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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="" 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/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="" 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/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 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fill="currentColor"></path></svg></button></div></div></aside></div></div></div></div><main 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">Synchronizing with Effects<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">Some components need to synchronize with external systems. For example, you might want to control a non-React component based on the React state, set up a server connection, or send an analytics log when a component appears on the screen. <em>Effects</em> let you run some code after rendering so that you can synchronize your component with some system outside of React.</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">What Effects are</li> <li class="leading-relaxed mb-1">How Effects are different from events</li> <li class="leading-relaxed mb-1">How to declare an Effect in your component</li> <li class="leading-relaxed mb-1">How to skip re-running an Effect unnecessarily</li> <li class="leading-relaxed mb-1">Why Effects run twice in development and how to fix them</li> </ul></div> <h2 id="what-are-effects-and-how-are-they-different-from-events" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">What are Effects and how are they different from events? <a href="#what-are-effects-and-how-are-they-different-from-events" aria-label="Link for What are Effects and how are they different from events? " title="Link for What are Effects and how are they different from events? " 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">Before getting to Effects, you need to be familiar with two types of logic inside React components:</p> <ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1"> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Rendering code</strong> (introduced in <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/describing-the-ui">Describing the UI</a>) lives at the top level of your component. This is where you take the props and state, transform them, and return the JSX you want to see on the screen. <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">Rendering code must be pure.</a> Like a math formula, it should only <em>calculate</em> the result, but not do anything else.</p> </li> <li class="leading-relaxed mb-1"> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Event handlers</strong> (introduced in <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/adding-interactivity">Adding Interactivity</a>) are nested functions inside your components that <em>do</em> things rather than just calculate them. An event handler might update an input field, submit an HTTP POST request to buy a product, or navigate the user to another screen. Event handlers contain <a href="https://en.wikipedia.org/wiki/Side_effect_(computer_science)" target="_blank" rel="nofollow noopener noreferrer" 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">“side effects”</a> (they change the program’s state) caused by a specific user action (for example, a button click or typing).</p> </li> </ul> <p class="whitespace-pre-wrap my-4">Sometimes this isn’t enough. Consider 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">ChatRoom</code> component that must connect to the chat server whenever it’s visible on the screen. Connecting to a server is not a pure calculation (it’s a side effect) so it can’t happen during rendering. However, there is no single particular event like a click that causes <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> to be displayed.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold"><em>Effects</em> let you specify side effects that are caused by rendering itself, rather than by a particular event.</strong> Sending a message in the chat is an <em>event</em> because it is directly caused by the user clicking a specific button. However, setting up a server connection is an <em>Effect</em> because it should happen no matter which interaction caused the component to appear. Effects run at the end of 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/render-and-commit">commit</a> after the screen updates. This is a good time to synchronize the React components with some external system (like network or a third-party library).</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-green-5 dark:bg-green-60 dark:bg-opacity-20 text-primary dark:text-primary-dark text-lg"><h3 class="text-2xl font-display font-bold text-green-60 dark:text-green-40"><svg class="inline me-2 mb-1 text-lg text-green-60 dark:text-green-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>Note</h3><div class="relative"><div class="py-2"><p class="whitespace-pre-wrap my-4">Here and later in this text, capitalized “Effect” refers to the React-specific definition above, i.e. a side effect caused by rendering. To refer to the broader programming concept, we’ll say “side effect”.</p></div></div></div> <h2 id="you-might-not-need-an-effect" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">You might not need an Effect <a href="#you-might-not-need-an-effect" aria-label="Link for You might not need an Effect " title="Link for You might not need an 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></h2> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Don’t rush to add Effects to your components.</strong> Keep in mind that Effects are typically used to “step out” of your React code and synchronize with some <em>external</em> system. This includes browser APIs, third-party widgets, network, and so on. If your Effect only adjusts some state based on other state, <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">you might not need an Effect.</a></p> <h2 id="how-to-write-an-effect" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">How to write an Effect <a href="#how-to-write-an-effect" aria-label="Link for How to write an Effect " title="Link for How to write an 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></h2> <p class="whitespace-pre-wrap my-4">To write an Effect, follow these three steps:</p> <ol class="ms-6 my-3 list-decimal"> <li class="leading-relaxed mb-1"><strong class="font-bold">Declare an Effect.</strong> By default, your Effect will run after every <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/render-and-commit">commit</a>.</li> <li class="leading-relaxed mb-1"><strong class="font-bold">Specify the Effect dependencies.</strong> Most Effects should only re-run <em>when needed</em> rather than after every render. For example, a fade-in animation should only trigger when a component appears. Connecting and disconnecting to a chat room should only happen when the component appears and disappears, or when the chat room changes. You will learn how to control this by specifying <em>dependencies.</em></li> <li class="leading-relaxed mb-1"><strong class="font-bold">Add cleanup if needed.</strong> Some Effects need to specify how to stop, undo, or clean up whatever they were doing. For example, “connect” needs “disconnect”, “subscribe” needs “unsubscribe”, and “fetch” needs either “cancel” or “ignore”. You will learn how to do this by returning a <em>cleanup function</em>.</li> </ol> <p class="whitespace-pre-wrap my-4">Let’s look at each of these steps in detail.</p> <h3 id="step-1-declare-an-effect" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Step 1: Declare an Effect <a href="#step-1-declare-an-effect" aria-label="Link for Step 1: Declare an Effect " title="Link for Step 1: Declare an 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></h3> <p class="whitespace-pre-wrap my-4">To declare an Effect in your component, import the <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/useEffect"><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">useEffect</code> Hook</a> from React:</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">import</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Then, call it at the top level of your component and put some code 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 "><span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">MyComponent</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">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-comment">// Code here will run after *every* render</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-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">div</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Every time your component renders, React will update the screen <em>and then</em> run the code inside <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">useEffect</code>. In other words, <strong class="font-bold"><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">useEffect</code> “delays” a piece of code from running until that render is reflected on the screen.</strong></p> <p class="whitespace-pre-wrap my-4">Let’s see how you can use an Effect to synchronize with an external system. Consider 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"><VideoPlayer></code> React component. It would be nice to control whether it’s playing or paused by passing 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">isPlaying</code> prop to 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-punctuation"><</span><span class="sp-syntax-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</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">Your custom <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">VideoPlayer</code> component renders the built-in browser <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video" target="_blank" rel="nofollow noopener noreferrer" 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"><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"><video></code></a> tag:</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">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">// TODO: do something with isPlaying</span><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">video</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">However, the browser <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"><video></code> tag does not have 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">isPlaying</code> prop. The only way to control it is to manually call the <a href="https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/play" target="_blank" rel="nofollow noopener noreferrer" 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"><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">play()</code></a> and <a href="https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/pause" target="_blank" rel="nofollow noopener noreferrer" 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"><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">pause()</code></a> methods on the DOM element. <strong class="font-bold">You need to synchronize the value 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">isPlaying</code> prop, which tells whether the video <em>should</em> currently be playing, with calls 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">play()</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">pause()</code>.</strong></p> <p class="whitespace-pre-wrap my-4">We’ll need to first <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/manipulating-the-dom-with-refs">get a ref</a> 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"><video></code> DOM node.</p> <p class="whitespace-pre-wrap my-4">You might be tempted to try to 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">play()</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">pause()</code> during rendering, but that isn’t correct:</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" 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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">useRef</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">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Calling these while rendering isn't allowed.</span> <span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-keyword">else</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Also, this crashes.</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">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsPlaying</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> <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">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">setIsPlaying</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isPlaying</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">isPlaying</span> ? <span class="sp-syntax-string">'Pause'</span> : <span class="sp-syntax-string">'Play'</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-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-string">"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"</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">The reason this code isn’t correct is that it tries to do something with the DOM node during rendering. In 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/keeping-components-pure">rendering should be a pure calculation</a> of JSX and should not contain side effects like modifying the DOM.</p> <p class="whitespace-pre-wrap my-4">Moreover, when <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">VideoPlayer</code> is called for the first time, its DOM does not exist yet! There isn’t a DOM node yet to 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">play()</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">pause()</code> on, because React doesn’t know what DOM to create until you return the JSX.