opacity-10 dark:bg-opacity-20 relative rounded px-1 py-[1.5px] border-b-[2px] border-opacity-60 bg-yellow-40 border-yellow-40 text-yellow-60 dark:text-yellow-30">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ So you must specify that reactive value as a dependency of your Effect</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4"><a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#all-variables-declared-in-the-component-body-are-reactive">Reactive values</a> include props and all variables and functions declared directly inside of your component. Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is a reactive value, you can’t remove it from the dependency list. The linter wouldn’t allow it:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// 🔴 React Hook useEffect has a missing dependency: 'roomId'</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">And the linter would be right! Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> may change over time, this would introduce a bug in your code.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">To remove a dependency, “prove” to the linter that it <em>doesn’t need</em> to be a dependency.</strong> For example, you can move <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> out of your component to prove that it’s not reactive and won’t change on re-renders:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'music'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Not a reactive value anymore</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Now that <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is not a reactive value (and can’t change on a re-render), it doesn’t need to be a dependency:</p> </div><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" 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sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId</span> = <span class="sp-syntax-string">'music'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div></div></div></div></div><div class="max-w-4xl ms-0 2xl:mx-auto"> <p class="whitespace-pre-wrap my-4">This is why you could now specify an <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#what-an-effect-with-empty-dependencies-means">empty (<code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code>) dependency list.</a> Your Effect <em>really doesn’t</em> depend on any reactive value anymore, so it <em>really doesn’t</em> need to re-run when any of the component’s props or state change.</p> <h3 id="to-change-the-dependencies-change-the-code" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">To change the dependencies, change the code <a href="#to-change-the-dependencies-change-the-code" aria-label="Link for To change the dependencies, change the code " title="Link for To change the dependencies, change the code " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">You might have noticed a pattern in your workflow:</p> <ol class="ms-6 my-3 list-decimal"> <li class="leading-relaxed mb-1">First, you <strong class="font-bold">change the code</strong> of your Effect or how your reactive values are declared.</li> <li class="leading-relaxed mb-1">Then, you follow the linter and adjust the dependencies to <strong class="font-bold">match the code you have changed.</strong></li> <li class="leading-relaxed mb-1">If you’re not happy with the list of dependencies, you <strong class="font-bold">go back to the first step</strong> (and change the code again).</li> </ol> <p class="whitespace-pre-wrap my-4">The last part is important. <strong class="font-bold">If you want to change the dependencies, change the surrounding code first.</strong> You can think of the dependency list as <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#react-verifies-that-you-specified-every-reactive-value-as-a-dependency">a list of all the reactive values used by your Effect’s code.</a> You don’t <em>choose</em> what to put on that list. The list <em>describes</em> your code. To change the dependency list, change the code.</p> <p class="whitespace-pre-wrap my-4">This might feel like solving an equation. You might start with a goal (for example, to remove a dependency), and you need to “find” the code matching that goal. Not everyone finds solving equations fun, and the same thing could be said about writing Effects! Luckily, there is a list of common recipes that you can try below.</p> <div class="expandable-callout pt-8 pb-4 px-5 sm:px-8 my-8 relative rounded-none shadow-inner-border -mx-5 sm:mx-auto sm:rounded-2xl bg-yellow-5 dark:bg-yellow-60 dark:bg-opacity-20"><h3 class="text-2xl font-display font-bold text-yellow-50 dark:text-yellow-40"><svg class="inline me-2 mb-1 text-lg text-yellow-50 dark:text-yellow-40" width="2em" height="2em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><g clip-path="url(#clip0_738_836)"><path fill-rule="evenodd" clip-rule="evenodd" d="M27 48L27 57.3409L40.0772 48L55.6975 48C57.1595 48 58.1986 47.0112 58.3851 45.8604C59.1824 40.9398 60 34.619 60 29.625C60 24.7282 59.2125 18.7546 58.4302 14.0813C58.2445 12.9721 57.2326 12 55.7805 12L16.2195 12C14.7674 12 13.7555 12.9721 13.5698 14.0813C12.7875 18.7546 12 24.7282 12 29.625C12 34.619 12.8176 40.9398 13.6149 45.8604C13.8014 47.0112 14.8404 48 16.3025 48H27ZM42 54H55.6975C59.9534 54 63.6271 51.0213 64.3078 46.8201C65.1161 41.8322 66 35.1209 66 29.625C66 24.2196 65.1449 17.8522 64.3478 13.0906C63.6513 8.93026 59.9987 6 55.7805 6H16.2195C12.0013 6 8.34867 8.93026 7.65218 13.0906C6.85505 17.8522 6 24.2196 6 29.625C6 35.1209 6.88391 41.8322 7.69215 46.8201C8.37291 51.0213 12.0466 54 16.3025 54H21L21 63.1704C21 65.6106 23.7581 67.0299 25.7437 65.6116L42 54ZM39 39.3686C39 40.9422 38 41.9912 36 41.9912C34 41.9912 33 40.9422 33 39.3686C33 37.7951 34 36.746 36 36.746C38 36.746 39 37.7951 39 39.3686ZM38.1771 20.2412C38.1771 18.9986 37.1697 17.9912 35.9271 17.9912C34.6845 17.9912 33.6771 18.9986 33.6771 20.2412V31.5956C33.6771 32.8382 34.6845 33.8456 35.9271 33.8456C37.1697 33.8456 38.1771 32.8382 38.1771 31.5956V20.2412Z" fill="currentColor"></path></g><defs><clipPath id="clip0_738_836"><rect width="72" height="72" fill="white"></rect></clipPath></defs></svg>Pitfall</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">If you have an existing codebase, you might have some Effects that suppress the linter like this:</p><div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🔴 Avoid suppressing the linter like this:</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// eslint-ignore-next-line react-hooks/exhaustive-deps</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4"><strong class="font-bold">When dependencies don’t match the code, there is a very high risk of introducing bugs.</strong> By suppressing the linter, you “lie” to React about the values your Effect depends on.</p><p class="whitespace-pre-wrap my-4">Instead, use the techniques below.</p></div></div></div> <details class="my-12 rounded-2xl shadow-inner-border dark:shadow-inner-border-dark relative dark:bg-opacity-20 dark:bg-purple-60 bg-purple-5"><summary class="list-none p-8" tabindex="-1"><h5 class="mb-4 uppercase font-bold flex items-center text-sm dark:text-purple-30 text-purple-50"><svg class="inline me-2 dark:text-purple-30 text-purple-40" width="1.5em" height="1.5em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><path fill-rule="evenodd" clip-rule="evenodd" d="M34.7409 59.7228L32.9567 58.9094C27.2672 56.3157 20.7328 56.3157 15.0433 58.9094C12.6018 60.0224 9.39163 59.0275 8.44602 56.0621C7.45647 52.9589 5.99975 46.5898 6 35.9997C6.00029 23.5648 8.00803 18.3599 9.11099 16.4196C9.67795 15.4222 10.5255 14.8455 11.2254 14.5264L12.0179 14.1651C19.6351 10.6926 28.4011 10.6738 36 14.1733C43.5989 10.6738 52.3649 10.6926 59.9821 14.1651L60.7746 14.5264C61.4745 14.8455 62.3221 15.4222 62.889 16.4196C63.992 18.3599 65.9997 23.5648 66 35.9997C66.0002 46.5898 64.5435 52.9589 63.554 56.0621C62.6084 59.0275 59.3982 60.0224 56.9567 58.9094C51.2672 56.3157 44.7328 56.3157 39.0433 58.9094L37.2591 59.7228C37.1986 59.7508 37.1373 59.7767 37.0753 59.8004C36.4484 60.0411 35.7556 60.0653 35.1102 59.8648C34.9847 59.8258 34.8613 59.7784 34.7409 59.7228ZM14.5068 19.6246C20.3733 16.9501 27.0874 16.8775 33 19.4067V52.473C26.7613 50.32 19.9378 50.471 13.7811 52.9261C13.0005 49.9843 11.9998 44.547 12 35.9998C12.0002 25.5786 13.4879 21.1893 14.1179 19.8018L14.5068 19.6246ZM39 52.473C45.2387 50.32 52.0622 50.471 58.2189 52.9261C58.9995 49.9843 60.0002 44.547 60 35.9998C59.9998 25.5786 58.5121 21.1893 57.8821 19.8018L57.4932 19.6246C51.6267 16.9501 44.9126 16.8775 39 19.4067V52.473Z" fill="currentColor"></path></svg>Deep Dive</h5><div class="mb-4"><h4 id="why-is-suppressing-the-dependency-linter-so-dangerous" class="mdx-heading text-xl font-bold text-primary dark:text-primary-dark text-xl font-display font-bold leading-9 my-4">Why is suppressing the dependency linter so dangerous? <a href="#why-is-suppressing-the-dependency-linter-so-dangerous" aria-label="Link for Why is suppressing the dependency linter so dangerous? " title="Link for Why is suppressing the dependency linter so dangerous? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4></div><button class="bg-purple-50 border-purple-50 hover:bg-purple-40 focus:bg-purple-50 active:bg-purple-50 text-base leading-tight font-bold rounded-full py-2 px-4 focus:outline focus:outline-offset-2 focus:outline-link dark:focus:outline-link-dark inline-flex items-center my-1 bg-link border-link text-white hover:bg-link focus:bg-link active:bg-link"><span class="me-1"><svg class="rotate-0" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></span>Show Details</button></summary><div class="p-8 border-t dark:border-purple-60 border-purple-10 "><p class="whitespace-pre-wrap my-4">Suppressing the linter leads to very unintuitive bugs that are hard to find and fix. Here’s one example:</p><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:R2cl887cq6:" aria-haspopup="true" aria-expanded="false" data-headlessui-state=""><span class="h-full py-2 px-1 mt-px -mb-px flex border-b text-link dark:text-link-dark border-link dark:border-link-dark items-center text-md leading-tight truncate" style="max-width:160px">App.js</span></button></div></div></div><div class="px-3 flex items-center justify-end text-start" translate="yes"><button class="text-sm text-primary dark:text-primary-dark inline-flex items-center hover:text-link duration-100 ease-in transition mx-1" title="Reset Sandbox" type="button"><svg width="1em" height="1em" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" class="inline mx-1 relative"><path d="M13.8982 5.20844C12.4626 4.88688 10.9686 4.93769 9.55821 5.35604L11.8524 3.06184C11.8989 3.0154 11.9357 2.96028 11.9608 2.89961C11.986 2.83894 11.9989 2.77391 11.9989 2.70824C11.9989 2.64256 11.986 2.57754 11.9608 2.51686C11.9357 2.45619 11.8989 2.40107 11.8524 2.35464L11.1456 1.64784C11.0992 1.60139 11.0441 1.56455 10.9834 1.53942C10.9227 1.51428 10.8577 1.50134 10.792 1.50134C10.7263 1.50134 10.6613 1.51428 10.6006 1.53942C10.54 1.56455 10.4848 1.60139 10.4384 1.64784L6.14571 5.94054C6.00654 6.07969 5.89615 6.2449 5.82083 6.42673C5.74551 6.60855 5.70675 6.80343 5.70675 7.00024C5.70675 7.19704 5.74551 7.39192 5.82083 7.57374C5.89615 7.75557 6.00654 7.92078 6.14571 8.05994L10.4387 12.3529C10.5325 12.4465 10.6595 12.4991 10.792 12.4991C10.9245 12.4991 11.0516 12.4465 11.1453 12.3529L11.8527 11.6455C11.9463 11.5518 11.9989 11.4247 11.9989 11.2922C11.9989 11.1598 11.9463 11.0327 11.8527 10.9389L8.77481 7.86104C9.99795 7.16236 11.415 6.8801 12.8125 7.05678C14.21 7.23347 15.5122 7.85953 16.523 8.84064C17.5338 9.82176 18.1983 11.1048 18.4165 12.4964C18.6347 13.888 18.3947 15.3129 17.7328 16.5562C17.0708 17.7996 16.0227 18.7942 14.7463 19.3902C13.47 19.9861 12.0345 20.1511 10.6563 19.8603C9.27798 19.5695 8.03152 18.8387 7.10469 17.778C6.17786 16.7172 5.62086 15.384 5.51761 13.9791C5.51156 13.8512 5.45689 13.7303 5.36477 13.6413C5.27265 13.5522 5.15001 13.5017 5.02191 13.5H4.02081C3.95297 13.4996 3.88574 13.5129 3.8232 13.5392C3.76065 13.5655 3.70408 13.6042 3.6569 13.6529C3.60972 13.7017 3.57291 13.7595 3.54869 13.8228C3.52448 13.8862 3.51336 13.9538 3.51601 14.0216C3.61349 15.5965 4.1473 17.1132 5.0577 18.4019C5.9681 19.6906 7.21917 20.7006 8.6709 21.3188C10.1226 21.937 11.7178 22.139 13.2778 21.9022C14.8378 21.6654 16.3011 20.9992 17.504 19.978C18.7069 18.9569 19.6019 17.6212 20.0889 16.1203C20.5759 14.6195 20.6356 13.0128 20.2614 11.4799C19.8872 9.94705 19.0938 8.54858 17.97 7.44098C16.8462 6.33339 15.4363 5.56037 13.8982 5.20844V5.20844Z" fill="currentColor"></path></svg> Reset</button><a href="https://codesandbox.io/api/v1/sandboxes/define?undefined&environment=create-react-app" rel="noreferrer noopener" target="_blank" title="Open in CodeSandbox" class="text-sm text-primary dark:text-primary-dark inline-flex items-center hover:text-link duration-100 ease-in transition mx-1 ms-2 md:ms-1"><svg width="1em" height="1em" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg" class="inline mx-1 relative top-[1px]"><path d="M20.5001 2H15.5001C15.3675 2 15.2403 2.05268 15.1465 2.14645C15.0528 2.24021 15.0001 2.36739 15.0001 2.5V3.5C15.0001 3.63261 15.0528 3.75979 15.1465 3.85355C15.2403 3.94732 15.3675 4 15.5001 4H18.5901L7.6501 14.94C7.60323 14.9865 7.56604 15.0418 7.54065 15.1027C7.51527 15.1636 7.5022 15.229 7.5022 15.295C7.5022 15.361 7.51527 15.4264 7.54065 15.4873C7.56604 15.5482 7.60323 15.6035 7.6501 15.65L8.3501 16.35C8.39658 16.3969 8.45188 16.4341 8.51281 16.4594C8.57374 16.4848 8.63909 16.4979 8.7051 16.4979C8.7711 16.4979 8.83646 16.4848 8.89738 16.4594C8.95831 16.4341 9.01362 16.3969 9.0601 16.35L20.0001 5.41V8.5C20.0001 8.63261 20.0528 8.75979 20.1465 8.85355C20.2403 8.94732 20.3675 9 20.5001 9H21.5001C21.6327 9 21.7599 8.94732 21.8537 8.85355C21.9474 8.75979 22.0001 8.63261 22.0001 8.5V3.5C22.0001 3.10218 21.8421 2.72064 21.5608 2.43934C21.2795 2.15804 20.8979 2 20.5001 2V2Z" fill="currentColor"></path><path d="M21.5 13H20.5C20.3674 13 20.2402 13.0527 20.1464 13.1464C20.0527 13.2402 20 13.3674 20 13.5V20H4V4H10.5C10.6326 4 10.7598 3.94732 10.8536 3.85355C10.9473 3.75979 11 3.63261 11 3.5V2.5C11 2.36739 10.9473 2.24021 10.8536 2.14645C10.7598 2.05268 10.6326 2 10.5 2H3.5C3.10218 2 2.72064 2.15804 2.43934 2.43934C2.15804 2.72064 2 3.10218 2 3.5V20.5C2 20.8978 2.15804 21.2794 2.43934 21.5607C2.72064 21.842 3.10218 22 3.5 22H20.5C20.8978 22 21.2794 21.842 21.5607 21.5607C21.842 21.2794 22 20.8978 22 20.5V13.5C22 13.3674 21.9473 13.2402 21.8536 13.1464C21.7598 13.0527 21.6326 13 21.5 13Z" fill="currentColor"></path></svg><span class="hidden md:block">Fork</span></a></div></div><div class=" sp-layout"><div class=" sp-editor sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Timer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">count</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCount</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">0</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">increment</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIncrement</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">1</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">onTick</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">setCount</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">count</span> + <span class="sp-syntax-plain">increment</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">id</span> = <span class="sp-syntax-definition">setInterval</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">onTick</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-static">1000</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">clearInterval</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">id</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// eslint-disable-next-line react-hooks/exhaustive-deps</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span> Counter: <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">count</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">button</span> <span class="sp-syntax-property">onClick</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setCount</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">0</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">></span>Reset<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">button</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">p</span><span class="sp-syntax-punctuation">></span> Every second, increment by: <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">button</span> <span class="sp-syntax-property">disabled</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">increment</span> === <span class="sp-syntax-static">0</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onClick</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">setIncrement</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">i</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">i</span> - <span class="sp-syntax-static">1</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">></span>–<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">button</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">b</span><span class="sp-syntax-punctuation">></span><span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">increment</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">b</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">button</span> <span class="sp-syntax-property">onClick</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-definition">setIncrement</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">i</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">i</span> + <span class="sp-syntax-static">1</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">></span>+<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">button</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">p</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><p class="whitespace-pre-wrap my-4">Let’s say that you wanted to run the Effect “only on mount”. You’ve read that <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#what-an-effect-with-empty-dependencies-means">empty (<code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code>) dependencies</a> do that, so you’ve decided to ignore the linter, and forcefully specified <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">[]</code> as the dependencies.</p><p class="whitespace-pre-wrap my-4">This counter was supposed to increment every second by the amount configurable with the two buttons. However, since you “lied” to React that this Effect doesn’t depend on anything, React forever keeps using the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onTick</code> function from the initial render. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/state-as-a-snapshot#rendering-takes-a-snapshot-in-time">During that render,</a> <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">count</code> was <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">0</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">increment</code> was <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">1</code>. This is why <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onTick</code> from that render always calls <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">setCount(0 + 1)</code> every second, and you always see <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">1</code>. Bugs like this are harder to fix when they’re spread across multiple components.</p><p class="whitespace-pre-wrap my-4">There’s always a better solution than ignoring the linter! To fix this code, you need to add <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onTick</code> to the dependency list. (To ensure the interval is only setup once, <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects#reading-latest-props-and-state-with-effect-events">make <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onTick</code> an Effect Event.</a>)</p><p class="whitespace-pre-wrap my-4"><strong class="font-bold">We recommend treating the dependency lint error as a compilation error. If you don’t suppress it, you will never see bugs like this.</strong> The rest of this page documents the alternatives for this and other cases.</p></div></details> <h2 id="removing-unnecessary-dependencies" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Removing unnecessary dependencies <a href="#removing-unnecessary-dependencies" aria-label="Link for Removing unnecessary dependencies " title="Link for Removing unnecessary dependencies " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2> <p class="whitespace-pre-wrap my-4">Every time you adjust the Effect’s dependencies to reflect the code, look at the dependency list. Does it make sense for the Effect to re-run when any of these dependencies change? Sometimes, the answer is “no”:</p> <ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1">You might want to re-execute <em>different parts</em> of your Effect under different conditions.</li> <li class="leading-relaxed mb-1">You might want to only read the <em>latest value</em> of some dependency instead of “reacting” to its changes.</li> <li class="leading-relaxed mb-1">A dependency may change too often <em>unintentionally</em> because it’s an object or a function.</li> </ul> <p class="whitespace-pre-wrap my-4">To find the right solution, you’ll need to answer a few questions about your Effect. Let’s walk through them.</p> <h3 id="should-this-code-move-to-an-event-handler" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Should this code move to an event handler? <a href="#should-this-code-move-to-an-event-handler" aria-label="Link for Should this code move to an event handler? " title="Link for Should this code move to an event handler? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">The first thing you should think about is whether this code should be an Effect at all.</p> <p class="whitespace-pre-wrap my-4">Imagine a form. On submit, you set the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">submitted</code> state variable to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">true</code>. You need to send a POST request and show a notification. You’ve put this logic inside an Effect that “reacts” to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">submitted</code> being <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">true</code>:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Form</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setSubmitted</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🔴 Avoid: Event-specific logic inside an Effect</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">post</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'/api/register'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">showNotification</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'Successfully registered!'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">handleSubmit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setSubmitted</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Later, you want to style the notification message according to the current theme, so you read the current theme. Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">theme</code> is declared in the component body, it is a reactive value, so you add it as a dependency:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Form</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setSubmitted</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">theme</span> = <span class="sp-syntax-definition">useContext</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">ThemeContext</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// 🔴 Avoid: Event-specific logic inside an Effect</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">post</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'/api/register'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">showNotification</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'Successfully registered!'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">theme</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">submitted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">theme</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">handleSubmit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setSubmitted</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span> <br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">By doing this, you’ve introduced a bug. Imagine you submit the form first and then switch between Dark and Light themes. The <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">theme</code> will change, the Effect will re-run, and so it will display the same notification again!</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">The problem here is that this shouldn’t be an Effect in the first place.</strong> You want to send this POST request and show the notification in response to <em>submitting the form,</em> which is a particular interaction. To run some code in response to particular interaction, put that logic directly into the corresponding event handler:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Form</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">theme</span> = <span class="sp-syntax-definition">useContext</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">ThemeContext</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">handleSubmit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ✅ Good: Event-specific logic is called from event handlers</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">post</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'/api/register'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">showNotification</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'Successfully registered!'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">theme</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span> <br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Now that the code is in an event handler, it’s not reactive—so it will only run when the user submits the form. Read more about <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects#reactive-values-and-reactive-logic">choosing between event handlers and Effects</a> and <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/you-might-not-need-an-effect">how to delete unnecessary Effects.</a></p> <h3 id="is-your-effect-doing-several-unrelated-things" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Is your Effect doing several unrelated things? <a href="#is-your-effect-doing-several-unrelated-things" aria-label="Link for Is your Effect doing several unrelated things? " title="Link for Is your Effect doing several unrelated things? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">The next question you should ask yourself is whether your Effect is doing several unrelated things.</p> <p class="whitespace-pre-wrap my-4">Imagine you’re creating a shipping form where the user needs to choose their city and area. You fetch the list of <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">cities</code> from the server according to the selected <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">country</code> to show them in a dropdown:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ShippingForm</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">country</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">cities</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCities</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCity</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">let</span> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">fetch</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/cities?country=</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">`</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">response</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">response</span>.<span class="sp-syntax-property">json</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">ignore</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setCities</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">This is a good example of <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/you-might-not-need-an-effect#fetching-data">fetching data in an Effect.</a> You are synchronizing the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">cities</code> state with the network according to the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">country</code> prop. You can’t do this in an event handler because you need to fetch as soon as <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ShippingForm</code> is displayed and whenever the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">country</code> changes (no matter which interaction causes it).</p> <p class="whitespace-pre-wrap my-4">Now let’s say you’re adding a second select box for city areas, which should fetch the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">areas</code> for the currently selected <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">city</code>. You might start by adding a second <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">fetch</code> call for the list of areas inside the same Effect:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ShippingForm</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">country</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">cities</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCities</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCity</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">areas</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setAreas</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">let</span> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">fetch</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/cities?country=</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">`</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">response</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">response</span>.<span class="sp-syntax-property">json</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">ignore</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setCities</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🔴 Avoid: A single Effect synchronizes two independent processes</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">fetch</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/areas?city=</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">`</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">response</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">response</span>.<span class="sp-syntax-property">json</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">ignore</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setAreas</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">However, since the Effect now uses the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">city</code> state variable, you’ve had to add <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">city</code> to the list of dependencies. That, in turn, introduced a problem: when the user selects a different city, the Effect will re-run and call <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">fetchCities(country)</code>. As a result, you will be unnecessarily refetching the list of cities many times.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">The problem with this code is that you’re synchronizing two different unrelated things:</strong></p> <ol class="ms-6 my-3 list-decimal"> <li class="leading-relaxed mb-1">You want to synchronize the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">cities</code> state to the network based on the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">country</code> prop.</li> <li class="leading-relaxed mb-1">You want to synchronize the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">areas</code> state to the network based on the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">city</code> state.</li> </ol> <p class="whitespace-pre-wrap my-4">Split the logic into two Effects, each of which reacts to the prop that it needs to synchronize with:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ShippingForm</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">country</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">cities</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCities</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">let</span> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">fetch</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/cities?country=</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">`</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">response</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">response</span>.<span class="sp-syntax-property">json</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line "> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">ignore</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setCities</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">country</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCity</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">areas</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setAreas</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">null</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">let</span> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">fetch</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/areas?city=</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">`</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">response</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">response</span>.<span class="sp-syntax-property">json</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> .<span class="sp-syntax-property">then</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">ignore</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setAreas</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">json</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">ignore</span> = <span class="sp-syntax-static">true</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">city</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Now the first Effect only re-runs if the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">country</code> changes, while the second Effect re-runs when the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">city</code> changes. You’ve separated them by purpose: two different things are synchronized by two separate Effects. Two separate Effects have two separate dependency lists, so they won’t trigger each other unintentionally.</p> <p class="whitespace-pre-wrap my-4">The final code is longer than the original, but splitting these Effects is still correct. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#each-effect-represents-a-separate-synchronization-process">Each Effect should represent an independent synchronization process.</a> In this example, deleting one Effect doesn’t break the other Effect’s logic. This means they <em>synchronize different things,</em> and it’s good to split them up. If you’re concerned about duplication, you can improve this code by <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/reusing-logic-with-custom-hooks#when-to-use-custom-hooks">extracting repetitive logic into a custom Hook.</a></p> <h3 id="are-you-reading-some-state-to-calculate-the-next-state" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Are you reading some state to calculate the next state? <a href="#are-you-reading-some-state-to-calculate-the-next-state" aria-label="Link for Are you reading some state to calculate the next state? " title="Link for Are you reading some state to calculate the next state? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">This Effect updates the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> state variable with a newly created array every time a new message arrives:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">It uses the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> variable to <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/updating-arrays-in-state">create a new array</a> starting with all the existing messages and adds the new message at the end. However, since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> is a reactive value read by an Effect, it must be a dependency:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">And making <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> a dependency introduces a problem.</p> <p class="whitespace-pre-wrap my-4">Every time you receive a message, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">setMessages()</code> causes the component to re-render with a new <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> array that includes the received message. However, since this Effect now depends on <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code>, this will <em>also</em> re-synchronize the Effect. So every new message will make the chat re-connect. The user would not like that!</p> <p class="whitespace-pre-wrap my-4">To fix the issue, don’t read <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> inside the Effect. Instead, pass an <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/reference/react/useState#updating-state-based-on-the-previous-state">updater function</a> to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">setMessages</code>:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">msgs</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">msgs</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Notice how your Effect does not read the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> variable at all now.</strong> You only need to pass an updater function like <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">msgs => [...msgs, receivedMessage]</code>. React <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/queueing-a-series-of-state-updates">puts your updater function in a queue</a> and will provide the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">msgs</code> argument to it during the next render. This is why the Effect itself doesn’t need to depend on <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">messages</code> anymore. As a result of this fix, receiving a chat message will no longer make the chat re-connect.</p> <h3 id="do-you-want-to-read-a-value-without-reacting-to-its-changes" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Do you want to read a value without “reacting” to its changes? <a href="#do-you-want-to-read-a-value-without-reacting-to-its-changes" aria-label="Link for Do you want to read a value without “reacting” to its changes? " title="Link for Do you want to read a value without “reacting” to its changes? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <div class="expandable-callout pt-8 pb-4 px-5 sm:px-8 my-8 relative rounded-none shadow-inner-border -mx-5 sm:mx-auto sm:rounded-2xl bg-yellow-5 dark:bg-yellow-60 dark:bg-opacity-20"><h3 class="text-2xl font-display font-bold text-yellow-50 dark:text-yellow-40"><svg class="inline me-2 mb-1 text-lg text-yellow-50 dark:text-yellow-40" width="2em" height="2em" viewBox="0 0 72 72" fill="none" xmlns="http://www.w3.org/2000/svg"><g clip-path="url(#clip0_40_48064)"><path d="M24 27C24 25.3431 25.3431 24 27 24H45C46.6569 24 48 25.3431 48 27C48 28.6569 46.6569 30 45 30H27C25.3431 30 24 28.6569 24 27Z" fill="currentColor"></path><path d="M24 39C24 37.3431 25.3431 36 27 36H39C40.6569 36 42 37.3431 42 39C42 40.6569 40.6569 42 39 42H27C25.3431 42 24 40.6569 24 39Z" fill="currentColor"></path><path fill-rule="evenodd" clip-rule="evenodd" d="M12 18C12 13.0294 16.0294 9 21 9H51C55.9706 9 60 13.0294 60 18V54C60 58.9706 55.9706 63 51 63H21C16.0294 63 12 58.9706 12 54V18ZM21 15H51C52.6569 15 54 16.3431 54 18V54C54 55.6569 52.6569 57 51 57H21C19.3431 57 18 55.6569 18 54V18C18 16.3431 19.3431 15 21 15Z" fill="currentColor"></path></g><defs><clipPath id="clip0_40_48064"><rect width="72" height="72" fill="white"></rect></clipPath></defs></svg>Under Construction</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">This section describes an <strong class="font-bold">experimental API that has not yet been released</strong> in a stable version of React.