</p> <p class="whitespace-pre-wrap my-4">The solution here is to <strong class="font-bold">wrap the side effect with <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">useEffect</code> to move it out of the rendering calculation:</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-keyword">import</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">useRef</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">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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 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 "> <span class="sp-syntax-keyword">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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-keyword">else</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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 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-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">By wrapping the DOM update in an Effect, you let React update the screen first. Then your Effect runs.</p> <p class="whitespace-pre-wrap my-4">When your <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">VideoPlayer</code> component renders (either the first time or if it re-renders), a few things will happen. First, React will update the screen, ensuring 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"><video></code> tag is in the DOM with the right props. Then React will run your Effect. Finally, your Effect will 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">play()</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">pause()</code> depending on the value 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">isPlaying</code>.</p> <p class="whitespace-pre-wrap my-4">Press Play/Pause multiple times and see how the video player stays synchronized 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">isPlaying</code> value:</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" 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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">useRef</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">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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">else</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsPlaying</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> <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">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">setIsPlaying</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isPlaying</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">isPlaying</span> ? <span class="sp-syntax-string">'Pause'</span> : <span class="sp-syntax-string">'Play'</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-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-string">"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"</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 this example, the “external system” you synchronized to React state was the browser media API. You can use a similar approach to wrap legacy non-React code (like jQuery plugins) into declarative React components.</p> <p class="whitespace-pre-wrap my-4">Note that controlling a video player is much more complex in practice. Calling <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">play()</code> may fail, the user might play or pause using the built-in browser controls, and so on. This example is very simplified and incomplete.</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">By default, Effects run after <em>every</em> render. This is why code like this will <strong class="font-bold">produce an infinite loop:</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-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><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-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><br/></div><div class="cm-line "><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">Effects run as a <em>result</em> of rendering. Setting state <em>triggers</em> rendering. Setting state immediately in an Effect is like plugging a power outlet into itself. The Effect runs, it sets the state, which causes a re-render, which causes the Effect to run, it sets the state again, this causes another re-render, and so on.</p><p class="whitespace-pre-wrap my-4">Effects should usually synchronize your components with an <em>external</em> system. If there’s no external system and you only want to adjust some state based on other state, <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">you might not need an Effect.</a></p></div></div></div> <h3 id="step-2-specify-the-effect-dependencies" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Step 2: Specify the Effect dependencies <a href="#step-2-specify-the-effect-dependencies" aria-label="Link for Step 2: Specify the Effect dependencies " title="Link for Step 2: Specify the Effect 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></h3> <p class="whitespace-pre-wrap my-4">By default, Effects run after <em>every</em> render. Often, this is <strong class="font-bold">not what you want:</strong></p> <ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1">Sometimes, it’s slow. Synchronizing with an external system is not always instant, so you might want to skip doing it unless it’s necessary. For example, you don’t want to reconnect to the chat server on every keystroke.</li> <li class="leading-relaxed mb-1">Sometimes, it’s wrong. For example, you don’t want to trigger a component fade-in animation on every keystroke. The animation should only play once when the component appears for the first time.</li> </ul> <p class="whitespace-pre-wrap my-4">To demonstrate the issue, here is the previous example with a few <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">console.log</code> calls and a text input that updates the parent component’s state. Notice how typing causes the Effect to re-run:</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></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:Rj56cq6:" aria-haspopup="true" aria-expanded="false" data-headlessui-state=""><span class="h-full py-2 px-1 mt-px -mb-px flex border-b 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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">useRef</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">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</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">'Calling video.play()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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">else</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">'Calling video.pause()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsPlaying</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> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">text</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setText</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-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">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">text</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">setText</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">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">setIsPlaying</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isPlaying</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">isPlaying</span> ? <span class="sp-syntax-string">'Pause'</span> : <span class="sp-syntax-string">'Play'</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-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-string">"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"</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">You can tell React to <strong class="font-bold">skip unnecessarily re-running the Effect</strong> by specifying an array of <em>dependencies</em> as the second argument 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">useEffect</code> call. Start by adding an 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> array to the above example on line 14:</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-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">You should see an error saying <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">React Hook useEffect has a missing dependency: 'isPlaying'</code>:</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 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class="sp-syntax-punctuation">;</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</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">'Calling video.play()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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">else</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">'Calling video.pause()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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-punctuation">;</span> <span class="sp-syntax-comment">// This causes an error</span> <span class="sp-syntax-keyword">return</span> <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-tag">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsPlaying</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> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">text</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setText</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-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">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">text</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">setText</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">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">setIsPlaying</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isPlaying</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">isPlaying</span> ? <span class="sp-syntax-string">'Pause'</span> : <span class="sp-syntax-string">'Play'</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-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-string">"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"</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 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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"> <p class="whitespace-pre-wrap my-4">The problem is that the code inside of your Effect <em>depends on</em> 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">isPlaying</code> prop to decide what to do, but this dependency was not explicitly declared. To fix this issue, 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">isPlaying</code> to the dependency array:</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 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">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// It's used here...</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> <span class="sp-syntax-keyword">else</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><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">isPlaying</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 it must be declared here!</span></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">Now all dependencies are declared, so there is no error. Specifying <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">[isPlaying]</code> as the dependency array tells React that it should skip re-running your Effect 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">isPlaying</code> is the same as it was during the previous render. With this change, typing into the input doesn’t cause the Effect to re-run, but pressing Play/Pause does:</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></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:Rj5acq6:" aria-haspopup="true" aria-expanded="false" data-headlessui-state=""><span 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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">useRef</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">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</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">'Calling video.play()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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">else</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">'Calling video.pause()'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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">isPlaying</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">video</span> <span class="sp-syntax-property">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">src</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">loop</span> <span class="sp-syntax-property">playsInline</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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setIsPlaying</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> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">text</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setText</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-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">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">text</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">setText</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">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">setIsPlaying</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">isPlaying</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">isPlaying</span> ? <span class="sp-syntax-string">'Pause'</span> : <span class="sp-syntax-string">'Play'</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-definition">VideoPlayer</span> <span class="sp-syntax-property">isPlaying</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">src</span>=<span class="sp-syntax-string">"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4"</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">The dependency array can contain multiple dependencies. React will only skip re-running the Effect if <em>all</em> of the dependencies you specify have exactly the same values as they had during the previous render. React compares the dependency values using the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is" target="_blank" rel="nofollow noopener noreferrer" 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"><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">Object.is</code></a> comparison. See the <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/useEffect#reference"><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">useEffect</code> reference</a> for details.