</p></div></div></div> <p class="whitespace-pre-wrap my-4">Suppose that you want to play a sound when the user receives a new message unless <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code> is <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">true</code>:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsMuted</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">msgs</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">msgs</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">playSound</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Since your Effect now uses <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code> in its code, you have to add it to the dependencies:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsMuted</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">msgs</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">msgs</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">playSound</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">The problem is that every time <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code> changes (for example, when the user presses the “Muted” toggle), the Effect will re-synchronize, and reconnect to the chat. This is not the desired user experience! (In this example, even disabling the linter would not work—if you do that, <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code> would get “stuck” with its old value.)</p> <p class="whitespace-pre-wrap my-4">To solve this problem, you need to extract the logic that shouldn’t be reactive out of the Effect. You don’t want this Effect to “react” to the changes in <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code>. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects#declaring-an-effect-event">Move this non-reactive piece of logic into an Effect Event:</a></p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffectEvent</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsMuted</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">false</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">onMessage</span> = <span class="sp-syntax-definition">useEffectEvent</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">setMessages</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">msgs</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">...</span><span class="sp-syntax-plain">msgs</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isMuted</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">playSound</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">onMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Effect Events let you split an Effect into reactive parts (which should “react” to reactive values like <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and their changes) and non-reactive parts (which only read their latest values, like <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onMessage</code> reads <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code>). <strong class="font-bold">Now that you read <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">isMuted</code> inside an Effect Event, it doesn’t need to be a dependency of your Effect.</strong> As a result, the chat won’t re-connect when you toggle the “Muted” setting on and off, solving the original issue!</p> <h4 id="wrapping-an-event-handler-from-the-props" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Wrapping an event handler from the props <a href="#wrapping-an-event-handler-from-the-props" aria-label="Link for Wrapping an event handler from the props " title="Link for Wrapping an event handler from the props " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">You might run into a similar problem when your component receives an event handler as a prop:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">onReceiveMessage</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">onReceiveMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">onReceiveMessage</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Suppose that the parent component passes a <em>different</em> <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onReceiveMessage</code> function on every render:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span><br/></div><div class="cm-line "> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">onReceiveMessage</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">receivedMessage</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">/></span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">onReceiveMessage</code> is a dependency, it would cause the Effect to re-synchronize after every parent re-render. This would make it re-connect to the chat. To solve this, wrap the call in an Effect Event:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">onReceiveMessage</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">messages</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessages</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">onMessage</span> = <span class="sp-syntax-definition">useEffectEvent</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">onReceiveMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">on</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'message'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">onMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">receivedMessage</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Effect Events aren’t reactive, so you don’t need to specify them as dependencies. As a result, the chat will no longer re-connect even if the parent component passes a function that’s different on every re-render.</p> <h4 id="separating-reactive-and-non-reactive-code" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Separating reactive and non-reactive code <a href="#separating-reactive-and-non-reactive-code" aria-label="Link for Separating reactive and non-reactive code " title="Link for Separating reactive and non-reactive code " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">In this example, you want to log a visit every time <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> changes. You want to include the current <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">notificationCount</code> with every log, but you <em>don’t</em> want a change to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">notificationCount</code> to trigger a log event.</p> <p class="whitespace-pre-wrap my-4">The solution is again to split out the non-reactive code into an Effect Event:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Chat</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">notificationCount</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">onVisit</span> = <span class="sp-syntax-definition">useEffectEvent</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">visitedRoomId</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">logVisit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">visitedRoomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">notificationCount</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">onVisit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">}</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">You want your logic to be reactive with regards to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>, so you read <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> inside of your Effect. However, you don’t want a change to <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">notificationCount</code> to log an extra visit, so you read <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">notificationCount</code> inside of the Effect Event. <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/separating-events-from-effects#reading-latest-props-and-state-with-effect-events">Learn more about reading the latest props and state from Effects using Effect Events.</a></p> <h3 id="does-some-reactive-value-change-unintentionally" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Does some reactive value change unintentionally? <a href="#does-some-reactive-value-change-unintentionally" aria-label="Link for Does some reactive value change unintentionally? " title="Link for Does some reactive value change unintentionally? " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h3> <p class="whitespace-pre-wrap my-4">Sometimes, you <em>do</em> want your Effect to “react” to a certain value, but that value changes more often than you’d like—and might not reflect any actual change from the user’s perspective. For example, let’s say that you create an <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code> object in the body of your component, and then read that object from inside of your Effect:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">This object is declared in the component body, so it’s a <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/lifecycle-of-reactive-effects#effects-react-to-reactive-values">reactive value.</a> When you read a reactive value like this inside an Effect, you declare it as a dependency. This ensures your Effect “reacts” to its changes:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "> <span class="sp-syntax-comment">// ...</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">It is important to declare it as a dependency! This ensures, for example, that if the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> changes, your Effect will re-connect to the chat with the new <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code>. However, there is also a problem with the code above. To see it, try typing into the input in the sandbox below, and watch what happens in the console:</p> </div><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 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13.1464C20.0527 13.2402 20 13.3674 20 13.5V20H4V4H10.5C10.6326 4 10.7598 3.94732 10.8536 3.85355C10.9473 3.75979 11 3.63261 11 3.5V2.5C11 2.36739 10.9473 2.24021 10.8536 2.14645C10.7598 2.05268 10.6326 2 10.5 2H3.5C3.10218 2 2.72064 2.15804 2.43934 2.43934C2.15804 2.72064 2 3.10218 2 3.5V20.5C2 20.8978 2.15804 21.2794 2.43934 21.5607C2.72064 21.842 3.10218 22 3.5 22H20.5C20.8978 22 21.2794 21.842 21.5607 21.5607C21.842 21.2794 22 20.8978 22 20.5V13.5C22 13.3674 21.9473 13.2402 21.8536 13.1464C21.7598 13.0527 21.6326 13 21.5 13Z" fill="currentColor"></path></svg><span class="hidden md:block">Fork</span></a></div></div><div class=" sp-layout"><div class=" sp-editor sp-stack"><div class=" sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for App.js" aria-multiline="true" class="sp-pristine sp-javascript sp-cm" role="textbox" tabindex="0" translate="no"><pre class=" sp-pre-placeholder" style="margin-left:var(--sp-space-11)"><span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Temporarily disable the linter to demonstrate the problem</span> <span class="sp-syntax-comment">// eslint-disable-next-line react-hooks/exhaustive-deps</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><div class="max-w-4xl ms-0 2xl:mx-auto"> <p class="whitespace-pre-wrap my-4">In the sandbox above, the input only updates the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">message</code> state variable. From the user’s perspective, this should not affect the chat connection. However, every time you update the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">message</code>, your component re-renders. When your component re-renders, the code inside of it runs again from scratch.</p> <p class="whitespace-pre-wrap my-4">A new <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code> object is created from scratch on every re-render of the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> component. React sees that the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code> object is a <em>different object</em> from the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code> object created during the last render. This is why it re-synchronizes your Effect (which depends on <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code>), and the chat re-connects as you type.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">This problem only affects objects and functions. In JavaScript, each newly created object and function is considered distinct from all the others. It doesn’t matter that the contents inside of them may be the same!</strong></p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-comment">// During the first render</span><br/></div><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options1</span> = <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'music'</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-comment">// During the next render</span><br/></div><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options2</span> = <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'music'</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-comment">// These are two different objects!</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-plain">console</span>.<span class="sp-syntax-property">log</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">Object</span>.<span class="sp-syntax-property">is</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options1</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">options2</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// false</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Object and function dependencies can make your Effect re-synchronize more often than you need.</strong></p> <p class="whitespace-pre-wrap my-4">This is why, whenever possible, you should try to avoid objects and functions as your Effect’s dependencies. Instead, try moving them outside the component, inside the Effect, or extracting primitive values out of them.</p> <h4 id="move-static-objects-and-functions-outside-your-component" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Move static objects and functions outside your component <a href="#move-static-objects-and-functions-outside-your-component" aria-label="Link for Move static objects and functions outside your component " title="Link for Move static objects and functions outside your component " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">If the object does not depend on any props and state, you can move that object outside your component:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'music'</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">This way, you <em>prove</em> to the linter that it’s not reactive. It can’t change as a result of a re-render, so it doesn’t need to be a dependency. Now re-rendering <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">ChatRoom</code> won’t cause your Effect to re-synchronize.</p> <p class="whitespace-pre-wrap my-4">This works for functions too:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">createOptions</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'music'</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-definition">createOptions</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">createOptions</code> is declared outside your component, it’s not a reactive value. This is why it doesn’t need to be specified in your Effect’s dependencies, and why it won’t ever cause your Effect to re-synchronize.</p> <h4 id="move-dynamic-objects-and-functions-inside-your-effect" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Move dynamic objects and functions inside your Effect <a href="#move-dynamic-objects-and-functions-inside-your-effect" aria-label="Link for Move dynamic objects and functions inside your Effect " title="Link for Move dynamic objects and functions inside your Effect " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">If your object depends on some reactive value that may change as a result of a re-render, like a <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> prop, you can’t pull it <em>outside</em> your component. You can, however, move its creation <em>inside</em> of your Effect’s code:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Now that <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options</code> is declared inside of your Effect, it is no longer a dependency of your Effect. Instead, the only reactive value used by your Effect is <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code>. Since <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> is not an object or function, you can be sure that it won’t be <em>unintentionally</em> different. In JavaScript, numbers and strings are compared by their content:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-comment">// During the first render</span><br/></div><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId1</span> = <span class="sp-syntax-string">'music'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-comment">// During the next render</span><br/></div><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">roomId2</span> = <span class="sp-syntax-string">'music'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-comment">// These two strings are the same!</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-plain">console</span>.<span class="sp-syntax-property">log</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">Object</span>.<span class="sp-syntax-property">is</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">roomId1</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">roomId2</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// true</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Thanks to this fix, the chat no longer re-connects if you edit the input:</p> </div><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button><button aria-selected="false" class=" sp-tab-button" data-active="false" role="tab" title="/src/chat.js" type="button">chat.js</button></div></div></div><button class="absolute top-0 start-[2px]" 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<span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">createConnection</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'./chat.js'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Welcome to the <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> room!<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setMessage</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">App</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setRoomId</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> Choose the chat room:<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-string">' '</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">select</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">onChange</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">e</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-definition">setRoomId</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span>.<span class="sp-syntax-property">target</span>.<span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"general"</span><span class="sp-syntax-punctuation">></span>general<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"travel"</span><span class="sp-syntax-punctuation">></span>travel<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">option</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-string">"music"</span><span class="sp-syntax-punctuation">></span>music<span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">option</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">select</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">label</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">hr</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-punctuation">/></span> <span class="sp-syntax-punctuation"></</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" style="height:15px;z-index:-1"></iframe></div></div></div><button translate="yes" class="sandpack-expand flex text-base justify-between dark:border-card-dark bg-wash dark:bg-card-dark items-center z-10 p-1 w-full order-2 xl:order-last border-b-1 relative top-0"><span class="flex p-2 focus:outline-none text-primary dark:text-primary-dark leading-[20px]"><svg class="rotate-0 inline me-1.5 text-xl" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg>Show more</span></button></div></div></div></div><div class="max-w-4xl ms-0 2xl:mx-auto"> <p class="whitespace-pre-wrap my-4">However, it <em>does</em> re-connect when you change the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> dropdown, as you would expect.