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">Notice that you can’t “choose” your dependencies.</strong> You will get a lint error if the dependencies you specified don’t match what React expects based on the code inside your Effect. This helps catch many bugs in your code. If you don’t want some code to re-run, <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-to-do-when-you-dont-want-to-re-synchronize"><em>edit the Effect code itself</em> to not “need” that dependency.</a></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">The behaviors without the dependency array and with an <em>empty</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">[]</code> dependency array are different:</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">// This runs after every render</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-comment">// This runs only on mount (when the component appears)</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><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-comment">// This runs on mount *and also* if either a or b have changed since the last render</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">a</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">b</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">We’ll take a close look at what “mount” means in the next step.</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-was-the-ref-omitted-from-the-dependency-array" class="mdx-heading text-xl font-bold text-primary dark:text-primary-dark text-xl font-display font-bold leading-9 my-4">Why was the ref omitted from the dependency array? <a href="#why-was-the-ref-omitted-from-the-dependency-array" aria-label="Link for Why was the ref omitted from the dependency array? " title="Link for Why was the ref omitted from the dependency array? " 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">This Effect uses <em>both</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">ref</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">isPlaying</code>, but only <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">isPlaying</code> is declared 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">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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-keyword">else</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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 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">isPlaying</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">This is because 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">ref</code> object has a <em>stable identity:</em> React guarantees <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/useRef#returns">you’ll always get the same object</a> from the same <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">useRef</code> call on every render. It never changes, so it will never by itself cause the Effect to re-run. Therefore, it does not matter whether you include it or not. Including it is fine 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">function</span> <span class="sp-syntax-definition">VideoPlayer</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">src</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">isPlaying</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">ref</span> = <span class="sp-syntax-definition">useRef</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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">isPlaying</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">play</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-keyword">else</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span>.<span class="sp-syntax-property">pause</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 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">isPlaying</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">ref</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">The <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#setstate"><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">set</code> functions</a> returned by <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">useState</code> also have stable identity, so you will often see them omitted from the dependencies too. If the linter lets you omit a dependency without errors, it is safe to do.</p><p class="whitespace-pre-wrap my-4">Omitting always-stable dependencies only works when the linter can “see” that the object is stable. For example, 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">ref</code> was passed from a parent component, you would have to specify it in the dependency array. However, this is good because you can’t know whether the parent component always passes the same ref, or passes one of several refs conditionally. So your Effect <em>would</em> depend on which ref is passed.</p></div></details> <h3 id="step-3-add-cleanup-if-needed" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Step 3: Add cleanup if needed <a href="#step-3-add-cleanup-if-needed" aria-label="Link for Step 3: Add cleanup if needed " title="Link for Step 3: Add cleanup if needed " 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">Consider a different example. You’re writing 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">ChatRoom</code> component that needs to connect to the chat server when it appears. You are given 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">createConnection()</code> API that returns an object with <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">connect()</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">disconnect()</code> methods. How do you keep the component connected while it is displayed to the user?</p> <p class="whitespace-pre-wrap my-4">Start by writing the Effect logic:</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-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-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">It would be slow to connect to the chat after every re-render, so you add the dependency array:</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-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-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">The code inside the Effect does not use any props or state, so your dependency array 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">[]</code> (empty). This tells React to only run this code when the component “mounts”, i.e. appears on the screen for the first time.</strong></p> <p class="whitespace-pre-wrap my-4">Let’s try running this code:</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]" id="headlessui-listbox-button-:Rj5ccq6:" 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 class="ms-2"><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 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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" 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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">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">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-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-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 chat!<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 Effect only runs on mount, so you might expect <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">"✅ Connecting..."</code> to be printed once in the console. <strong class="font-bold">However, if you check the console, <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">"✅ Connecting..."</code> gets printed twice. Why does it happen?</strong></p> <p class="whitespace-pre-wrap my-4">Imagine 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 is a part of a larger app with many different screens. The user starts their journey 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">ChatRoom</code> page. The component mounts and 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">connection.connect()</code>. Then imagine the user navigates to another screen—for example, to the Settings page. 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 unmounts. Finally, the user clicks Back 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">ChatRoom</code> mounts again. This would set up a second connection—but the first connection was never destroyed! As the user navigates across the app, the connections would keep piling up.</p> <p class="whitespace-pre-wrap my-4">Bugs like this are easy to miss without extensive manual testing. To help you spot them quickly, in development React remounts every component once immediately after its initial mount.</p> <p class="whitespace-pre-wrap my-4">Seeing 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">"✅ Connecting..."</code> log twice helps you notice the real issue: your code doesn’t close the connection when the component unmounts.</p> <p class="whitespace-pre-wrap my-4">To fix the issue, return a <em>cleanup function</em> from 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-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-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">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><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">React will call your cleanup function each time before the Effect runs again, and one final time when the component unmounts (gets removed). Let’s see what happens when the cleanup function is implemented:</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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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=" rounded-b-lg overflow-hidden 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">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-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 chat!<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">Now you get three console logs in development:</p> <ol class="ms-6 my-3 list-decimal"> <li class="leading-relaxed mb-1"><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">"✅ Connecting..."</code></li> <li class="leading-relaxed mb-1"><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">"❌ Disconnected."</code></li> <li class="leading-relaxed mb-1"><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">"✅ Connecting..."</code></li> </ol> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">This is the correct behavior in development.</strong> By remounting your component, React verifies that navigating away and back would not break your code. Disconnecting and then connecting again is exactly what should happen! When you implement the cleanup well, there should be no user-visible difference between running the Effect once vs running it, cleaning it up, and running it again. There’s an extra connect/disconnect call pair because React is probing your code for bugs in development. This is normal—don’t try to make it go away!</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">In production, you would only 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">"✅ Connecting..."</code> printed once.</strong> Remounting components only happens in development to help you find Effects that need cleanup. You can turn off <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/StrictMode">Strict Mode</a> to opt out of the development behavior, but we recommend keeping it on. This lets you find many bugs like the one above.</p> <h2 id="how-to-handle-the-effect-firing-twice-in-development" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">How to handle the Effect firing twice in development? <a href="#how-to-handle-the-effect-firing-twice-in-development" aria-label="Link for How to handle the Effect firing twice in development? " title="Link for How to handle the Effect firing twice in development? " 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">React intentionally remounts your components in development to find bugs like in the last example. <strong class="font-bold">The right question isn’t “how to run an Effect once”, but “how to fix my Effect so that it works after remounting”.</strong></p> <p class="whitespace-pre-wrap my-4">Usually, the answer is to implement the cleanup function. The cleanup function should stop or undo whatever the Effect was doing. The rule of thumb is that the user shouldn’t be able to distinguish between the Effect running once (as in production) and a <em>setup → cleanup → setup</em> sequence (as you’d see in development).</p> <p class="whitespace-pre-wrap my-4">Most of the Effects you’ll write will fit into one of the common patterns 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"><h4 id="dont-use-refs-to-prevent-effects-from-firing" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Don’t use refs to prevent Effects from firing <a href="#dont-use-refs-to-prevent-effects-from-firing" aria-label="Link for Don’t use refs to prevent Effects from firing " title="Link for Don’t use refs to prevent Effects from firing " 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">A common pitfall for preventing Effects firing twice in development is to use 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">ref</code> to prevent the Effect from running more than once. For example, you could “fix” the above bug with 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">useRef</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 bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-keyword">const</span> <span class="sp-syntax-plain">connectionRef</span> = <span class="sp-syntax-definition">useRef</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 bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🚩 This wont fix the bug!!!