</p> <p class="whitespace-pre-wrap my-4">This works for functions, too:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">serverUrl</span> = <span class="sp-syntax-string">'https://localhost:1234'</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">createOptions</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">options</span> = <span class="sp-syntax-definition">createOptions</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">You can write your own functions to group pieces of logic inside your Effect. As long as you also declare them <em>inside</em> your Effect, they’re not reactive values, and so they don’t need to be dependencies of your Effect.</p> <h4 id="read-primitive-values-from-objects" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Read primitive values from objects <a href="#read-primitive-values-from-objects" aria-label="Link for Read primitive values from objects " title="Link for Read primitive values from objects " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">Sometimes, you may receive an object from props:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">options</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">The risk here is that the parent component will create the object during rendering:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span><br/></div><div class="cm-line "> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">options</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">/></span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">This would cause your Effect to re-connect every time the parent component re-renders. To fix this, read information from the object <em>outside</em> the Effect, and avoid having object and function dependencies:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">options</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">serverUrl</span> <span class="sp-syntax-punctuation">}</span> = <span class="sp-syntax-plain">options</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">The logic gets a little repetitive (you read some values from an object outside an Effect, and then create an object with the same values inside the Effect). But it makes it very explicit what information your Effect <em>actually</em> depends on. If an object is re-created unintentionally by the parent component, the chat would not re-connect. However, if <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options.roomId</code> or <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">options.serverUrl</code> really are different, the chat would re-connect.</p> <h4 id="calculate-primitive-values-from-functions" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Calculate primitive values from functions <a href="#calculate-primitive-values-from-functions" aria-label="Link for Calculate primitive values from functions " title="Link for Calculate primitive values from functions " class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4> <p class="whitespace-pre-wrap my-4">The same approach can work for functions. For example, suppose the parent component passes a function:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line "><span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">ChatRoom</span><br/></div><div class="cm-line "> <span class="sp-syntax-property">roomId</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">getOptions</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">}</span><br/></div><div class="cm-line "><span class="sp-syntax-punctuation">/></span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">To avoid making it a dependency (and causing it to re-connect on re-renders), call it outside the Effect. This gives you the <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">roomId</code> and <code dir="ltr" class="inline text-code text-secondary dark:text-secondary-dark px-1 rounded-md no-underline bg-gray-30 bg-opacity-10 py-px">serverUrl</code> values that aren’t objects, and that you can read from inside your Effect:</p> <div dir="ltr" class="sandpack sandpack--codeblock rounded-2xl h-full w-full overflow-x-auto flex items-center bg-wash dark:bg-gray-95 shadow-lg my-8" style="contain:content"><div class="sp-wrapper"><div class="sp-stack"><div class="sp-code-editor"><pre class="sp-cm sp-pristine sp-javascript flex align-start"><code class="sp-pre-placeholder grow-[2]"><div class="cm-line bg-github-highlight dark:bg-opacity-10"><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">ChatRoom</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">getOptions</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">message</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setMessage</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">''</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">serverUrl</span> <span class="sp-syntax-punctuation">}</span> = <span class="sp-syntax-definition">getOptions</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connection</span> = <span class="sp-syntax-definition">createConnection</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-property">roomId</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span><br/></div><div class="cm-line "> <span class="sp-syntax-property">serverUrl</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-plain">serverUrl</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">connect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-plain">connection</span>.<span class="sp-syntax-property">disconnect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">roomId</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">serverUrl</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// ✅ All dependencies declared</span><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// ...</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">This only works for <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="/learn/keeping-components-pure">pure</a> functions because they are safe to call during rendering. If your function is an event handler, but you don’t want its changes to re-synchronize your Effect, <a class="inline text-link dark:text-link-dark border-b border-link border-opacity-0 hover:border-opacity-100 duration-100 ease-in transition leading-normal" href="#do-you-want-to-read-a-value-without-reacting-to-its-changes">wrap it into an Effect Event instead.</a></p> <section><h2 id="recap" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Recap<a href="#recap" aria-label="Link for Recap" title="Link for Recap" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2><ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1">Dependencies should always match the code.</li> <li class="leading-relaxed mb-1">When you’re not happy with your dependencies, what you need to edit is the code.</li> <li class="leading-relaxed mb-1">Suppressing the linter leads to very confusing bugs, and you should always avoid it.</li> <li class="leading-relaxed mb-1">To remove a dependency, you need to “prove” to the linter that it’s not necessary.</li> <li class="leading-relaxed mb-1">If some code should run in response to a specific interaction, move that code to an event handler.</li> <li class="leading-relaxed mb-1">If different parts of your Effect should re-run for different reasons, split it into several Effects.</li> <li class="leading-relaxed mb-1">If you want to update some state based on the previous state, pass an updater function.</li> <li class="leading-relaxed mb-1">If you want to read the latest value without “reacting” it, extract an Effect Event from your Effect.</li> <li class="leading-relaxed mb-1">In JavaScript, objects and functions are considered different if they were created at different times.</li> <li class="leading-relaxed mb-1">Try to avoid object and function dependencies. Move them outside the component or inside the Effect.</li> </ul></section> </div><div class="max-w-7xl mx-auto py-4 w-full"><div class="border-gray-10 bg-card dark:bg-card-dark shadow-inner rounded-none -mx-5 sm:mx-auto sm:rounded-2xl"><div class="py-2 px-5 sm:px-8 pb-0 md:pb-0"><h2 id="challenges" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6 mb-2 leading-10 relative text-3xl text-link">Try out some challenges<a href="#challenges" aria-label="Link for Try out some challenges" title="Link for Try out some challenges" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h2><div class="flex items-center justify-between"><div class="overflow-hidden"><div class="flex relative transition-transform content-box overflow-x-auto"><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis text-link border-link hover:text-link dark:text-link-dark dark:border-link-dark dark:hover:text-link-dark">1. Fix a resetting interval </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">2. Fix a retriggering animation </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">3. Fix a reconnecting chat </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">4. Fix a reconnecting chat, again </button></div></div><div class="flex z-10 pb-2 ps-2"><button aria-label="Scroll left" class="bg-secondary-button dark:bg-secondary-button-dark h-8 px-2 rounded-l border-gray-20 border-r rtl:rotate-180 text-gray-30"><svg class="rotate-90 rtl:-rotate-90" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></button><button aria-label="Scroll right" class="bg-secondary-button dark:bg-secondary-button-dark h-8 px-2 rounded-e rtl:rotate-180 text-primary dark:text-primary-dark"><svg class="-rotate-90 rtl:rotate-90" xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 20 20"><g fill="none" fill-rule="evenodd" transform="translate(-446 -398)"><path fill="currentColor" fill-rule="nonzero" d="M95.8838835,240.366117 C95.3957281,239.877961 94.6042719,239.877961 94.1161165,240.366117 C93.6279612,240.854272 93.6279612,241.645728 94.1161165,242.133883 L98.6161165,246.633883 C99.1042719,247.122039 99.8957281,247.122039 100.383883,246.633883 L104.883883,242.133883 C105.372039,241.645728 105.372039,240.854272 104.883883,240.366117 C104.395728,239.877961 103.604272,239.877961 103.116117,240.366117 L99.5,243.982233 L95.8838835,240.366117 Z" transform="translate(356.5 164.5)"></path><polygon points="446 418 466 418 466 398 446 398"></polygon></g></svg></button></div></div></div><div class="p-5 sm:py-8 sm:px-8"><div><h4 id="fix-a-resetting-interval" class="mdx-heading text-xl text-primary dark:text-primary-dark mb-2 mt-0 font-medium text-xl font-display font-bold leading-9 my-4"><div class="font-bold block md:inline">Challenge 1 of 4<span class="text-primary dark:text-primary-dark">: </span></div>Fix a resetting interval <a href="#fix-a-resetting-interval" aria-label="Link for this heading" title="Link for this heading" class="mdx-header-anchor inline-block"><svg width="1em" height="1em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg" class="text-gray-70 ms-2 h-5 w-5"><g fill="currentColor" fill-rule="evenodd"><path d="M7.778 7.975a2.5 2.5 0 0 0 .347-3.837L6.017 2.03a2.498 2.498 0 0 0-3.542-.007 2.5 2.5 0 0 0 .006 3.543l1.153 1.15c.07-.29.154-.563.25-.773.036-.077.084-.16.14-.25L3.18 4.85a1.496 1.496 0 0 1 .002-2.12 1.496 1.496 0 0 1 2.12 0l2.124 2.123a1.496 1.496 0 0 1-.333 2.37c.16.246.42.504.685.752z"></path><path d="M5.657 4.557a2.5 2.5 0 0 0-.347 3.837l2.108 2.108a2.498 2.498 0 0 0 3.542.007 2.5 2.5 0 0 0-.006-3.543L9.802 5.815c-.07.29-.154.565-.25.774-.036.076-.084.16-.14.25l.842.84c.585.587.59 1.532 0 2.122-.587.585-1.532.59-2.12 0L6.008 7.68a1.496 1.496 0 0 1 .332-2.372c-.16-.245-.42-.503-.685-.75z"></path></g></svg></a></h4><p class="whitespace-pre-wrap my-4">This Effect sets up an interval that ticks every second. You’ve noticed something strange happening: it seems like the interval gets destroyed and re-created every time it ticks. Fix the code so that the interval doesn’t get constantly re-created.</p><div class="sandpack sandpack--playground w-full my-8" dir="ltr"><div class="sp-wrapper"><div class="shadow-lg dark:shadow-lg-dark rounded-lg" style="contain:content"><div class="bg-wash dark:bg-card-dark flex justify-between items-center relative z-10 border-b border-border dark:border-border-dark rounded-t-lg text-lg"><div class="flex-1 grow min-w-0 px-4 lg:px-6"><div><div class="relative overflow-hidden"><div class="w-[fit-content] invisible"><div class=" sp-tabs" translate="no"><div aria-label="Select active file" class=" sp-tabs-scrollable-container" role="tablist"><button aria-selected="true" class=" sp-tab-button" data-active="true" role="tab" title="/src/App.js" type="button">App.js</button></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:R4pd3ccq6:" aria-haspopup="true" aria-expanded="false" data-headlessui-state=""><span class="h-full py-2 px-1 mt-px -mb-px flex 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class="sp-syntax-keyword">import</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">useState</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useEffect</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">from</span> <span class="sp-syntax-string">'react'</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">export</span> <span class="sp-syntax-keyword">default</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">Timer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">count</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setCount</span><span class="sp-syntax-punctuation">]</span> = <span class="sp-syntax-definition">useState</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-static">0</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">useEffect</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">console</span>.<span class="sp-syntax-property">log</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'✅ Creating an interval'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">id</span> = <span class="sp-syntax-definition">setInterval</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">console</span>.<span class="sp-syntax-property">log</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'⏰ Interval tick'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">setCount</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">count</span> + <span class="sp-syntax-static">1</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-static">1000</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">=></span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">console</span>.<span class="sp-syntax-property">log</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'❌ Clearing an interval'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">clearInterval</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">id</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">count</span><span class="sp-syntax-punctuation">]</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span>Counter: <span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">count</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation"></</span><span class="sp-syntax-tag">h1</span><span class="sp-syntax-punctuation">></span> <span class="sp-syntax-punctuation">}</span> </pre></div></div></div><div class=" order-last xl:order-2 sp-stack"><div class="p-0 sm:p-2 md:p-4 lg:p-8 bg-card dark:bg-wash-dark h-full relative md:rounded-b-lg lg:rounded-b-none"><div style="position:relative"><iframe class="rounded-t-none bg-white md:shadow-md sm:rounded-lg w-full max-w-full transition-opacity absolute opacity-0 pointer-events-none duration-75" title="Sandbox Preview" 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dark:group-focus:text-link-dark group-focus:text-opacity-100">Next</span><span class="text-lg break-words group-hover:underline">Reusing Logic with Custom Hooks</span></div></a></div></div></div></article><div class="self-stretch w-full"><div class="w-full px-5 pt-10 mx-auto sm:px-12 md:px-12 md:pt-12 lg:pt-10"><hr class="mx-auto max-w-7xl border-border dark:border-border-dark"/></div><div class="py-12 px-5 sm:px-12 md:px-12 sm:py-12 md:py-16 lg:py-14"><footer class="text-secondary dark:text-secondary-dark"><div class="grid grid-cols-2 md:grid-cols-3 xl:grid-cols-5 gap-x-12 gap-y-8 max-w-7xl mx-auto"><div class="col-span-2 md:col-span-1 justify-items-start mt-3.5"><a href="https://opensource.fb.com/" target="_blank" rel="noopener" aria-label="Meta Open Source"><div><svg width="160" height="19" viewBox="0 0 160 19" fill="none" xmlns="http://www.w3.org/2000/svg" class="text-primary dark:text-primary-dark"><path d="M22.0605 3.62598H24.3349L28.202 10.6212L32.0691 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text-secondary dark:text-secondary-dark px-4 w-full">On this page</h2><div class="h-full overflow-y-auto ps-4 max-h-[calc(100vh-7.5rem)]" style="overscroll-behavior:contain"><ul class="space-y-2 pb-16"><li class="text-sm px-2 rounded-s-xl bg-highlight dark:bg-highlight-dark"><a class="text-link dark:text-link-dark font-bold block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#">Overview</a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#dependencies-should-match-the-code">Dependencies should match the code </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#to-remove-a-dependency-prove-that-its-not-a-dependency">To remove a dependency, prove that it’s not a dependency </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#to-change-the-dependencies-change-the-code">To change the dependencies, change the code </a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#removing-unnecessary-dependencies">Removing unnecessary dependencies </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#should-this-code-move-to-an-event-handler">Should this code move to an event handler? </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#is-your-effect-doing-several-unrelated-things">Is your Effect doing several unrelated things? </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#are-you-reading-some-state-to-calculate-the-next-state">Are you reading some state to calculate the next state? </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#do-you-want-to-read-a-value-without-reacting-to-its-changes">Do you want to read a value without “reacting” to its changes? </a></li><li class="text-sm px-2 rounded-s-xl ps-4"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#does-some-reactive-value-change-unintentionally">Does some reactive value change unintentionally? </a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#recap">Recap</a></li><li class="text-sm px-2 rounded-s-xl"><a class="text-secondary dark:text-secondary-dark block hover:text-link dark:hover:text-link-dark leading-normal py-2" href="#challenges">Challenges</a></li></ul></div></nav></div></div></div><script id="__NEXT_DATA__" type="application/json">{"props":{"pageProps":{"toc":"[{\"url\":\"#\",\"text\":\"Overview\",\"depth\":2},{\"url\":\"#dependencies-should-match-the-code\",\"depth\":2,\"text\":\"Dependencies should match the code \"},{\"url\":\"#to-remove-a-dependency-prove-that-its-not-a-dependency\",\"depth\":3,\"text\":\"To remove a dependency, prove that it’s not a dependency \"},{\"url\":\"#to-change-the-dependencies-change-the-code\",\"depth\":3,\"text\":\"To change the dependencies, change the code \"},{\"url\":\"#removing-unnecessary-dependencies\",\"depth\":2,\"text\":\"Removing unnecessary dependencies \"},{\"url\":\"#should-this-code-move-to-an-event-handler\",\"depth\":3,\"text\":\"Should this code move to an event handler? \"},{\"url\":\"#is-your-effect-doing-several-unrelated-things\",\"depth\":3,\"text\":\"Is your Effect doing several unrelated things? \"},{\"url\":\"#are-you-reading-some-state-to-calculate-the-next-state\",\"depth\":3,\"text\":\"Are you reading some state to calculate the next state? \"},{\"url\":\"#do-you-want-to-read-a-value-without-reacting-to-its-changes\",\"depth\":3,\"text\":\"Do you want to read a value without “reacting” to its changes? \"},{\"url\":\"#does-some-reactive-value-change-unintentionally\",\"depth\":3,\"text\":\"Does some reactive value change unintentionally? \"},{\"url\":\"#recap\",\"depth\":2,\"text\":\"Recap\"},{\"url\":\"#challenges\",\"depth\":2,\"text\":\"Challenges\"}]","content":"[[\"$r\",\"MaxWidth\",\"12\",{\"children\":[[\"$r\",\"Intro\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"When you write an Effect, the linter will verify that you’ve included every reactive value (like props and state) that the Effect reads in the list of your Effect’s dependencies. This ensures that your Effect remains synchronized with the latest props and state of your component. Unnecessary dependencies may cause your Effect to run too often, or even create an infinite loop. Follow this guide to review and remove unnecessary dependencies from your Effects.