</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">connectionRef</span>.<span class="sp-syntax-property">current</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">connectionRef</span>.<span class="sp-syntax-property">current</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">connectionRef</span>.<span class="sp-syntax-property">current</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-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></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">This makes it so you only 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">"✅ Connecting..."</code> once in development, but it doesn’t fix the bug.</p><p class="whitespace-pre-wrap my-4">When the user navigates away, the connection still isn’t closed and when they navigate back, a new connection is created. As the user navigates across the app, the connections would keep piling up, the same as it would before the “fix”.</p><p class="whitespace-pre-wrap my-4">To fix the bug, it is not enough to just make the Effect run once. The effect needs to work after re-mounting, which means the connection needs to be cleaned up like in the solution above.</p><p class="whitespace-pre-wrap my-4">See the examples below for how to handle common patterns.</p></div></div></div> <h3 id="controlling-non-react-widgets" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Controlling non-React widgets <a href="#controlling-non-react-widgets" aria-label="Link for Controlling non-React widgets " title="Link for Controlling non-React widgets " 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 need to add UI widgets that aren’t written in React. For example, let’s say you’re adding a map component to your page. It has 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">setZoomLevel()</code> method, and you’d like to keep the zoom level in sync with 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">zoomLevel</code> state variable in your React code. Your Effect would look similar to 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-keyword">const</span> <span class="sp-syntax-plain">map</span> = <span class="sp-syntax-plain">mapRef</span>.<span class="sp-syntax-property">current</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">map</span>.<span class="sp-syntax-property">setZoomLevel</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">zoomLevel</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">zoomLevel</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">Note that there is no cleanup needed in this case. In development, React will call the Effect twice, but this is not a problem because calling <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">setZoomLevel</code> twice with the same value does not do anything. It may be slightly slower, but this doesn’t matter because it won’t remount needlessly in production.</p> <p class="whitespace-pre-wrap my-4">Some APIs may not allow you to call them twice in a row. For example, the <a href="https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement/showModal" target="_blank" rel="nofollow noopener noreferrer" 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"><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">showModal</code></a> method of the built-in <a href="https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement" target="_blank" rel="nofollow noopener noreferrer" 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"><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"><dialog></code></a> element throws if you call it twice. Implement the cleanup function and make it close the dialog:</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-keyword">const</span> <span class="sp-syntax-plain">dialog</span> = <span class="sp-syntax-plain">dialogRef</span>.<span class="sp-syntax-property">current</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">dialog</span>.<span class="sp-syntax-property">showModal</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">dialog</span>.<span class="sp-syntax-property">close</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">In development, your Effect will 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">showModal()</code>, then immediately <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">close()</code>, and then <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">showModal()</code> again. This has the same user-visible behavior as calling <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">showModal()</code> once, as you would see in production.</p> <h3 id="subscribing-to-events" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Subscribing to events <a href="#subscribing-to-events" aria-label="Link for Subscribing to events " title="Link for Subscribing to events " 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">If your Effect subscribes to something, the cleanup function should unsubscribe:</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-keyword">function</span> <span class="sp-syntax-definition">handleScroll</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">e</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span><br/></div><div class="cm-line "> <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">window</span>.<span class="sp-syntax-property">scrollX</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">window</span>.<span class="sp-syntax-property">scrollY</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-plain">window</span>.<span class="sp-syntax-property">addEventListener</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'scroll'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">handleScroll</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">window</span>.<span class="sp-syntax-property">removeEventListener</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">'scroll'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">handleScroll</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">In development, your Effect will 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">addEventListener()</code>, then immediately <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">removeEventListener()</code>, and then <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">addEventListener()</code> again with the same handler. So there would be only one active subscription at a time. This has the same user-visible behavior as calling <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">addEventListener()</code> once, as in production.</p> <h3 id="triggering-animations" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Triggering animations <a href="#triggering-animations" aria-label="Link for Triggering animations " title="Link for Triggering animations " 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">If your Effect animates something in, the cleanup function should reset the animation to the initial values:</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-keyword">const</span> <span class="sp-syntax-plain">node</span> = <span class="sp-syntax-plain">ref</span>.<span class="sp-syntax-property">current</span><span class="sp-syntax-punctuation">;</span><br/></div><div class="cm-line "> <span class="sp-syntax-plain">node</span>.<span class="sp-syntax-property">style</span>.<span class="sp-syntax-property">opacity</span> = <span class="sp-syntax-static">1</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Trigger the animation</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">node</span>.<span class="sp-syntax-property">style</span>.<span class="sp-syntax-property">opacity</span> = <span class="sp-syntax-static">0</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Reset to the initial value</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><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">In development, opacity will be set 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">1</code>, then 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">0</code>, and then 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">1</code> again. This should have the same user-visible behavior as setting it 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">1</code> directly, which is what would happen in production. If you use a third-party animation library with support for tweening, your cleanup function should reset the timeline to its initial state.</p> <h3 id="fetching-data" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Fetching data <a href="#fetching-data" aria-label="Link for Fetching data " title="Link for Fetching data " 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">If your Effect fetches something, the cleanup function should either <a href="https://developer.mozilla.org/en-US/docs/Web/API/AbortController" target="_blank" rel="nofollow noopener noreferrer" 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">abort the fetch</a> or ignore its result:</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 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 "><br/></div><div class="cm-line "> <span class="sp-syntax-keyword">async</span> <span class="sp-syntax-keyword">function</span> <span class="sp-syntax-definition">startFetching</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">json</span> = <span class="sp-syntax-keyword">await</span> <span class="sp-syntax-definition">fetchTodos</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">userId</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 "> <span class="sp-syntax-definition">setTodos</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><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <span class="sp-syntax-definition">startFetching</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">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 "><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-plain">userId</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">You can’t “undo” a network request that already happened, but your cleanup function should ensure that the fetch that’s <em>not relevant anymore</em> does not keep affecting your application. 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">userId</code> changes from <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">'Alice'</code> 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">'Bob'</code>, cleanup ensures 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">'Alice'</code> response is ignored even if it arrives after <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">'Bob'</code>.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">In development, you will see two fetches in the Network tab.</strong> There is nothing wrong with that. With the approach above, the first Effect will immediately get cleaned up so its copy 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">ignore</code> variable will be set 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>. So even though there is an extra request, it won’t affect the state thanks 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">if (!ignore)</code> check.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">In production, there will only be one request.</strong> If the second request in development is bothering you, the best approach is to use a solution that deduplicates requests and caches their responses between components:</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">TodoList</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">todos</span> = <span class="sp-syntax-definition">useSomeDataLibrary</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-string">`/api/user/</span><span class="sp-syntax-punctuation">${</span><span class="sp-syntax-plain">userId</span><span class="sp-syntax-punctuation">}</span><span class="sp-syntax-string">/todos`</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 will not only improve the development experience, but also make your application feel faster. For example, the user pressing the Back button won’t have to wait for some data to load again because it will be cached. You can either build such a cache yourself or use one of the many alternatives to manual fetching in Effects.</p> <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="what-are-good-alternatives-to-data-fetching-in-effects" class="mdx-heading text-xl font-bold text-primary dark:text-primary-dark text-xl font-display font-bold leading-9 my-4">What are good alternatives to data fetching in Effects? <a href="#what-are-good-alternatives-to-data-fetching-in-effects" aria-label="Link for What are good alternatives to data fetching in Effects? " title="Link for What are good alternatives to data fetching in Effects? " 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">Writing <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> calls inside Effects is a <a href="https://www.robinwieruch.de/react-hooks-fetch-data/" target="_blank" rel="nofollow noopener noreferrer" 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">popular way to fetch data</a>, especially in fully client-side apps. This is, however, a very manual approach and it has significant downsides:</p><ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1"><strong class="font-bold">Effects don’t run on the server.</strong> This means that the initial server-rendered HTML will only include a loading state with no data. The client computer will have to download all JavaScript and render your app only to discover that now it needs to load the data. This is not very efficient.</li> <li class="leading-relaxed mb-1"><strong class="font-bold">Fetching directly in Effects makes it easy to create “network waterfalls”.</strong> You render the parent component, it fetches some data, renders the child components, and then they start fetching their data. If the network is not very fast, this is significantly slower than fetching all data in parallel.</li> <li class="leading-relaxed mb-1"><strong class="font-bold">Fetching directly in Effects usually means you don’t preload or cache data.</strong> For example, if the component unmounts and then mounts again, it would have to fetch the data again.</li> <li class="leading-relaxed mb-1"><strong class="font-bold">It’s not very ergonomic.</strong> There’s quite a bit of boilerplate code involved when writing <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> calls in a way that doesn’t suffer from bugs like <a href="https://maxrozen.com/race-conditions-fetching-data-react-with-useeffect" target="_blank" rel="nofollow noopener noreferrer" 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">race conditions.</a></li> </ul><p class="whitespace-pre-wrap my-4">This list of downsides is not specific to React. It applies to fetching data on mount with any library. Like with routing, data fetching is not trivial to do well, so we recommend the following approaches:</p><ul class="ms-6 my-3 list-disc"> <li class="leading-relaxed mb-1"><strong class="font-bold">If you use 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/start-a-new-react-project#production-grade-react-frameworks">framework</a>, use its built-in data fetching mechanism.</strong> Modern React frameworks have integrated data fetching mechanisms that are efficient and don’t suffer from the above pitfalls.</li> <li class="leading-relaxed mb-1"><strong class="font-bold">Otherwise, consider using or building a client-side cache.</strong> Popular open source solutions include <a href="https://tanstack.com/query/latest" target="_blank" rel="nofollow noopener noreferrer" 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">React Query</a>, <a href="https://swr.vercel.app/" target="_blank" rel="nofollow noopener noreferrer" 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">useSWR</a>, and <a href="https://beta.reactrouter.com/en/main/start/overview" target="_blank" rel="nofollow noopener noreferrer" 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">React Router 6.4+.</a> You can build your own solution too, in which case you would use Effects under the hood, but add logic for deduplicating requests, caching responses, and avoiding network waterfalls (by preloading data or hoisting data requirements to routes).</li> </ul><p class="whitespace-pre-wrap my-4">You can continue fetching data directly in Effects if neither of these approaches suit you.</p></div></details> <h3 id="sending-analytics" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Sending analytics <a href="#sending-analytics" aria-label="Link for Sending analytics " title="Link for Sending analytics " 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">Consider this code that sends an analytics event on the page visit:</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-definition">logVisit</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">url</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-comment">// Sends a POST request</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">url</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">In development, <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">logVisit</code> will be called twice for every URL, so you might be tempted to try to fix that. <strong class="font-bold">We recommend keeping this code as is.</strong> Like with earlier examples, there is no <em>user-visible</em> behavior difference between running it once and running it twice. From a practical point of view, <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">logVisit</code> should not do anything in development because you don’t want the logs from the development machines to skew the production metrics. Your component remounts every time you save its file, so it logs extra visits in development anyway.</p> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">In production, there will be no duplicate visit logs.</strong></p> <p class="whitespace-pre-wrap my-4">To debug the analytics events you’re sending, you can deploy your app to a staging environment (which runs in production mode) or temporarily opt out 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="/reference/react/StrictMode">Strict Mode</a> and its development-only remounting checks. You may also send analytics from the route change event handlers instead of Effects. For more precise analytics, <a href="https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API" target="_blank" rel="nofollow noopener noreferrer" 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">intersection observers</a> can help track which components are in the viewport and how long they remain visible.</p> <h3 id="not-an-effect-initializing-the-application" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Not an Effect: Initializing the application <a href="#not-an-effect-initializing-the-application" aria-label="Link for Not an Effect: Initializing the application " title="Link for Not an Effect: Initializing the application " 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">Some logic should only run once when the application starts. You can put it outside your components:</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">if</span> <span class="sp-syntax-punctuation">(</span><span class="sp-syntax-keyword">typeof</span> <span class="sp-syntax-plain">window</span> !== <span class="sp-syntax-string">'undefined'</span><span class="sp-syntax-punctuation">)</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-comment">// Check if we're running in the browser.</span><br/></div><div class="cm-line bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-definition">checkAuthToken</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">loadDataFromLocalStorage</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 "><br/></div><div class="cm-line "><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><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">This guarantees that such logic only runs once after the browser loads the page.</p> <h3 id="not-an-effect-buying-a-product" class="mdx-heading text-2xl font-display leading-9 text-primary dark:text-primary-dark font-bold my-6">Not an Effect: Buying a product <a href="#not-an-effect-buying-a-product" aria-label="Link for Not an Effect: Buying a product " title="Link for Not an Effect: Buying a product " 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, even if you write a cleanup function, there’s no way to prevent user-visible consequences of running the Effect twice. For example, maybe your Effect sends a POST request like buying a product:</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 bg-github-highlight dark:bg-opacity-10"> <span class="sp-syntax-comment">// 🔴 Wrong: This Effect fires twice in development, exposing a problem in the code.</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/buy'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">method</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'POST'</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4">You wouldn’t want to buy the product twice. However, this is also why you shouldn’t put this logic in an Effect. What if the user goes to another page and then presses Back? Your Effect would run again. You don’t want to buy the product when the user <em>visits</em> a page; you want to buy it when the user <em>clicks</em> the Buy button.</p> <p class="whitespace-pre-wrap my-4">Buying is not caused by rendering; it’s caused by a specific interaction. It should run only when the user presses the button. <strong class="font-bold">Delete the Effect and move your <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">/api/buy</code> request into the Buy button event handler:</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-keyword">function</span> <span class="sp-syntax-definition">handleClick</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-comment">// ✅ Buying is an event because it is caused by a particular interaction.</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/buy'</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">method</span><span class="sp-syntax-punctuation">:</span> <span class="sp-syntax-string">'POST'</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></div></code></pre></div></div></div></div> <p class="whitespace-pre-wrap my-4"><strong class="font-bold">This illustrates that if remounting breaks the logic of your application, this usually uncovers existing bugs.</strong> From a user’s perspective, visiting a page shouldn’t be different from visiting it, clicking a link, then pressing Back to view the page again. React verifies that your components abide by this principle by remounting them once in development.</p> <h2 id="putting-it-all-together" class="mdx-heading text-3xl font-display leading-10 text-primary dark:text-primary-dark font-bold my-6">Putting it all together <a href="#putting-it-all-together" aria-label="Link for Putting it all together " title="Link for Putting it all together " 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">This playground can help you “get a feel” for how Effects work in practice.</p> <p class="whitespace-pre-wrap my-4">This example uses <a href="https://developer.mozilla.org/en-US/docs/Web/API/setTimeout" target="_blank" rel="nofollow noopener noreferrer" 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"><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">setTimeout</code></a> to schedule a console log with the input text to appear three seconds after the Effect runs. The cleanup function cancels the pending timeout. Start by pressing “Mount the component”:</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></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:Rj5gcq6:" 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 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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 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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">function</span> <span class="sp-syntax-definition">Playground</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">text</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setText</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">'a'</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">function</span> <span class="sp-syntax-definition">onTimeout</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">'⏰ '</span> + <span class="sp-syntax-plain">text</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">'🔵 Schedule "'</span> + <span class="sp-syntax-plain">text</span> + <span class="sp-syntax-string">'" log'</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">timeoutId</span> = <span class="sp-syntax-definition">setTimeout</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">onTimeout</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-static">3000</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">'🟡 Cancel "'</span> + <span class="sp-syntax-plain">text</span> + <span class="sp-syntax-string">'" log'</span><span class="sp-syntax-punctuation">)</span><span class="sp-syntax-punctuation">;</span> <span class="sp-syntax-definition">clearTimeout</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-plain">timeoutId</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">text</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">label</span><span class="sp-syntax-punctuation">></span> What to log:<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">input</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">text</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">setText</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">label</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-plain">text</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-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">show</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">setShow</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> <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">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">setShow</span><span class="sp-syntax-punctuation">(</span>!<span class="sp-syntax-plain">show</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">show</span> ? <span class="sp-syntax-string">'Unmount'</span> : <span class="sp-syntax-string">'Mount'</span><span class="sp-syntax-punctuation">}</span> the component <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-plain">show</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-punctuation">{</span><span class="sp-syntax-plain">show</span> && <span class="sp-syntax-punctuation"><</span><span class="sp-syntax-definition">Playground</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">You will see three logs at first: <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">Schedule "a" log</code>, <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">Cancel "a" log</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">Schedule "a" log</code> again. Three second later there will also be a log saying <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">a</code>. As you learned earlier, the extra schedule/cancel pair is because React remounts the component once in development to verify that you’ve implemented cleanup well.