\"}]}],\"\\n\",[\"$r\",\"YouWillLearn\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How to fix infinite Effect dependency loops\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"What to do when you want to remove a dependency\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How to read a value from your Effect without “reacting” to it\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How and why to avoid object and function dependencies\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Why suppressing the dependency linter is dangerous, and what to do instead\"}],\"\\n\"]}]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"dependencies-should-match-the-code\",\"children\":\"Dependencies should match the code \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"When you write an Effect, you first specify how to \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#the-lifecycle-of-an-effect\",\"children\":\"start and stop\"}],\" whatever you want your Effect to be doing:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{5-7}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n \\t// ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Then, if you leave the Effect dependencies empty (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\"), the linter will suggest the correct dependencies:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // \u003c-- Fix the mistake here!\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"20\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Fill them in according to what the linter says:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6}\",\"children\":\"function ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#effects-react-to-reactive-values\",\"children\":\"Effects “react” to reactive values.\"}],\" Since \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is a reactive value (it can change due to a re-render), the linter verifies that you’ve specified it as a dependency. If \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" receives a different value, React will re-synchronize your Effect. This ensures that the chat stays connected to the selected room and “reacts” to the dropdown:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]);\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"40\",{\"children\":[\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"to-remove-a-dependency-prove-that-its-not-a-dependency\",\"children\":\"To remove a dependency, prove that it’s not a dependency \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Notice that you can’t “choose” the dependencies of your Effect. Every \",[\"$r\",\"CodeStep\",null,{\"step\":2,\"children\":\"reactive value\"}],\" used by your Effect’s code must be declared in your dependency list. The dependency list is determined by the surrounding code:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"[[2, 3, \\\"roomId\\\"], [2, 5, \\\"roomId\\\"], [2, 8, \\\"roomId\\\"]]\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) { // This is a reactive value\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId); // This Effect reads that reactive value\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ So you must specify that reactive value as a dependency of your Effect\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#all-variables-declared-in-the-component-body-are-reactive\",\"children\":\"Reactive values\"}],\" include props and all variables and functions declared directly inside of your component. Since \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is a reactive value, you can’t remove it from the dependency list. The linter wouldn’t allow it:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{8}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // 🔴 React Hook useEffect has a missing dependency: 'roomId'\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"And the linter would be right! Since \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" may change over time, this would introduce a bug in your code.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"To remove a dependency, “prove” to the linter that it \",[\"$r\",\"em\",null,{\"children\":\"doesn’t need\"}],\" to be a dependency.\"]}],\" For example, you can move \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" out of your component to prove that it’s not reactive and won’t change on re-renders:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2,9}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\nconst roomId = 'music'; // Not a reactive value anymore\\n\\nfunction ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now that \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is not a reactive value (and can’t change on a re-render), it doesn’t need to be a dependency:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\nconst roomId = 'music';\\n\\nexport default function ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection(serverUrl, roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []);\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection(serverUrl, roomId) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"192\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This is why you could now specify an \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#what-an-effect-with-empty-dependencies-means\",\"children\":[\"empty (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\") dependency list.\"]}],\" Your Effect \",[\"$r\",\"em\",null,{\"children\":\"really doesn’t\"}],\" depend on any reactive value anymore, so it \",[\"$r\",\"em\",null,{\"children\":\"really doesn’t\"}],\" need to re-run when any of the component’s props or state change.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"to-change-the-dependencies-change-the-code\",\"children\":\"To change the dependencies, change the code \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"You might have noticed a pattern in your workflow:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"First, you \",[\"$r\",\"strong\",null,{\"children\":\"change the code\"}],\" of your Effect or how your reactive values are declared.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Then, you follow the linter and adjust the dependencies to \",[\"$r\",\"strong\",null,{\"children\":\"match the code you have changed.\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"If you’re not happy with the list of dependencies, you \",[\"$r\",\"strong\",null,{\"children\":\"go back to the first step\"}],\" (and change the code again).\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The last part is important. \",[\"$r\",\"strong\",null,{\"children\":\"If you want to change the dependencies, change the surrounding code first.\"}],\" You can think of the dependency list as \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#react-verifies-that-you-specified-every-reactive-value-as-a-dependency\",\"children\":\"a list of all the reactive values used by your Effect’s code.\"}],\" You don’t \",[\"$r\",\"em\",null,{\"children\":\"choose\"}],\" what to put on that list. The list \",[\"$r\",\"em\",null,{\"children\":\"describes\"}],\" your code. To change the dependency list, change the code.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This might feel like solving an equation. You might start with a goal (for example, to remove a dependency), and you need to “find” the code matching that goal. Not everyone finds solving equations fun, and the same thing could be said about writing Effects! Luckily, there is a list of common recipes that you can try below.\"}],\"\\n\",[\"$r\",\"Pitfall\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":\"If you have an existing codebase, you might have some Effects that suppress the linter like this:\"}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-4}\",\"children\":\"useEffect(() =\u003e {\\n // ...\\n // 🔴 Avoid suppressing the linter like this:\\n // eslint-ignore-next-line react-hooks/exhaustive-deps\\n}, []);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"When dependencies don’t match the code, there is a very high risk of introducing bugs.\"}],\" By suppressing the linter, you “lie” to React about the values your Effect depends on.\"]}],[\"$r\",\"p\",null,{\"children\":\"Instead, use the techniques below.\"}]]}],\"\\n\",[\"$r\",\"DeepDive\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"why-is-suppressing-the-dependency-linter-so-dangerous\",\"children\":\"Why is suppressing the dependency linter so dangerous? \"}],[\"$r\",\"p\",null,{\"children\":\"Suppressing the linter leads to very unintuitive bugs that are hard to find and fix. Here’s one example:\"}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function Timer() {\\n const [count, setCount] = useState(0);\\n const [increment, setIncrement] = useState(1);\\n\\n function onTick() {\\n\\tsetCount(count + increment);\\n }\\n\\n useEffect(() =\u003e {\\n const id = setInterval(onTick, 1000);\\n return () =\u003e clearInterval(id);\\n // eslint-disable-next-line react-hooks/exhaustive-deps\\n }, []);\\n\\n return (\\n \u003c\u003e\\n \u003ch1\u003e\\n Counter: {count}\\n \u003cbutton onClick={() =\u003e setCount(0)}\u003eReset\u003c/button\u003e\\n \u003c/h1\u003e\\n \u003chr /\u003e\\n \u003cp\u003e\\n Every second, increment by:\\n \u003cbutton disabled={increment === 0} onClick={() =\u003e {\\n setIncrement(i =\u003e i - 1);\\n }}\u003e–\u003c/button\u003e\\n \u003cb\u003e{increment}\u003c/b\u003e\\n \u003cbutton onClick={() =\u003e {\\n setIncrement(i =\u003e i + 1);\\n }}\u003e+\u003c/button\u003e\\n \u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"button { margin: 10px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Let’s say that you wanted to run the Effect “only on mount”. You’ve read that \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#what-an-effect-with-empty-dependencies-means\",\"children\":[\"empty (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\") dependencies\"]}],\" do that, so you’ve decided to ignore the linter, and forcefully specified \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" as the dependencies.\"]}],[\"$r\",\"p\",null,{\"children\":[\"This counter was supposed to increment every second by the amount configurable with the two buttons. However, since you “lied” to React that this Effect doesn’t depend on anything, React forever keeps using the \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" function from the initial render. \",[\"$r\",\"a\",null,{\"href\":\"/learn/state-as-a-snapshot#rendering-takes-a-snapshot-in-time\",\"children\":\"During that render,\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\" was \",[\"$r\",\"code\",null,{\"children\":\"0\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"increment\"}],\" was \",[\"$r\",\"code\",null,{\"children\":\"1\"}],\". This is why \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" from that render always calls \",[\"$r\",\"code\",null,{\"children\":\"setCount(0 + 1)\"}],\" every second, and you always see \",[\"$r\",\"code\",null,{\"children\":\"1\"}],\". Bugs like this are harder to fix when they’re spread across multiple components.\"]}],[\"$r\",\"p\",null,{\"children\":[\"There’s always a better solution than ignoring the linter! To fix this code, you need to add \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" to the dependency list. (To ensure the interval is only setup once, \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects#reading-latest-props-and-state-with-effect-events\",\"children\":[\"make \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" an Effect Event.\"]}],\")\"]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"We recommend treating the dependency lint error as a compilation error. If you don’t suppress it, you will never see bugs like this.\"}],\" The rest of this page documents the alternatives for this and other cases.\"]}]]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"removing-unnecessary-dependencies\",\"children\":\"Removing unnecessary dependencies \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Every time you adjust the Effect’s dependencies to reflect the code, look at the dependency list. Does it make sense for the Effect to re-run when any of these dependencies change? Sometimes, the answer is “no”:\"}],\"\\n\",[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You might want to re-execute \",[\"$r\",\"em\",null,{\"children\":\"different parts\"}],\" of your Effect under different conditions.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You might want to only read the \",[\"$r\",\"em\",null,{\"children\":\"latest value\"}],\" of some dependency instead of “reacting” to its changes.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"A dependency may change too often \",[\"$r\",\"em\",null,{\"children\":\"unintentionally\"}],\" because it’s an object or a function.\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"To find the right solution, you’ll need to answer a few questions about your Effect. Let’s walk through them.\"}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"should-this-code-move-to-an-event-handler\",\"children\":\"Should this code move to an event handler? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The first thing you should think about is whether this code should be an Effect at all.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Imagine a form. On submit, you set the \",[\"$r\",\"code\",null,{\"children\":\"submitted\"}],\" state variable to \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\". You need to send a POST request and show a notification. You’ve put this logic inside an Effect that “reacts” to \",[\"$r\",\"code\",null,{\"children\":\"submitted\"}],\" being \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\":\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6-8}\",\"children\":\"function Form() {\\n const [submitted, setSubmitted] = useState(false);\\n\\n useEffect(() =\u003e {\\n if (submitted) {\\n // 🔴 Avoid: Event-specific logic inside an Effect\\n post('/api/register');\\n showNotification('Successfully registered!');\\n }\\n }, [submitted]);\\n\\n function handleSubmit() {\\n setSubmitted(true);\\n }\\n\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Later, you want to style the notification message according to the current theme, so you read the current theme. Since \",[\"$r\",\"code\",null,{\"children\":\"theme\"}],\" is declared in the component body, it is a reactive value, so you add it as a dependency:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,9,11}\",\"children\":\"function Form() {\\n const [submitted, setSubmitted] = useState(false);\\n const theme = useContext(ThemeContext);\\n\\n useEffect(() =\u003e {\\n if (submitted) {\\n // 🔴 Avoid: Event-specific logic inside an Effect\\n post('/api/register');\\n showNotification('Successfully registered!', theme);\\n }\\n }, [submitted, theme]); // ✅ All dependencies declared\\n\\n function handleSubmit() {\\n setSubmitted(true);\\n } \\n\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"By doing this, you’ve introduced a bug. Imagine you submit the form first and then switch between Dark and Light themes. The \",[\"$r\",\"code\",null,{\"children\":\"theme\"}],\" will change, the Effect will re-run, and so it will display the same notification again!\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"The problem here is that this shouldn’t be an Effect in the first place.\"}],\" You want to send this POST request and show the notification in response to \",[\"$r\",\"em\",null,{\"children\":\"submitting the form,\"}],\" which is a particular interaction. To run some code in response to particular interaction, put that logic directly into the corresponding event handler:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6-7}\",\"children\":\"function Form() {\\n const theme = useContext(ThemeContext);\\n\\n function handleSubmit() {\\n // ✅ Good: Event-specific logic is called from event handlers\\n post('/api/register');\\n showNotification('Successfully registered!', theme);\\n } \\n\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now that the code is in an event handler, it’s not reactive—so it will only run when the user submits the form. Read more about \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects#reactive-values-and-reactive-logic\",\"children\":\"choosing between event handlers and Effects\"}],\" and \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect\",\"children\":\"how to delete unnecessary Effects.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"is-your-effect-doing-several-unrelated-things\",\"children\":\"Is your Effect doing several unrelated things? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The next question you should ask yourself is whether your Effect is doing several unrelated things.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Imagine you’re creating a shipping form where the user needs to choose their city and area. You fetch the list of \",[\"$r\",\"code\",null,{\"children\":\"cities\"}],\" from the server according to the selected \",[\"$r\",\"code\",null,{\"children\":\"country\"}],\" to show them in a dropdown:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"function ShippingForm({ country }) {\\n const [cities, setCities] = useState(null);\\n const [city, setCity] = useState(null);\\n\\n useEffect(() =\u003e {\\n let ignore = false;\\n fetch(`/api/cities?country=${country}`)\\n .then(response =\u003e response.json())\\n .then(json =\u003e {\\n if (!ignore) {\\n setCities(json);\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n };\\n }, [country]); // ✅ All dependencies declared\\n\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This is a good example of \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect#fetching-data\",\"children\":\"fetching data in an Effect.\"}],\" You are synchronizing the \",[\"$r\",\"code\",null,{\"children\":\"cities\"}],\" state with the network according to the \",[\"$r\",\"code\",null,{\"children\":\"country\"}],\" prop. You can’t do this in an event handler because you need to fetch as soon as \",[\"$r\",\"code\",null,{\"children\":\"ShippingForm\"}],\" is displayed and whenever the \",[\"$r\",\"code\",null,{\"children\":\"country\"}],\" changes (no matter which interaction causes it).\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now let’s say you’re adding a second select box for city areas, which should fetch the \",[\"$r\",\"code\",null,{\"children\":\"areas\"}],\" for the currently selected \",[\"$r\",\"code\",null,{\"children\":\"city\"}],\". You might start by adding a second \",[\"$r\",\"code\",null,{\"children\":\"fetch\"}],\" call for the list of areas inside the same Effect:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{15-24,28}\",\"children\":\"function ShippingForm({ country }) {\\n const [cities, setCities] = useState(null);\\n const [city, setCity] = useState(null);\\n const [areas, setAreas] = useState(null);\\n\\n useEffect(() =\u003e {\\n let ignore = false;\\n fetch(`/api/cities?country=${country}`)\\n .then(response =\u003e response.json())\\n .then(json =\u003e {\\n if (!ignore) {\\n setCities(json);\\n }\\n });\\n // 🔴 Avoid: A single Effect synchronizes two independent processes\\n if (city) {\\n fetch(`/api/areas?city=${city}`)\\n .then(response =\u003e response.json())\\n .then(json =\u003e {\\n if (!ignore) {\\n setAreas(json);\\n }\\n });\\n }\\n return () =\u003e {\\n ignore = true;\\n };\\n }, [country, city]); // ✅ All dependencies declared\\n\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"However, since the Effect now uses the \",[\"$r\",\"code\",null,{\"children\":\"city\"}],\" state variable, you’ve had to add \",[\"$r\",\"code\",null,{\"children\":\"city\"}],\" to the list of dependencies. That, in turn, introduced a problem: when the user selects a different city, the Effect will re-run and call \",[\"$r\",\"code\",null,{\"children\":\"fetchCities(country)\"}],\". As a result, you will be unnecessarily refetching the list of cities many times.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"The problem with this code is that you’re synchronizing two different unrelated things:\"}]}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You want to synchronize the \",[\"$r\",\"code\",null,{\"children\":\"cities\"}],\" state to the network based on the \",[\"$r\",\"code\",null,{\"children\":\"country\"}],\" prop.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"You want to synchronize the \",[\"$r\",\"code\",null,{\"children\":\"areas\"}],\" state to the network based on the \",[\"$r\",\"code\",null,{\"children\":\"city\"}],\" state.\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Split the logic into two Effects, each of which reacts to the prop that it needs to synchronize with:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{19-33}\",\"children\":\"function ShippingForm({ country }) {\\n const [cities, setCities] = useState(null);\\n useEffect(() =\u003e {\\n let ignore = false;\\n fetch(`/api/cities?country=${country}`)\\n .then(response =\u003e response.json())\\n .then(json =\u003e {\\n if (!ignore) {\\n setCities(json);\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n };\\n }, [country]); // ✅ All dependencies declared\\n\\n const [city, setCity] = useState(null);\\n const [areas, setAreas] = useState(null);\\n useEffect(() =\u003e {\\n if (city) {\\n let ignore = false;\\n fetch(`/api/areas?city=${city}`)\\n .then(response =\u003e response.json())\\n .then(json =\u003e {\\n if (!ignore) {\\n setAreas(json);\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n };\\n }\\n }, [city]); // ✅ All dependencies declared\\n\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now the first Effect only re-runs if the \",[\"$r\",\"code\",null,{\"children\":\"country\"}],\" changes, while the second Effect re-runs when the \",[\"$r\",\"code\",null,{\"children\":\"city\"}],\" changes. You’ve separated them by purpose: two different things are synchronized by two separate Effects. Two separate Effects have two separate dependency lists, so they won’t trigger each other unintentionally.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The final code is longer than the original, but splitting these Effects is still correct. \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#each-effect-represents-a-separate-synchronization-process\",\"children\":\"Each Effect should represent an independent synchronization process.\"}],\" In this example, deleting one Effect doesn’t break the other Effect’s logic. This means they \",[\"$r\",\"em\",null,{\"children\":\"synchronize different things,\"}],\" and it’s good to split them up. If you’re concerned about duplication, you can improve this code by \",[\"$r\",\"a\",null,{\"href\":\"/learn/reusing-logic-with-custom-hooks#when-to-use-custom-hooks\",\"children\":\"extracting repetitive logic into a custom Hook.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"are-you-reading-some-state-to-calculate-the-next-state\",\"children\":\"Are you reading some state to calculate the next state? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This Effect updates the \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" state variable with a newly created array every time a new message arrives:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2,6-8}\",\"children\":\"function ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n setMessages([...messages, receivedMessage]);\\n });\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"It uses the \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" variable to \",[\"$r\",\"a\",null,{\"href\":\"/learn/updating-arrays-in-state\",\"children\":\"create a new array\"}],\" starting with all the existing messages and adds the new message at the end. However, since \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" is a reactive value read by an Effect, it must be a dependency:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7,10}\",\"children\":\"function ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n setMessages([...messages, receivedMessage]);\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId, messages]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"And making \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" a dependency introduces a problem.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Every time you receive a message, \",[\"$r\",\"code\",null,{\"children\":\"setMessages()\"}],\" causes the component to re-render with a new \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" array that includes the received message. However, since this Effect now depends on \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\", this will \",[\"$r\",\"em\",null,{\"children\":\"also\"}],\" re-synchronize the Effect. So every new message will make the chat re-connect. The user would not like that!\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To fix the issue, don’t read \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" inside the Effect. Instead, pass an \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useState#updating-state-based-on-the-previous-state\",\"children\":\"updater function\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"setMessages\"}],\":\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7,10}\",\"children\":\"function ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n setMessages(msgs =\u003e [...msgs, receivedMessage]);\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"Notice how your Effect does not read the \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" variable at all now.\"]}],\" You only need to pass an updater function like \",[\"$r\",\"code\",null,{\"children\":\"msgs =\u003e [...msgs, receivedMessage]\"}],\". React \",[\"$r\",\"a\",null,{\"href\":\"/learn/queueing-a-series-of-state-updates\",\"children\":\"puts your updater function in a queue\"}],\" and will provide the \",[\"$r\",\"code\",null,{\"children\":\"msgs\"}],\" argument to it during the next render. This is why the Effect itself doesn’t need to depend on \",[\"$r\",\"code\",null,{\"children\":\"messages\"}],\" anymore. As a result of this fix, receiving a chat message will no longer make the chat re-connect.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"do-you-want-to-read-a-value-without-reacting-to-its-changes\",\"children\":\"Do you want to read a value without “reacting” to its changes? \"}],\"\\n\",[\"$r\",\"Wip\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"This section describes an \",[\"$r\",\"strong\",null,{\"children\":\"experimental API that has not yet been released\"}],\" in a stable version of React.\"]}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Suppose that you want to play a sound when the user receives a new message unless \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\" is \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\":\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,10-12}\",\"children\":\"function ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n const [isMuted, setIsMuted] = useState(false);\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n setMessages(msgs =\u003e [...msgs, receivedMessage]);\\n if (!isMuted) {\\n playSound();\\n }\\n });\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Since your Effect now uses \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\" in its code, you have to add it to the dependencies:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{10,15}\",\"children\":\"function ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n const [isMuted, setIsMuted] = useState(false);\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n setMessages(msgs =\u003e [...msgs, receivedMessage]);\\n if (!isMuted) {\\n playSound();\\n }\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId, isMuted]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The problem is that every time \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\" changes (for example, when the user presses the “Muted” toggle), the Effect will re-synchronize, and reconnect to the chat. This is not the desired user experience! (In this example, even disabling the linter would not work—if you do that, \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\" would get “stuck” with its old value.)\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To solve this problem, you need to extract the logic that shouldn’t be reactive out of the Effect. You don’t want this Effect to “react” to the changes in \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\". \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects#declaring-an-effect-event\",\"children\":\"Move this non-reactive piece of logic into an Effect Event:\"}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,7-12,18,21}\",\"children\":\"import { useState, useEffect, useEffectEvent } from 'react';\\n\\nfunction ChatRoom({ roomId }) {\\n const [messages, setMessages] = useState([]);\\n const [isMuted, setIsMuted] = useState(false);\\n\\n const onMessage = useEffectEvent(receivedMessage =\u003e {\\n setMessages(msgs =\u003e [...msgs, receivedMessage]);\\n if (!isMuted) {\\n playSound();\\n }\\n });\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n onMessage(receivedMessage);\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Effect Events let you split an Effect into reactive parts (which should “react” to reactive values like \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and their changes) and non-reactive parts (which only read their latest values, like \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\" reads \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\"). \",[\"$r\",\"strong\",null,{\"children\":[\"Now that you read \",[\"$r\",\"code\",null,{\"children\":\"isMuted\"}],\" inside an Effect Event, it doesn’t need to be a dependency of your Effect.\"]}],\" As a result, the chat won’t re-connect when you toggle the “Muted” setting on and off, solving the original issue!\"]}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"wrapping-an-event-handler-from-the-props\",\"children\":\"Wrapping an event handler from the props \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"You might run into a similar problem when your component receives an event handler as a prop:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,8,11}\",\"children\":\"function ChatRoom({ roomId, onReceiveMessage }) {\\n const [messages, setMessages] = useState([]);\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n onReceiveMessage(receivedMessage);\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId, onReceiveMessage]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Suppose that the parent component passes a \",[\"$r\",\"em\",null,{\"children\":\"different\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"onReceiveMessage\"}],\" function on every render:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-5}\",\"children\":\"\u003cChatRoom\\n roomId={roomId}\\n onReceiveMessage={receivedMessage =\u003e {\\n // ...\\n }}\\n/\u003e\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Since \",[\"$r\",\"code\",null,{\"children\":\"onReceiveMessage\"}],\" is a dependency, it would cause the Effect to re-synchronize after every parent re-render. This would make it re-connect to the chat. To solve this, wrap the call in an Effect Event:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4-6,12,15}\",\"children\":\"function ChatRoom({ roomId, onReceiveMessage }) {\\n const [messages, setMessages] = useState([]);\\n\\n const onMessage = useEffectEvent(receivedMessage =\u003e {\\n onReceiveMessage(receivedMessage);\\n });\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n connection.on('message', (receivedMessage) =\u003e {\\n onMessage(receivedMessage);\\n });\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Effect Events aren’t reactive, so you don’t need to specify them as dependencies. As a result, the chat will no longer re-connect even if the parent component passes a function that’s different on every re-render.\"}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"separating-reactive-and-non-reactive-code\",\"children\":\"Separating reactive and non-reactive code \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In this example, you want to log a visit every time \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" changes. You want to include the current \",[\"$r\",\"code\",null,{\"children\":\"notificationCount\"}],\" with every log, but you \",[\"$r\",\"em\",null,{\"children\":\"don’t\"}],\" want a change to \",[\"$r\",\"code\",null,{\"children\":\"notificationCount\"}],\" to trigger a log event.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The solution is again to split out the non-reactive code into an Effect Event:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-4,7}\",\"children\":\"function Chat({ roomId, notificationCount }) {\\n const onVisit = useEffectEvent(visitedRoomId =\u003e {\\n logVisit(visitedRoomId, notificationCount);\\n });\\n\\n useEffect(() =\u003e {\\n onVisit(roomId);\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You want your logic to be reactive with regards to \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\", so you read \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" inside of your Effect. However, you don’t want a change to \",[\"$r\",\"code\",null,{\"children\":\"notificationCount\"}],\" to log an extra visit, so you read \",[\"$r\",\"code\",null,{\"children\":\"notificationCount\"}],\" inside of the Effect Event. \",[\"$r\",\"a\",null,{\"href\":\"/learn/separating-events-from-effects#reading-latest-props-and-state-with-effect-events\",\"children\":\"Learn more about reading the latest props and state from Effects using Effect Events.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"does-some-reactive-value-change-unintentionally\",\"children\":\"Does some reactive value change unintentionally? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Sometimes, you \",[\"$r\",\"em\",null,{\"children\":\"do\"}],\" want your Effect to “react” to a certain value, but that value changes more often than you’d like—and might not reflect any actual change from the user’s perspective. For example, let’s say that you create an \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" object in the body of your component, and then read that object from inside of your Effect:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-6,9}\",\"children\":\"function ChatRoom({ roomId }) {\\n // ...\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This object is declared in the component body, so it’s a \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#effects-react-to-reactive-values\",\"children\":\"reactive value.\"}],\" When you read a reactive value like this inside an Effect, you declare it as a dependency. This ensures your Effect “reacts” to its changes:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,6}\",\"children\":\" // ...\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [options]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"It is important to declare it as a dependency! This ensures, for example, that if the \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" changes, your Effect will re-connect to the chat with the new \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\". However, there is also a problem with the code above. To see it, try typing into the input in the sandbox below, and watch what happens in the console:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n // Temporarily disable the linter to demonstrate the problem\\n // eslint-disable-next-line react-hooks/exhaustive-deps\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [options]);\\n\\n return (\\n \u003c\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003cinput value={message} onChange={e =\u003e setMessage(e.target.value)} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"232\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In the sandbox above, the input only updates the \",[\"$r\",\"code\",null,{\"children\":\"message\"}],\" state variable. From the user’s perspective, this should not affect the chat connection. However, every time you update the \",[\"$r\",\"code\",null,{\"children\":\"message\"}],\", your component re-renders. When your component re-renders, the code inside of it runs again from scratch.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"A new \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" object is created from scratch on every re-render of the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component. React sees that the \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" object is a \",[\"$r\",\"em\",null,{\"children\":\"different object\"}],\" from the \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" object created during the last render. This is why it re-synchronizes your Effect (which depends on \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\"), and the chat re-connects as you type.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"This problem only affects objects and functions. In JavaScript, each newly created object and function is considered distinct from all the others. It doesn’t matter that the contents inside of them may be the same!\"}]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7-8}\",\"children\":\"// During the first render\\nconst options1 = { serverUrl: 'https://localhost:1234', roomId: 'music' };\\n\\n// During the next render\\nconst options2 = { serverUrl: 'https://localhost:1234', roomId: 'music' };\\n\\n// These are two different objects!\\nconsole.log(Object.is(options1, options2)); // false\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"Object and function dependencies can make your Effect re-synchronize more often than you need.\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This is why, whenever possible, you should try to avoid objects and functions as your Effect’s dependencies. Instead, try moving them outside the component, inside the Effect, or extracting primitive values out of them.\"}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"move-static-objects-and-functions-outside-your-component\",\"children\":\"Move static objects and functions outside your component \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"If the object does not depend on any props and state, you can move that object outside your component:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1-4,13}\",\"children\":\"const options = {\\n serverUrl: 'https://localhost:1234',\\n roomId: 'music'\\n};\\n\\nfunction ChatRoom() {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This way, you \",[\"$r\",\"em\",null,{\"children\":\"prove\"}],\" to the linter that it’s not reactive. It can’t change as a result of a re-render, so it doesn’t need to be a dependency. Now re-rendering \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" won’t cause your Effect to re-synchronize.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This works for functions too:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1-6,12}\",\"children\":\"function createOptions() {\\n return {\\n serverUrl: 'https://localhost:1234',\\n roomId: 'music'\\n };\\n}\\n\\nfunction ChatRoom() {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const options = createOptions();\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Since \",[\"$r\",\"code\",null,{\"children\":\"createOptions\"}],\" is declared outside your component, it’s not a reactive value. This is why it doesn’t need to be specified in your Effect’s dependencies, and why it won’t ever cause your Effect to re-synchronize.\"]}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"move-dynamic-objects-and-functions-inside-your-effect\",\"children\":\"Move dynamic objects and functions inside your Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"If your object depends on some reactive value that may change as a result of a re-render, like a \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" prop, you can’t pull it \",[\"$r\",\"em\",null,{\"children\":\"outside\"}],\" your component. You can, however, move its creation \",[\"$r\",\"em\",null,{\"children\":\"inside\"}],\" of your Effect’s code:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7-10,11,14}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now that \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" is declared inside of your Effect, it is no longer a dependency of your Effect. Instead, the only reactive value used by your Effect is \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\". Since \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" is not an object or function, you can be sure that it won’t be \",[\"$r\",\"em\",null,{\"children\":\"unintentionally\"}],\" different. In JavaScript, numbers and strings are compared by their content:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7-8}\",\"children\":\"// During the first render\\nconst roomId1 = 'music';\\n\\n// During the next render\\nconst roomId2 = 'music';\\n\\n// These two strings are the same!