</p> <p class="whitespace-pre-wrap my-4">Now edit the input to say <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">abc</code>. If you do it fast enough, you’ll 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">Schedule "ab" log</code> immediately followed by <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">Cancel "ab" log</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">Schedule "abc" log</code>. <strong class="font-bold">React always cleans up the previous render’s Effect before the next render’s Effect.</strong> This is why even if you type into the input fast, there is at most one timeout scheduled at a time. Edit the input a few times and watch the console to get a feel for how Effects get cleaned up.</p> <p class="whitespace-pre-wrap my-4">Type something into the input and then immediately press “Unmount the component”. Notice how unmounting cleans up the last render’s Effect. Here, it clears the last timeout before it has a chance to fire.</p> <p class="whitespace-pre-wrap my-4">Finally, edit the component above and comment out the cleanup function so that the timeouts don’t get cancelled. Try typing <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">abcde</code> fast. What do you expect to happen in three seconds? Will <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">console.log(text)</code> inside the timeout print the <em>latest</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">text</code> and produce five <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">abcde</code> logs? Give it a try to check your intuition!</p> <p class="whitespace-pre-wrap my-4">Three seconds later, you should see a sequence of logs (<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">a</code>, <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">ab</code>, <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">abc</code>, <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">abcd</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">abcde</code>) rather than five <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">abcde</code> logs. <strong class="font-bold">Each Effect “captures” 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">text</code> value from its corresponding render.</strong> It doesn’t matter 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">text</code> state changed: an Effect from the render with <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">text = 'ab'</code> will 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">'ab'</code>. In other words, Effects from each render are isolated from each other. If you’re curious how this works, you can read about <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures" target="_blank" rel="nofollow noopener noreferrer" 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">closures</a>.</p> <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="each-render-has-its-own-effects" class="mdx-heading text-xl font-bold text-primary dark:text-primary-dark text-xl font-display font-bold leading-9 my-4">Each render has its own Effects <a href="#each-render-has-its-own-effects" aria-label="Link for Each render has its own Effects " title="Link for Each render has its own Effects " 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">You can think 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">useEffect</code> as “attaching” a piece of behavior to the render output. Consider this 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">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-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">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 "> <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><br/></div><div class="cm-line "><br/></div><div class="cm-line "> <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 <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-tag">h1</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></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">Let’s see what exactly happens as the user navigates around the app.</p><h4 id="initial-render" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Initial render <a href="#initial-render" aria-label="Link for Initial render " title="Link for Initial render " 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 user visits <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 roomId="general" /></code>. Let’s <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">mentally substitute</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> with <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">'general'</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-comment">// JSX for the first render (roomId = "general")</span><br/></div><div class="cm-line "> <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 general!<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></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4"><strong class="font-bold">The Effect is <em>also</em> a part of the rendering output.</strong> The first render’s Effect becomes:</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">// Effect for the first render (roomId = "general")</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><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-string">'general'</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><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// Dependencies for the first render (roomId = "general")</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">]</span></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">React runs this Effect, which connects 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">'general'</code> chat room.</p><h4 id="re-render-with-same-dependencies" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Re-render with same dependencies <a href="#re-render-with-same-dependencies" aria-label="Link for Re-render with same dependencies " title="Link for Re-render with same 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></h4><p class="whitespace-pre-wrap my-4">Let’s say <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 roomId="general" /></code> re-renders. The JSX output is the same:</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">// JSX for the second render (roomId = "general")</span><br/></div><div class="cm-line "> <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 general!<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></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">React sees that the rendering output has not changed, so it doesn’t update the DOM.</p><p class="whitespace-pre-wrap my-4">The Effect from the second render looks 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-comment">// Effect for the second render (roomId = "general")</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><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-string">'general'</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><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// Dependencies for the second render (roomId = "general")</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-string">'general'</span><span class="sp-syntax-punctuation">]</span></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">React compares <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">['general']</code> from the second render with <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">['general']</code> from the first render. <strong class="font-bold">Because all dependencies are the same, React <em>ignores</em> the Effect from the second render.</strong> It never gets called.</p><h4 id="re-render-with-different-dependencies" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Re-render with different dependencies <a href="#re-render-with-different-dependencies" aria-label="Link for Re-render with different dependencies " title="Link for Re-render with different 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></h4><p class="whitespace-pre-wrap my-4">Then, the user visits <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 roomId="travel" /></code>. This time, the component returns different JSX:</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">// JSX for the third render (roomId = "travel")</span><br/></div><div class="cm-line "> <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 travel!<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></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">React updates the DOM to change <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">"Welcome to general"</code> into <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">"Welcome to travel"</code>.</p><p class="whitespace-pre-wrap my-4">The Effect from the third render looks 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-comment">// Effect for the third render (roomId = "travel")</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><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-string">'travel'</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><br/></div><div class="cm-line "> <span class="sp-syntax-comment">// Dependencies for the third render (roomId = "travel")</span><br/></div><div class="cm-line "> <span class="sp-syntax-punctuation">[</span><span class="sp-syntax-string">'travel'</span><span class="sp-syntax-punctuation">]</span></div></code></pre></div></div></div></div><p class="whitespace-pre-wrap my-4">React compares <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">['travel']</code> from the third render with <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">['general']</code> from the second render. One dependency is different: <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">Object.is('travel', 'general')</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">false</code>. The Effect can’t be skipped.</p><p class="whitespace-pre-wrap my-4"><strong class="font-bold">Before React can apply the Effect from the third render, it needs to clean up the last Effect that <em>did</em> run.</strong> The second render’s Effect was skipped, so React needs to clean up the first render’s Effect. If you scroll up to the first render, you’ll see that its cleanup 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">disconnect()</code> on the connection that was created with <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">createConnection('general')</code>. This disconnects the app 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">'general'</code> chat room.</p><p class="whitespace-pre-wrap my-4">After that, React runs the third render’s Effect. It connects 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">'travel'</code> chat room.</p><h4 id="unmount" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Unmount <a href="#unmount" aria-label="Link for Unmount " title="Link for Unmount " 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">Finally, let’s say the user navigates away, and 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 unmounts. React runs the last Effect’s cleanup function. The last Effect was from the third render. The third render’s cleanup destroys 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">createConnection('travel')</code> connection. So the app disconnects 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">'travel'</code> room.</p><h4 id="development-only-behaviors" class="mdx-heading text-xl font-display font-bold leading-9 my-4">Development-only behaviors <a href="#development-only-behaviors" aria-label="Link for Development-only behaviors " title="Link for Development-only behaviors " 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">When <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/StrictMode">Strict Mode</a> is on, React remounts every component once after mount (state and DOM are preserved). This <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="#step-3-add-cleanup-if-needed">helps you find Effects that need cleanup</a> and exposes bugs like race conditions early. Additionally, React will remount the Effects whenever you save a file in development. Both of these behaviors are development-only.</p></div></details> <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">Unlike events, Effects are caused by rendering itself rather than a particular interaction.</li> <li class="leading-relaxed mb-1">Effects let you synchronize a component with some external system (third-party API, network, etc).</li> <li class="leading-relaxed mb-1">By default, Effects run after every render (including the initial one).</li> <li class="leading-relaxed mb-1">React will skip the Effect if all of its dependencies have the same values as during the last render.</li> <li class="leading-relaxed mb-1">You can’t “choose” your dependencies. They are determined by the code inside the Effect.</li> <li class="leading-relaxed mb-1">Empty dependency array (<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>) corresponds to the component “mounting”, i.e. being added to the screen.</li> <li class="leading-relaxed mb-1">In Strict Mode, React mounts components twice (in development only!) to stress-test your Effects.</li> <li class="leading-relaxed mb-1">If your Effect breaks because of remounting, you need to implement a cleanup function.</li> <li class="leading-relaxed mb-1">React will call your cleanup function before the Effect runs next time, and during the unmount.</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. Focus a field on mount </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">2. Focus a field conditionally </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">3. Fix an interval that fires twice </button><button class="py-2 me-4 text-base border-b-4 duration-100 ease-in transition whitespace-nowrap text-ellipsis">4. Fix fetching inside an Effect </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="focus-a-field-on-mount" 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>Focus a field on mount <a href="#focus-a-field-on-mount" 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">In this example, the form renders 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"><MyInput /></code> component.