\\nconsole.log(Object.is(roomId1, roomId2)); // true\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Thanks to this fix, the chat no longer re-connects if you edit the input:\"}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nconst serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]);\\n\\n return (\\n \u003c\u003e\\n \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e\\n \u003cinput value={message} onChange={e =\u003e setMessage(e.target.value)} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [roomId, setRoomId] = useState('general');\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom roomId={roomId} /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\nbutton { margin-left: 10px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"272\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"However, it \",[\"$r\",\"em\",null,{\"children\":\"does\"}],\" re-connect when you change the \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" dropdown, as you would expect.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This works for functions, too:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{7-12,14}\",\"children\":\"const serverUrl = 'https://localhost:1234';\\n\\nfunction ChatRoom({ roomId }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n function createOptions() {\\n return {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n }\\n\\n const options = createOptions();\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You can write your own functions to group pieces of logic inside your Effect. As long as you also declare them \",[\"$r\",\"em\",null,{\"children\":\"inside\"}],\" your Effect, they’re not reactive values, and so they don’t need to be dependencies of your Effect.\"]}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"read-primitive-values-from-objects\",\"children\":\"Read primitive values from objects \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Sometimes, you may receive an object from props:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,5,8}\",\"children\":\"function ChatRoom({ options }) {\\n const [message, setMessage] = useState('');\\n\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [options]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The risk here is that the parent component will create the object during rendering:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-6}\",\"children\":\"\u003cChatRoom\\n roomId={roomId}\\n options={{\\n serverUrl: serverUrl,\\n roomId: roomId\\n }}\\n/\u003e\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This would cause your Effect to re-connect every time the parent component re-renders. To fix this, read information from the object \",[\"$r\",\"em\",null,{\"children\":\"outside\"}],\" the Effect, and avoid having object and function dependencies:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4,7-8,12}\",\"children\":\"function ChatRoom({ options }) {\\n const [message, setMessage] = useState('');\\n\\n const { roomId, serverUrl } = options;\\n useEffect(() =\u003e {\\n const connection = createConnection({\\n roomId: roomId,\\n serverUrl: serverUrl\\n });\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, serverUrl]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The logic gets a little repetitive (you read some values from an object outside an Effect, and then create an object with the same values inside the Effect). But it makes it very explicit what information your Effect \",[\"$r\",\"em\",null,{\"children\":\"actually\"}],\" depends on. If an object is re-created unintentionally by the parent component, the chat would not re-connect. However, if \",[\"$r\",\"code\",null,{\"children\":\"options.roomId\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"options.serverUrl\"}],\" really are different, the chat would re-connect.\"]}],\"\\n\",[\"$r\",\"h4\",null,{\"id\":\"calculate-primitive-values-from-functions\",\"children\":\"Calculate primitive values from functions \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"The same approach can work for functions. For example, suppose the parent component passes a function:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3-8}\",\"children\":\"\u003cChatRoom\\n roomId={roomId}\\n getOptions={() =\u003e {\\n return {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n }}\\n/\u003e\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To avoid making it a dependency (and causing it to re-connect on re-renders), call it outside the Effect. This gives you the \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" values that aren’t objects, and that you can read from inside your Effect:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,4}\",\"children\":\"function ChatRoom({ getOptions }) {\\n const [message, setMessage] = useState('');\\n\\n const { roomId, serverUrl } = getOptions();\\n useEffect(() =\u003e {\\n const connection = createConnection({\\n roomId: roomId,\\n serverUrl: serverUrl\\n });\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, serverUrl]); // ✅ All dependencies declared\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This only works for \",[\"$r\",\"a\",null,{\"href\":\"/learn/keeping-components-pure\",\"children\":\"pure\"}],\" functions because they are safe to call during rendering. If your function is an event handler, but you don’t want its changes to re-synchronize your Effect, \",[\"$r\",\"a\",null,{\"href\":\"#do-you-want-to-read-a-value-without-reacting-to-its-changes\",\"children\":\"wrap it into an Effect Event instead.\"}]]}],\"\\n\",[\"$r\",\"Recap\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Dependencies should always match the code.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"When you’re not happy with your dependencies, what you need to edit is the code.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Suppressing the linter leads to very confusing bugs, and you should always avoid it.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"To remove a dependency, you need to “prove” to the linter that it’s not necessary.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"If some code should run in response to a specific interaction, move that code to an event handler.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"If different parts of your Effect should re-run for different reasons, split it into several Effects.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"If you want to update some state based on the previous state, pass an updater function.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"If you want to read the latest value without “reacting” it, extract an Effect Event from your Effect.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"In JavaScript, objects and functions are considered different if they were created at different times.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Try to avoid object and function dependencies. Move them outside the component or inside the Effect.\"}],\"\\n\"]}]}],\"\\n\"]}],[\"$r\",\"Challenges\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"fix-a-resetting-interval\",\"children\":\"Fix a resetting interval \"}],[\"$r\",\"p\",null,{\"children\":\"This Effect sets up an interval that ticks every second. You’ve noticed something strange happening: it seems like the interval gets destroyed and re-created every time it ticks. Fix the code so that the interval doesn’t get constantly re-created.\"}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"It seems like this Effect’s code depends on \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\". Is there some way to not need this dependency? There should be a way to update the \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\" state based on its previous value without adding a dependency on that value.\"]}]}],[\"$r\",\"Sandpack\",null,{\"children\":[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function Timer() {\\n const [count, setCount] = useState(0);\\n\\n useEffect(() =\u003e {\\n console.log('✅ Creating an interval');\\n const id = setInterval(() =\u003e {\\n console.log('⏰ Interval tick');\\n setCount(count + 1);\\n }, 1000);\\n return () =\u003e {\\n console.log('❌ Clearing an interval');\\n clearInterval(id);\\n };\\n }, [count]);\\n\\n return \u003ch1\u003eCounter: {count}\u003c/h1\u003e\\n}\\n\"}]}]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"You want to update the \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\" state to be \",[\"$r\",\"code\",null,{\"children\":\"count + 1\"}],\" from inside the Effect. However, this makes your Effect depend on \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\", which changes with every tick, and that’s why your interval gets re-created on every tick.\"]}],[\"$r\",\"p\",null,{\"children\":[\"To solve this, use the \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useState#updating-state-based-on-the-previous-state\",\"children\":\"updater function\"}],\" and write \",[\"$r\",\"code\",null,{\"children\":\"setCount(c =\u003e c + 1)\"}],\" instead of \",[\"$r\",\"code\",null,{\"children\":\"setCount(count + 1)\"}],\":\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function Timer() {\\n const [count, setCount] = useState(0);\\n\\n useEffect(() =\u003e {\\n console.log('✅ Creating an interval');\\n const id = setInterval(() =\u003e {\\n console.log('⏰ Interval tick');\\n setCount(c =\u003e c + 1);\\n }, 1000);\\n return () =\u003e {\\n console.log('❌ Clearing an interval');\\n clearInterval(id);\\n };\\n }, []);\\n\\n return \u003ch1\u003eCounter: {count}\u003c/h1\u003e\\n}\\n\"}]}]}],[\"$r\",\"p\",null,{\"children\":[\"Instead of reading \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\" inside the Effect, you pass a \",[\"$r\",\"code\",null,{\"children\":\"c =\u003e c + 1\"}],\" instruction (“increment this number!”) to React. React will apply it on the next render. And since you don’t need to read the value of \",[\"$r\",\"code\",null,{\"children\":\"count\"}],\" inside your Effect anymore, so you can keep your Effect’s dependencies empty (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\"). This prevents your Effect from re-creating the interval on every tick.\"]}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-a-retriggering-animation\",\"children\":\"Fix a retriggering animation \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, when you press “Show”, a welcome message fades in. The animation takes a second. When you press “Remove”, the welcome message immediately disappears. The logic for the fade-in animation is implemented in the \",[\"$r\",\"code\",null,{\"children\":\"animation.js\"}],\" file as plain JavaScript \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"animation loop.\"}],\" You don’t need to change that logic. You can treat it as a third-party library. Your Effect creates an instance of \",[\"$r\",\"code\",null,{\"children\":\"FadeInAnimation\"}],\" for the DOM node, and then calls \",[\"$r\",\"code\",null,{\"children\":\"start(duration)\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"stop()\"}],\" to control the animation. The \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" is controlled by a slider. Adjust the slider and see how the animation changes.\"]}],[\"$r\",\"p\",null,{\"children\":[\"This code already works, but there is something you want to change. Currently, when you move the slider that controls the \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" state variable, it retriggers the animation. Change the behavior so that the Effect does not “react” to the \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" variable. When you press “Show”, the Effect should use the current \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" on the slider. However, moving the slider itself should not by itself retrigger the animation.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"Is there a line of code inside the Effect that should not be reactive? How can you move non-reactive code out of the Effect?\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-json\",\"meta\":\"package.json hidden\",\"children\":\"{\\n \\\"dependencies\\\": {\\n \\\"react\\\": \\\"experimental\\\",\\n \\\"react-dom\\\": \\\"experimental\\\",\\n \\\"react-scripts\\\": \\\"latest\\\"\\n },\\n \\\"scripts\\\": {\\n \\\"start\\\": \\\"react-scripts start\\\",\\n \\\"build\\\": \\\"react-scripts build\\\",\\n \\\"test\\\": \\\"react-scripts test --env=jsdom\\\",\\n \\\"eject\\\": \\\"react-scripts eject\\\"\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect, useRef } from 'react';\\nimport { experimental_useEffectEvent as useEffectEvent } from 'react';\\nimport { FadeInAnimation } from './animation.js';\\n\\nfunction Welcome({ duration }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n const animation = new FadeInAnimation(ref.current);\\n animation.start(duration);\\n return () =\u003e {\\n animation.stop();\\n };\\n }, [duration]);\\n\\n return (\\n \u003ch1\\n ref={ref}\\n style={{\\n opacity: 0,\\n color: 'white',\\n padding: 50,\\n textAlign: 'center',\\n fontSize: 50,\\n backgroundImage: 'radial-gradient(circle, rgba(63,94,251,1) 0%, rgba(252,70,107,1) 100%)'\\n }}\\n \u003e\\n Welcome\\n \u003c/h1\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [duration, setDuration] = useState(1000);\\n const [show, setShow] = useState(false);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"range\\\"\\n min=\\\"100\\\"\\n max=\\\"3000\\\"\\n value={duration}\\n onChange={e =\u003e setDuration(Number(e.target.value))}\\n /\u003e\\n \u003cbr /\u003e\\n Fade in duration: {duration} ms\\n \u003c/label\u003e\\n \u003cbutton onClick={() =\u003e setShow(!show)}\u003e\\n {show ? 'Remove' : 'Show'}\\n \u003c/button\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 \u003cWelcome duration={duration} /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/animation.js\",\"children\":\"export class FadeInAnimation {\\n constructor(node) {\\n this.node = node;\\n }\\n start(duration) {\\n this.duration = duration;\\n if (this.duration === 0) {\\n // Jump to end immediately\\n this.onProgress(1);\\n } else {\\n this.onProgress(0);\\n // Start animating\\n this.startTime = performance.now();\\n this.frameId = requestAnimationFrame(() =\u003e this.onFrame());\\n }\\n }\\n onFrame() {\\n const timePassed = performance.now() - this.startTime;\\n const progress = Math.min(timePassed / this.duration, 1);\\n this.onProgress(progress);\\n if (progress \u003c 1) {\\n // We still have more frames to paint\\n this.frameId = requestAnimationFrame(() =\u003e this.onFrame());\\n }\\n }\\n onProgress(progress) {\\n this.node.style.opacity = progress;\\n }\\n stop() {\\n cancelAnimationFrame(this.frameId);\\n this.startTime = null;\\n this.frameId = null;\\n this.duration = 0;\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 20px; }\\nhtml, body { min-height: 300px; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"Your Effect needs to read the latest value of \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\", but you don’t want it to “react” to changes in \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\". You use \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" to start the animation, but starting animation isn’t reactive. Extract the non-reactive line of code into an Effect Event, and call that function from your Effect.\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-json\",\"meta\":\"package.json hidden\",\"children\":\"{\\n \\\"dependencies\\\": {\\n \\\"react\\\": \\\"experimental\\\",\\n \\\"react-dom\\\": \\\"experimental\\\",\\n \\\"react-scripts\\\": \\\"latest\\\"\\n },\\n \\\"scripts\\\": {\\n \\\"start\\\": \\\"react-scripts start\\\",\\n \\\"build\\\": \\\"react-scripts build\\\",\\n \\\"test\\\": \\\"react-scripts test --env=jsdom\\\",\\n \\\"eject\\\": \\\"react-scripts eject\\\"\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect, useRef } from 'react';\\nimport { FadeInAnimation } from './animation.js';\\nimport { experimental_useEffectEvent as useEffectEvent } from 'react';\\n\\nfunction Welcome({ duration }) {\\n const ref = useRef(null);\\n\\n const onAppear = useEffectEvent(animation =\u003e {\\n animation.start(duration);\\n });\\n\\n useEffect(() =\u003e {\\n const animation = new FadeInAnimation(ref.current);\\n onAppear(animation);\\n return () =\u003e {\\n animation.stop();\\n };\\n }, []);\\n\\n return (\\n \u003ch1\\n ref={ref}\\n style={{\\n opacity: 0,\\n color: 'white',\\n padding: 50,\\n textAlign: 'center',\\n fontSize: 50,\\n backgroundImage: 'radial-gradient(circle, rgba(63,94,251,1) 0%, rgba(252,70,107,1) 100%)'\\n }}\\n \u003e\\n Welcome\\n \u003c/h1\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [duration, setDuration] = useState(1000);\\n const [show, setShow] = useState(false);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"range\\\"\\n min=\\\"100\\\"\\n max=\\\"3000\\\"\\n value={duration}\\n onChange={e =\u003e setDuration(Number(e.target.value))}\\n /\u003e\\n \u003cbr /\u003e\\n Fade in duration: {duration} ms\\n \u003c/label\u003e\\n \u003cbutton onClick={() =\u003e setShow(!show)}\u003e\\n {show ? 'Remove' : 'Show'}\\n \u003c/button\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 \u003cWelcome duration={duration} /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/animation.js\",\"children\":\"export class FadeInAnimation {\\n constructor(node) {\\n this.node = node;\\n }\\n start(duration) {\\n this.duration = duration;\\n this.onProgress(0);\\n this.startTime = performance.now();\\n this.frameId = requestAnimationFrame(() =\u003e this.onFrame());\\n }\\n onFrame() {\\n const timePassed = performance.now() - this.startTime;\\n const progress = Math.min(timePassed / this.duration, 1);\\n this.onProgress(progress);\\n if (progress \u003c 1) {\\n // We still have more frames to paint\\n this.frameId = requestAnimationFrame(() =\u003e this.onFrame());\\n }\\n }\\n onProgress(progress) {\\n this.node.style.opacity = progress;\\n }\\n stop() {\\n cancelAnimationFrame(this.frameId);\\n this.startTime = null;\\n this.frameId = null;\\n this.duration = 0;\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 20px; }\\nhtml, body { min-height: 300px; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Effect Events like \",[\"$r\",\"code\",null,{\"children\":\"onAppear\"}],\" are not reactive, so you can read \",[\"$r\",\"code\",null,{\"children\":\"duration\"}],\" inside without retriggering the animation.\"]}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-a-reconnecting-chat\",\"children\":\"Fix a reconnecting chat \"}],[\"$r\",\"p\",null,{\"children\":\"In this example, every time you press “Toggle theme”, the chat re-connects. Why does this happen? Fix the mistake so that the chat re-connects only when you edit the Server URL or choose a different chat room.\"}],[\"$r\",\"p\",null,{\"children\":[\"Treat \",[\"$r\",\"code\",null,{\"children\":\"chat.js\"}],\" as an external third-party library: you can consult it to check its API, but don’t edit it.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"There’s more than one way to fix this, but ultimately you want to avoid having an object as your dependency.\"}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\n\\nexport default function App() {\\n const [isDark, setIsDark] = useState(false);\\n const [roomId, setRoomId] = useState('general');\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234');\\n\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n\\n return (\\n \u003cdiv className={isDark ? 