</p><p class="whitespace-pre-wrap my-4">Use the input’s <a href="https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/focus" target="_blank" rel="nofollow noopener noreferrer" 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"><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">focus()</code></a> method to 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">MyInput</code> automatically focus when it appears on the screen. There is already a commented out implementation, but it doesn’t quite work. Figure out why it doesn’t work, and fix it. (If you’re familiar with 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">autoFocus</code> attribute, pretend that it does not exist: we are reimplementing the same functionality from scratch.)</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/MyInput.js" type="button">MyInput.js</button></div></div></div><button class="absolute top-0 start-[2px]" id="headlessui-listbox-button-:Rj5e6icq6:" 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">MyInput.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 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sp-code-editor"><div aria-autocomplete="list" aria-label="Code Editor for MyInput.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">useEffect</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-plain">useRef</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">MyInput</span><span class="sp-syntax-punctuation">(</span><span class="sp-syntax-punctuation">{</span> <span class="sp-syntax-property">value</span><span class="sp-syntax-punctuation">,</span> <span class="sp-syntax-property">onChange</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">ref</span> = <span class="sp-syntax-definition">useRef</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> <span class="sp-syntax-comment">// TODO: This doesn't quite work. Fix it.</span> <span class="sp-syntax-comment">// ref.current.focus() </span> <span class="sp-syntax-keyword">return</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">ref</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">ref</span><span class="sp-syntax-punctuation">}</span> <span class="sp-syntax-property">value</span>=<span class="sp-syntax-punctuation">{</span><span class="sp-syntax-plain">value</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">onChange</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">To verify that your solution works, press “Show form” and verify that the input receives focus (becomes highlighted and the cursor is placed inside). Press “Hide form” and “Show form” again. Verify the input is highlighted again.</p><p class="whitespace-pre-wrap my-4"><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">MyInput</code> should only focus <em>on mount</em> rather than after every render. To verify that the behavior is right, press “Show form” and then repeatedly press the “Make it uppercase” checkbox. Clicking the checkbox should <em>not</em> focus the input above it.</p></div><div class="flex justify-between items-center mt-4"><button class="me-2 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-transparent text-primary dark:text-primary-dark active:text-primary shadow-secondary-button-stroke dark:shadow-secondary-button-stroke-dark hover:bg-gray-40/5 active:bg-gray-40/10 hover:dark:bg-gray-60/5 active:dark:bg-gray-60/10"><svg class="inline me-1.5" width="0.75em" height="0.75em" viewBox="0 0 13 13" xmlns="http://www.w3.org/2000/svg"><path d="M2.21908 8.74479V12.7448H0.885742V0.078125H7.14041C7.26418 0.0781911 7.3855 0.112714 7.49076 0.177827C7.59602 0.242939 7.68108 0.336071 7.73641 0.446792L8.21908 1.41146H12.2191C12.3959 1.41146 12.5655 1.4817 12.6905 1.60672C12.8155 1.73174 12.8857 1.90131 12.8857 2.07812V9.41146C12.8857 9.58827 12.8155 9.75784 12.6905 9.88286C12.5655 10.0079 12.3959 10.0781 12.2191 10.0781H7.96441C7.84063 10.0781 7.71932 10.0435 7.61406 9.97842C7.50879 9.91331 7.42374 9.82018 7.36841 9.70946L6.88574 8.74479H2.21908ZM2.21908 1.41146V7.41146H7.70974L8.37641 8.74479H11.5524V2.74479H7.39508L6.72841 1.41146H2.21908Z" fill="currentColor"></path></svg> Show solution</button><button class="bg-link dark:bg-link-dark 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">Next Challenge<svg width="1em" height="1em" viewBox="0 0 20 20" fill="none" xmlns="http://www.w3.org/2000/svg" class="block ms-1.5"><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></button></div></div></div></div></div><div class="grid grid-cols-1 gap-4 py-4 mx-auto max-w-7xl md:grid-cols-2 md:py-12"><a class="flex gap-x-4 md:gap-x-6 items-center w-full md:min-w-80 md:w-fit md:max-w-md px-4 md:px-5 py-6 border-2 border-transparent text-base leading-base text-link dark:text-link-dark rounded-lg group focus:text-link dark:focus:text-link-dark focus:bg-highlight focus:border-link dark:focus:bg-highlight-dark dark:focus:border-link-dark focus:border-opacity-100 focus:border-2 focus:ring-1 focus:ring-offset-4 focus:ring-blue-40 active:ring-0 active:ring-offset-0 hover:bg-gray-5 dark:hover:bg-gray-80" href="/learn/manipulating-the-dom-with-refs"><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 inline text-tertiary dark:text-tertiary-dark group-focus:text-link dark:group-focus:text-link-dark" 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><div class="flex flex-col overflow-hidden"><span class="text-sm font-bold tracking-wide no-underline uppercase text-secondary dark:text-secondary-dark group-focus:text-link dark:group-focus:text-link-dark group-focus:text-opacity-100">Previous</span><span class="text-lg break-words group-hover:underline">Manipulating the DOM with Refs</span></div></a><a class="flex gap-x-4 md:gap-x-6 items-center w-full md:min-w-80 md:w-fit md:max-w-md px-4 md:px-5 py-6 border-2 border-transparent text-base leading-base text-link dark:text-link-dark rounded-lg group focus:text-link dark:focus:text-link-dark focus:bg-highlight focus:border-link dark:focus:bg-highlight-dark dark:focus:border-link-dark focus:border-opacity-100 focus:border-2 focus:ring-1 focus:ring-offset-4 focus:ring-blue-40 active:ring-0 active:ring-offset-0 hover:bg-gray-5 dark:hover:bg-gray-80 flex-row-reverse justify-self-end text-end" href="/learn/you-might-not-need-an-effect"><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 inline text-tertiary dark:text-tertiary-dark group-focus:text-link dark:group-focus:text-link-dark" 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><div class="flex flex-col overflow-hidden"><span class="text-sm font-bold tracking-wide no-underline uppercase text-secondary dark:text-secondary-dark group-focus:text-link dark:group-focus:text-link-dark group-focus:text-opacity-100">Next</span><span class="text-lg break-words group-hover:underline">You Might Not Need an Effect</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 3.62598H34.2942V15.1206H32.4387V6.31077L29.0476 12.4111H27.307L23.9162 6.31077V15.1206H22.0605V3.62598Z" 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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="#what-are-effects-and-how-are-they-different-from-events">What are Effects and how are they different from events? </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="#you-might-not-need-an-effect">You might not need an Effect </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="#how-to-write-an-effect">How to write an Effect </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="#step-1-declare-an-effect">Step 1: Declare an Effect </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="#step-2-specify-the-effect-dependencies">Step 2: Specify the Effect 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="#step-3-add-cleanup-if-needed">Step 3: Add cleanup if needed </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="#how-to-handle-the-effect-firing-twice-in-development">How to handle the Effect firing twice in development? </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="#controlling-non-react-widgets">Controlling non-React widgets </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="#subscribing-to-events">Subscribing to events </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="#triggering-animations">Triggering animations </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="#fetching-data">Fetching data </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="#sending-analytics">Sending analytics </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="#not-an-effect-initializing-the-application">Not an Effect: Initializing the application </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="#not-an-effect-buying-a-product">Not an Effect: Buying a product </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="#putting-it-all-together">Putting it all together </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\":\"#what-are-effects-and-how-are-they-different-from-events\",\"depth\":2,\"text\":\"What are Effects and how are they different from events? \"},{\"url\":\"#you-might-not-need-an-effect\",\"depth\":2,\"text\":\"You might not need an Effect \"},{\"url\":\"#how-to-write-an-effect\",\"depth\":2,\"text\":\"How to write an Effect \"},{\"url\":\"#step-1-declare-an-effect\",\"depth\":3,\"text\":\"Step 1: Declare an Effect \"},{\"url\":\"#step-2-specify-the-effect-dependencies\",\"depth\":3,\"text\":\"Step 2: Specify the Effect dependencies \"},{\"url\":\"#step-3-add-cleanup-if-needed\",\"depth\":3,\"text\":\"Step 3: Add cleanup if needed 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\"},{\"url\":\"#recap\",\"depth\":2,\"text\":\"Recap\"},{\"url\":\"#challenges\",\"depth\":2,\"text\":\"Challenges\"}]","content":"[[\"$r\",\"MaxWidth\",\"54\",{\"children\":[[\"$r\",\"Intro\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"Some components need to synchronize with external systems. For example, you might want to control a non-React component based on the React state, set up a server connection, or send an analytics log when a component appears on the screen. \",[\"$r\",\"em\",null,{\"children\":\"Effects\"}],\" let you run some code after rendering so that you can synchronize your component with some system outside of React.\"]}]}],\"\\n\",[\"$r\",\"YouWillLearn\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"What Effects are\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How Effects are different from events\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How to declare an Effect in your component\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"How to skip re-running an Effect unnecessarily\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Why Effects run twice in development and how to fix them\"}],\"\\n\"]}]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"what-are-effects-and-how-are-they-different-from-events\",\"children\":\"What are Effects and how are they different from events? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Before getting to Effects, you need to be familiar with two types of logic inside React components:\"}],\"\\n\",[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Rendering code\"}],\" (introduced in \",[\"$r\",\"a\",null,{\"href\":\"/learn/describing-the-ui\",\"children\":\"Describing the UI\"}],\") lives at the top level of your component. This is where you take the props and state, transform them, and return the JSX you want to see on the screen. \",[\"$r\",\"a\",null,{\"href\":\"/learn/keeping-components-pure\",\"children\":\"Rendering code must be pure.\"}],\" Like a math formula, it should only \",[\"$r\",\"em\",null,{\"children\":\"calculate\"}],\" the result, but not do anything else.\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Event handlers\"}],\" (introduced in \",[\"$r\",\"a\",null,{\"href\":\"/learn/adding-interactivity\",\"children\":\"Adding Interactivity\"}],\") are nested functions inside your components that \",[\"$r\",\"em\",null,{\"children\":\"do\"}],\" things rather than just calculate them. An event handler might update an input field, submit an HTTP POST request to buy a product, or navigate the user to another screen. Event handlers contain \",[\"$r\",\"a\",null,{\"href\":\"https://en.wikipedia.org/wiki/Side_effect_(computer_science)\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"“side effects”\"}],\" (they change the program’s state) caused by a specific user action (for example, a button click or typing).\"]}],\"\\n\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Sometimes this isn’t enough. Consider a \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component that must connect to the chat server whenever it’s visible on the screen. Connecting to a server is not a pure calculation (it’s a side effect) so it can’t happen during rendering. However, there is no single particular event like a click that causes \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" to be displayed.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[[\"$r\",\"em\",null,{\"children\":\"Effects\"}],\" let you specify side effects that are caused by rendering itself, rather than by a particular event.\"]}],\" Sending a message in the chat is an \",[\"$r\",\"em\",null,{\"children\":\"event\"}],\" because it is directly caused by the user clicking a specific button. However, setting up a server connection is an \",[\"$r\",\"em\",null,{\"children\":\"Effect\"}],\" because it should happen no matter which interaction caused the component to appear. Effects run at the end of a \",[\"$r\",\"a\",null,{\"href\":\"/learn/render-and-commit\",\"children\":\"commit\"}],\" after the screen updates. This is a good time to synchronize the React components with some external system (like network or a third-party library).