'dark' : 'light'}\u003e\\n \u003cbutton onClick={() =\u003e setIsDark(!isDark)}\u003e\\n Toggle theme\\n \u003c/button\u003e\\n \u003clabel\u003e\\n Server URL:{' '}\\n \u003cinput\\n value={serverUrl}\\n onChange={e =\u003e setServerUrl(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom options={options} /\u003e\\n \u003c/div\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nexport default function ChatRoom({ options }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(options);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [options]);\\n\\n return \u003ch1\u003eWelcome to the {options.roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 5px; }\\n.dark { background: #222; color: #eee; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"Your Effect is re-running because it depends on the \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" object. Objects can be re-created unintentionally, you should try to avoid them as dependencies of your Effects whenever possible.\"]}],[\"$r\",\"p\",null,{\"children\":[\"The least invasive fix is to read \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" right outside the Effect, and then make the Effect depend on those primitive values (which can’t change unintentionally). Inside the Effect, create an object and pass it to \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\":\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\n\\nexport default function App() {\\n const [isDark, setIsDark] = useState(false);\\n const [roomId, setRoomId] = useState('general');\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234');\\n\\n const options = {\\n serverUrl: serverUrl,\\n roomId: roomId\\n };\\n\\n return (\\n \u003cdiv className={isDark ? 'dark' : 'light'}\u003e\\n \u003cbutton onClick={() =\u003e setIsDark(!isDark)}\u003e\\n Toggle theme\\n \u003c/button\u003e\\n \u003clabel\u003e\\n Server URL:{' '}\\n \u003cinput\\n value={serverUrl}\\n onChange={e =\u003e setServerUrl(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom options={options} /\u003e\\n \u003c/div\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nexport default function ChatRoom({ options }) {\\n const { roomId, serverUrl } = options;\\n useEffect(() =\u003e {\\n const connection = createConnection({\\n roomId: roomId,\\n serverUrl: serverUrl\\n });\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, serverUrl]);\\n\\n return \u003ch1\u003eWelcome to the {options.roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 5px; }\\n.dark { background: #222; color: #eee; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"It would be even better to replace the object \",[\"$r\",\"code\",null,{\"children\":\"options\"}],\" prop with the more specific \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"serverUrl\"}],\" props:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\n\\nexport default function App() {\\n const [isDark, setIsDark] = useState(false);\\n const [roomId, setRoomId] = useState('general');\\n const [serverUrl, setServerUrl] = useState('https://localhost:1234');\\n\\n return (\\n \u003cdiv className={isDark ? 'dark' : 'light'}\u003e\\n \u003cbutton onClick={() =\u003e setIsDark(!isDark)}\u003e\\n Toggle theme\\n \u003c/button\u003e\\n \u003clabel\u003e\\n Server URL:{' '}\\n \u003cinput\\n value={serverUrl}\\n onChange={e =\u003e setServerUrl(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n serverUrl={serverUrl}\\n /\u003e\\n \u003c/div\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nexport default function ChatRoom({ roomId, serverUrl }) {\\n useEffect(() =\u003e {\\n const connection = createConnection({\\n roomId: roomId,\\n serverUrl: serverUrl\\n });\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, serverUrl]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room at ' + serverUrl + '...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room at ' + serverUrl);\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 5px; }\\n.dark { background: #222; color: #eee; }\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":\"Sticking to primitive props where possible makes it easier to optimize your components later.\"}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-a-reconnecting-chat-again\",\"children\":\"Fix a reconnecting chat, again \"}],[\"$r\",\"p\",null,{\"children\":\"This example connects to the chat either with or without encryption. Toggle the checkbox and notice the different messages in the console when the encryption is on and off. Try changing the room. Then, try toggling the theme. When you’re connected to a chat room, you will receive new messages every few seconds. Verify that their color matches the theme you’ve picked.\"}],[\"$r\",\"p\",null,{\"children\":\"In this example, the chat re-connects every time you try to change the theme. Fix this. After the fix, changing the theme should not re-connect the chat, but toggling encryption settings or changing the room should re-connect.\"}],[\"$r\",\"p\",null,{\"children\":[\"Don’t change any code in \",[\"$r\",\"code\",null,{\"children\":\"chat.js\"}],\". Other than that, you can change any code as long as it results in the same behavior. For example, you may find it helpful to change which props are being passed down.\"]}],[\"$r\",\"Hint\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"You’re passing down two functions: \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\". Both of them are created from scratch every time \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" re-renders. They are considered to be new values every time, which is why they re-trigger your Effect.\"]}],[\"$r\",\"p\",null,{\"children\":\"One of these functions is an event handler. Do you know some way to call an event handler an Effect without “reacting” to the new values of the event handler function? That would come in handy!\"}],[\"$r\",\"p\",null,{\"children\":[\"Another of these functions only exists to pass some state to an imported API method. Is this function really necessary? What is the essential information that’s being passed down? You might need to move some imports from \",[\"$r\",\"code\",null,{\"children\":\"App.js\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom.js\"}],\".\"]}]]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-json\",\"meta\":\"package.json hidden\",\"children\":\"{\\n \\\"dependencies\\\": {\\n \\\"react\\\": \\\"experimental\\\",\\n \\\"react-dom\\\": \\\"experimental\\\",\\n \\\"react-scripts\\\": \\\"latest\\\",\\n \\\"toastify-js\\\": \\\"1.12.0\\\"\\n },\\n \\\"scripts\\\": {\\n \\\"start\\\": \\\"react-scripts start\\\",\\n \\\"build\\\": \\\"react-scripts build\\\",\\n \\\"test\\\": \\\"react-scripts test --env=jsdom\\\",\\n \\\"eject\\\": \\\"react-scripts eject\\\"\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\nimport {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\nimport { showNotification } from './notifications.js';\\n\\nexport default function App() {\\n const [isDark, setIsDark] = useState(false);\\n const [roomId, setRoomId] = useState('general');\\n const [isEncrypted, setIsEncrypted] = useState(false);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isDark}\\n onChange={e =\u003e setIsDark(e.target.checked)}\\n /\u003e\\n Use dark theme\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isEncrypted}\\n onChange={e =\u003e setIsEncrypted(e.target.checked)}\\n /\u003e\\n Enable encryption\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n onMessage={msg =\u003e {\\n showNotification('New message: ' + msg, isDark ? 'dark' : 'light');\\n }}\\n createConnection={() =\u003e {\\n const options = {\\n serverUrl: 'https://localhost:1234',\\n roomId: roomId\\n };\\n if (isEncrypted) {\\n return createEncryptedConnection(options);\\n } else {\\n return createUnencryptedConnection(options);\\n }\\n }}\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { experimental_useEffectEvent as useEffectEvent } from 'react';\\n\\nexport default function ChatRoom({ roomId, createConnection, onMessage }) {\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.on('message', (msg) =\u003e onMessage(msg));\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [createConnection, onMessage]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createEncryptedConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n let intervalId;\\n let messageCallback;\\n return {\\n connect() {\\n console.log('✅ 🔐 Connecting to \\\"' + roomId + '\\\" room... (encrypted)');\\n clearInterval(intervalId);\\n intervalId = setInterval(() =\u003e {\\n if (messageCallback) {\\n if (Math.random() \u003e 0.5) {\\n messageCallback('hey')\\n } else {\\n messageCallback('lol');\\n }\\n }\\n }, 3000);\\n },\\n disconnect() {\\n clearInterval(intervalId);\\n messageCallback = null;\\n console.log('❌ 🔐 Disconnected from \\\"' + roomId + '\\\" room (encrypted)');\\n },\\n on(event, callback) {\\n if (messageCallback) {\\n throw Error('Cannot add the handler twice.');\\n }\\n if (event !== 'message') {\\n throw Error('Only \\\"message\\\" event is supported.');\\n }\\n messageCallback = callback;\\n },\\n };\\n}\\n\\nexport function createUnencryptedConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n let intervalId;\\n let messageCallback;\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room (unencrypted)...');\\n clearInterval(intervalId);\\n intervalId = setInterval(() =\u003e {\\n if (messageCallback) {\\n if (Math.random() \u003e 0.5) {\\n messageCallback('hey')\\n } else {\\n messageCallback('lol');\\n }\\n }\\n }, 3000);\\n },\\n disconnect() {\\n clearInterval(intervalId);\\n messageCallback = null;\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room (unencrypted)');\\n },\\n on(event, callback) {\\n if (messageCallback) {\\n throw Error('Cannot add the handler twice.');\\n }\\n if (event !== 'message') {\\n throw Error('Only \\\"message\\\" event is supported.');\\n }\\n messageCallback = callback;\\n },\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/notifications.js\",\"children\":\"import Toastify from 'toastify-js';\\nimport 'toastify-js/src/toastify.css';\\n\\nexport function showNotification(message, theme) {\\n Toastify({\\n text: message,\\n duration: 2000,\\n gravity: 'top',\\n position: 'right',\\n style: {\\n background: theme === 'dark' ? 'black' : 'white',\\n color: theme === 'dark' ? 'white' : 'black',\\n },\\n }).showToast();\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 5px; }\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":\"There’s more than one correct way to solve this, but here is one possible solution.\"}],[\"$r\",\"p\",null,{\"children\":[\"In the original example, toggling the theme caused different \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" functions to be created and passed down. Since the Effect depended on these functions, the chat would re-connect every time you toggle the theme.\"]}],[\"$r\",\"p\",null,{\"children\":[\"To fix the problem with \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\", you needed to wrap it into an Effect Event:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,2,6}\",\"children\":\"export default function ChatRoom({ roomId, createConnection, onMessage }) {\\n const onReceiveMessage = useEffectEvent(onMessage);\\n\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.on('message', (msg) =\u003e onReceiveMessage(msg));\\n // ...\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"Unlike the \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\" prop, the \",[\"$r\",\"code\",null,{\"children\":\"onReceiveMessage\"}],\" Effect Event is not reactive. This is why it doesn’t need to be a dependency of your Effect. As a result, changes to \",[\"$r\",\"code\",null,{\"children\":\"onMessage\"}],\" won’t cause the chat to re-connect.\"]}],[\"$r\",\"p\",null,{\"children\":[\"You can’t do the same with \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" because it \",[\"$r\",\"em\",null,{\"children\":\"should\"}],\" be reactive. You \",[\"$r\",\"em\",null,{\"children\":\"want\"}],\" the Effect to re-trigger if the user switches between an encrypted and an unencryption connection, or if the user switches the current room. However, because \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" is a function, you can’t check whether the information it reads has \",[\"$r\",\"em\",null,{\"children\":\"actually\"}],\" changed or not. To solve this, instead of passing \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" down from the \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\" component, pass the raw \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"isEncrypted\"}],\" values:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-3}\",\"children\":\" \u003cChatRoom\\n roomId={roomId}\\n isEncrypted={isEncrypted}\\n onMessage={msg =\u003e {\\n showNotification('New message: ' + msg, isDark ? 'dark' : 'light');\\n }}\\n /\u003e\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"Now you can move the \",[\"$r\",\"code\",null,{\"children\":\"createConnection\"}],\" function \",[\"$r\",\"em\",null,{\"children\":\"inside\"}],\" the Effect instead of passing it down from the \",[\"$r\",\"code\",null,{\"children\":\"App\"}],\":\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1-4,6,10-20}\",\"children\":\"import {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\n\\nexport default function ChatRoom({ roomId, isEncrypted, onMessage }) {\\n const onReceiveMessage = useEffectEvent(onMessage);\\n\\n useEffect(() =\u003e {\\n function createConnection() {\\n const options = {\\n serverUrl: 'https://localhost:1234',\\n roomId: roomId\\n };\\n if (isEncrypted) {\\n return createEncryptedConnection(options);\\n } else {\\n return createUnencryptedConnection(options);\\n }\\n }\\n // ...\\n\"}]}],[\"$r\",\"p\",null,{\"children\":\"After these two changes, your Effect no longer depends on any function values:\"}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,8,10,21}\",\"children\":\"export default function ChatRoom({ roomId, isEncrypted, onMessage }) { // Reactive values\\n const onReceiveMessage = useEffectEvent(onMessage); // Not reactive\\n\\n useEffect(() =\u003e {\\n function createConnection() {\\n const options = {\\n serverUrl: 'https://localhost:1234',\\n roomId: roomId // Reading a reactive value\\n };\\n if (isEncrypted) { // Reading a reactive value\\n return createEncryptedConnection(options);\\n } else {\\n return createUnencryptedConnection(options);\\n }\\n }\\n\\n const connection = createConnection();\\n connection.on('message', (msg) =\u003e onReceiveMessage(msg));\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, isEncrypted]); // ✅ All dependencies declared\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"As a result, the chat re-connects only when something meaningful (\",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"isEncrypted\"}],\") changes:\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-json\",\"meta\":\"package.json hidden\",\"children\":\"{\\n \\\"dependencies\\\": {\\n \\\"react\\\": \\\"experimental\\\",\\n \\\"react-dom\\\": \\\"experimental\\\",\\n \\\"react-scripts\\\": \\\"latest\\\",\\n \\\"toastify-js\\\": \\\"1.12.0\\\"\\n },\\n \\\"scripts\\\": {\\n \\\"start\\\": \\\"react-scripts start\\\",\\n \\\"build\\\": \\\"react-scripts build\\\",\\n \\\"test\\\": \\\"react-scripts test --env=jsdom\\\",\\n \\\"eject\\\": \\\"react-scripts eject\\\"\\n }\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState } from 'react';\\nimport ChatRoom from './ChatRoom.js';\\n\\nimport { showNotification } from './notifications.js';\\n\\nexport default function App() {\\n const [isDark, setIsDark] = useState(false);\\n const [roomId, setRoomId] = useState('general');\\n const [isEncrypted, setIsEncrypted] = useState(false);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isDark}\\n onChange={e =\u003e setIsDark(e.target.checked)}\\n /\u003e\\n Use dark theme\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={isEncrypted}\\n onChange={e =\u003e setIsEncrypted(e.target.checked)}\\n /\u003e\\n Enable encryption\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Choose the chat room:{' '}\\n \u003cselect\\n value={roomId}\\n onChange={e =\u003e setRoomId(e.target.value)}\\n \u003e\\n \u003coption value=\\\"general\\\"\u003egeneral\u003c/option\u003e\\n \u003coption value=\\\"travel\\\"\u003etravel\u003c/option\u003e\\n \u003coption value=\\\"music\\\"\u003emusic\u003c/option\u003e\\n \u003c/select\u003e\\n \u003c/label\u003e\\n \u003chr /\u003e\\n \u003cChatRoom\\n roomId={roomId}\\n isEncrypted={isEncrypted}\\n onMessage={msg =\u003e {\\n showNotification('New message: ' + msg, isDark ? 'dark' : 'light');\\n }}\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/ChatRoom.js active\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { experimental_useEffectEvent as useEffectEvent } from 'react';\\nimport {\\n createEncryptedConnection,\\n createUnencryptedConnection,\\n} from './chat.js';\\n\\nexport default function ChatRoom({ roomId, isEncrypted, onMessage }) {\\n const onReceiveMessage = useEffectEvent(onMessage);\\n\\n useEffect(() =\u003e {\\n function createConnection() {\\n const options = {\\n serverUrl: 'https://localhost:1234',\\n roomId: roomId\\n };\\n if (isEncrypted) {\\n return createEncryptedConnection(options);\\n } else {\\n return createUnencryptedConnection(options);\\n }\\n }\\n\\n const connection = createConnection();\\n connection.on('message', (msg) =\u003e onReceiveMessage(msg));\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId, isEncrypted]);\\n\\n return \u003ch1\u003eWelcome to the {roomId} room!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createEncryptedConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n let intervalId;\\n let messageCallback;\\n return {\\n connect() {\\n console.log('✅ 🔐 Connecting to \\\"' + roomId + '\\\" room... (encrypted)');\\n clearInterval(intervalId);\\n intervalId = setInterval(() =\u003e {\\n if (messageCallback) {\\n if (Math.random() \u003e 0.5) {\\n messageCallback('hey')\\n } else {\\n messageCallback('lol');\\n }\\n }\\n }, 3000);\\n },\\n disconnect() {\\n clearInterval(intervalId);\\n messageCallback = null;\\n console.log('❌ 🔐 Disconnected from \\\"' + roomId + '\\\" room (encrypted)');\\n },\\n on(event, callback) {\\n if (messageCallback) {\\n throw Error('Cannot add the handler twice.');\\n }\\n if (event !== 'message') {\\n throw Error('Only \\\"message\\\" event is supported.');\\n }\\n messageCallback = callback;\\n },\\n };\\n}\\n\\nexport function createUnencryptedConnection({ serverUrl, roomId }) {\\n // A real implementation would actually connect to the server\\n if (typeof serverUrl !== 'string') {\\n throw Error('Expected serverUrl to be a string. Received: ' + serverUrl);\\n }\\n if (typeof roomId !== 'string') {\\n throw Error('Expected roomId to be a string. Received: ' + roomId);\\n }\\n let intervalId;\\n let messageCallback;\\n return {\\n connect() {\\n console.log('✅ Connecting to \\\"' + roomId + '\\\" room (unencrypted)...');\\n clearInterval(intervalId);\\n intervalId = setInterval(() =\u003e {\\n if (messageCallback) {\\n if (Math.random() \u003e 0.5) {\\n messageCallback('hey')\\n } else {\\n messageCallback('lol');\\n }\\n }\\n }, 3000);\\n },\\n disconnect() {\\n clearInterval(intervalId);\\n messageCallback = null;\\n console.log('❌ Disconnected from \\\"' + roomId + '\\\" room (unencrypted)');\\n },\\n on(event, callback) {\\n if (messageCallback) {\\n throw Error('Cannot add the handler twice.');\\n }\\n if (event !== 'message') {\\n throw Error('Only \\\"message\\\" event is supported.');\\n }\\n messageCallback = callback;\\n },\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/notifications.js\",\"children\":\"import Toastify from 'toastify-js';\\nimport 'toastify-js/src/toastify.css';\\n\\nexport function showNotification(message, theme) {\\n Toastify({\\n text: message,\\n duration: 2000,\\n gravity: 'top',\\n position: 'right',\\n style: {\\n background: theme === 'dark' ? 'black' : 'white',\\n color: theme === 'dark' ? 'white' : 'black',\\n },\\n }).showToast();\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label, button { display: block; margin-bottom: 5px; }\\n\"}]}]]}]]}]]}]]","meta":{"title":"Removing Effect Dependencies"},"languages":null},"__N_SSG":true},"page":"/[[...markdownPath]]","query":{"markdownPath":["learn","removing-effect-dependencies"]},"buildId":"_Fz6u7K2TvuR-_M7SFBnH","isFallback":false,"gsp":true,"scriptLoader":[]}</script></body></html>