\"]}],\"\\n\",[\"$r\",\"Note\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"Here and later in this text, capitalized “Effect” refers to the React-specific definition above, i.e. a side effect caused by rendering. To refer to the broader programming concept, we’ll say “side effect”.\"}]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"you-might-not-need-an-effect\",\"children\":\"You might not need an Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Don’t rush to add Effects to your components.\"}],\" Keep in mind that Effects are typically used to “step out” of your React code and synchronize with some \",[\"$r\",\"em\",null,{\"children\":\"external\"}],\" system. This includes browser APIs, third-party widgets, network, and so on. If your Effect only adjusts some state based on other state, \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect\",\"children\":\"you might not need an Effect.\"}]]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"how-to-write-an-effect\",\"children\":\"How to write an Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"To write an Effect, follow these three steps:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Declare an Effect.\"}],\" By default, your Effect will run after every \",[\"$r\",\"a\",null,{\"href\":\"/learn/render-and-commit\",\"children\":\"commit\"}],\".\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Specify the Effect dependencies.\"}],\" Most Effects should only re-run \",[\"$r\",\"em\",null,{\"children\":\"when needed\"}],\" rather than after every render. For example, a fade-in animation should only trigger when a component appears. Connecting and disconnecting to a chat room should only happen when the component appears and disappears, or when the chat room changes. You will learn how to control this by specifying \",[\"$r\",\"em\",null,{\"children\":\"dependencies.\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Add cleanup if needed.\"}],\" Some Effects need to specify how to stop, undo, or clean up whatever they were doing. For example, “connect” needs “disconnect”, “subscribe” needs “unsubscribe”, and “fetch” needs either “cancel” or “ignore”. You will learn how to do this by returning a \",[\"$r\",\"em\",null,{\"children\":\"cleanup function\"}],\".\"]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Let’s look at each of these steps in detail.\"}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"step-1-declare-an-effect\",\"children\":\"Step 1: Declare an Effect \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To declare an Effect in your component, import the \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useEffect\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" Hook\"]}],\" from React:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useEffect } from 'react';\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Then, call it at the top level of your component and put some code inside your Effect:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-4}\",\"children\":\"function MyComponent() {\\n useEffect(() =\u003e {\\n // Code here will run after *every* render\\n });\\n return \u003cdiv /\u003e;\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Every time your component renders, React will update the screen \",[\"$r\",\"em\",null,{\"children\":\"and then\"}],\" run the code inside \",[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\". In other words, \",[\"$r\",\"strong\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" “delays” a piece of code from running until that render is reflected on the screen.\"]}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Let’s see how you can use an Effect to synchronize with an external system. Consider a \",[\"$r\",\"code\",null,{\"children\":\"\u003cVideoPlayer\u003e\"}],\" React component. It would be nice to control whether it’s playing or paused by passing an \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" prop to it:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"\u003cVideoPlayer isPlaying={isPlaying} /\u003e;\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Your custom \",[\"$r\",\"code\",null,{\"children\":\"VideoPlayer\"}],\" component renders the built-in browser \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/HTML/Element/video\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"\u003cvideo\u003e\"}]}],\" tag:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"function VideoPlayer({ src, isPlaying }) {\\n // TODO: do something with isPlaying\\n return \u003cvideo src={src} /\u003e;\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"However, the browser \",[\"$r\",\"code\",null,{\"children\":\"\u003cvideo\u003e\"}],\" tag does not have an \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" prop. The only way to control it is to manually call the \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/play\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"play()\"}]}],\" and \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/pause\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"pause()\"}]}],\" methods on the DOM element. \",[\"$r\",\"strong\",null,{\"children\":[\"You need to synchronize the value of \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" prop, which tells whether the video \",[\"$r\",\"em\",null,{\"children\":\"should\"}],\" currently be playing, with calls like \",[\"$r\",\"code\",null,{\"children\":\"play()\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"pause()\"}],\".\"]}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"We’ll need to first \",[\"$r\",\"a\",null,{\"href\":\"/learn/manipulating-the-dom-with-refs\",\"children\":\"get a ref\"}],\" to the \",[\"$r\",\"code\",null,{\"children\":\"\u003cvideo\u003e\"}],\" DOM node.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You might be tempted to try to call \",[\"$r\",\"code\",null,{\"children\":\"play()\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"pause()\"}],\" during rendering, but that isn’t correct:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useRef, useEffect } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n if (isPlaying) {\\n ref.current.play(); // Calling these while rendering isn't allowed.\\n } else {\\n ref.current.pause(); // Also, this crashes.\\n }\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\\nexport default function App() {\\n const [isPlaying, setIsPlaying] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setIsPlaying(!isPlaying)}\u003e\\n {isPlaying ? 'Pause' : 'Play'}\\n \u003c/button\u003e\\n \u003cVideoPlayer\\n isPlaying={isPlaying}\\n src=\\\"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4\\\"\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"button { display: block; margin-bottom: 20px; }\\nvideo { width: 250px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"70\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The reason this code isn’t correct is that it tries to do something with the DOM node during rendering. In React, \",[\"$r\",\"a\",null,{\"href\":\"/learn/keeping-components-pure\",\"children\":\"rendering should be a pure calculation\"}],\" of JSX and should not contain side effects like modifying the DOM.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Moreover, when \",[\"$r\",\"code\",null,{\"children\":\"VideoPlayer\"}],\" is called for the first time, its DOM does not exist yet! There isn’t a DOM node yet to call \",[\"$r\",\"code\",null,{\"children\":\"play()\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"pause()\"}],\" on, because React doesn’t know what DOM to create until you return the JSX.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The solution here is to \",[\"$r\",\"strong\",null,{\"children\":[\"wrap the side effect with \",[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" to move it out of the rendering calculation:\"]}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6,12}\",\"children\":\"import { useEffect, useRef } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n ref.current.play();\\n } else {\\n ref.current.pause();\\n }\\n });\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"By wrapping the DOM update in an Effect, you let React update the screen first. Then your Effect runs.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"When your \",[\"$r\",\"code\",null,{\"children\":\"VideoPlayer\"}],\" component renders (either the first time or if it re-renders), a few things will happen. First, React will update the screen, ensuring the \",[\"$r\",\"code\",null,{\"children\":\"\u003cvideo\u003e\"}],\" tag is in the DOM with the right props. Then React will run your Effect. Finally, your Effect will call \",[\"$r\",\"code\",null,{\"children\":\"play()\"}],\" or \",[\"$r\",\"code\",null,{\"children\":\"pause()\"}],\" depending on the value of \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\".\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Press Play/Pause multiple times and see how the video player stays synchronized to the \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" value:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useRef, useEffect } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n ref.current.play();\\n } else {\\n ref.current.pause();\\n }\\n });\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\\nexport default function App() {\\n const [isPlaying, setIsPlaying] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setIsPlaying(!isPlaying)}\u003e\\n {isPlaying ? 'Pause' : 'Play'}\\n \u003c/button\u003e\\n \u003cVideoPlayer\\n isPlaying={isPlaying}\\n src=\\\"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4\\\"\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"button { display: block; margin-bottom: 20px; }\\nvideo { width: 250px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"86\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":\"In this example, the “external system” you synchronized to React state was the browser media API. You can use a similar approach to wrap legacy non-React code (like jQuery plugins) into declarative React components.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Note that controlling a video player is much more complex in practice. Calling \",[\"$r\",\"code\",null,{\"children\":\"play()\"}],\" may fail, the user might play or pause using the built-in browser controls, and so on. This example is very simplified and incomplete.\"]}],\"\\n\",[\"$r\",\"Pitfall\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"By default, Effects run after \",[\"$r\",\"em\",null,{\"children\":\"every\"}],\" render. This is why code like this will \",[\"$r\",\"strong\",null,{\"children\":\"produce an infinite loop:\"}]]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"const [count, setCount] = useState(0);\\nuseEffect(() =\u003e {\\n setCount(count + 1);\\n});\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"Effects run as a \",[\"$r\",\"em\",null,{\"children\":\"result\"}],\" of rendering. Setting state \",[\"$r\",\"em\",null,{\"children\":\"triggers\"}],\" rendering. Setting state immediately in an Effect is like plugging a power outlet into itself. The Effect runs, it sets the state, which causes a re-render, which causes the Effect to run, it sets the state again, this causes another re-render, and so on.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Effects should usually synchronize your components with an \",[\"$r\",\"em\",null,{\"children\":\"external\"}],\" system. If there’s no external system and you only want to adjust some state based on other state, \",[\"$r\",\"a\",null,{\"href\":\"/learn/you-might-not-need-an-effect\",\"children\":\"you might not need an Effect.\"}]]}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"step-2-specify-the-effect-dependencies\",\"children\":\"Step 2: Specify the Effect dependencies \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"By default, Effects run after \",[\"$r\",\"em\",null,{\"children\":\"every\"}],\" render. Often, this is \",[\"$r\",\"strong\",null,{\"children\":\"not what you want:\"}]]}],\"\\n\",[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Sometimes, it’s slow. Synchronizing with an external system is not always instant, so you might want to skip doing it unless it’s necessary. For example, you don’t want to reconnect to the chat server on every keystroke.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Sometimes, it’s wrong. For example, you don’t want to trigger a component fade-in animation on every keystroke. The animation should only play once when the component appears for the first time.\"}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To demonstrate the issue, here is the previous example with a few \",[\"$r\",\"code\",null,{\"children\":\"console.log\"}],\" calls and a text input that updates the parent component’s state. Notice how typing causes the Effect to re-run:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useRef, useEffect } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n console.log('Calling video.play()');\\n ref.current.play();\\n } else {\\n console.log('Calling video.pause()');\\n ref.current.pause();\\n }\\n });\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\\nexport default function App() {\\n const [isPlaying, setIsPlaying] = useState(false);\\n const [text, setText] = useState('');\\n return (\\n \u003c\u003e\\n \u003cinput value={text} onChange={e =\u003e setText(e.target.value)} /\u003e\\n \u003cbutton onClick={() =\u003e setIsPlaying(!isPlaying)}\u003e\\n {isPlaying ? 'Pause' : 'Play'}\\n \u003c/button\u003e\\n \u003cVideoPlayer\\n isPlaying={isPlaying}\\n src=\\\"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4\\\"\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input, button { display: block; margin-bottom: 20px; }\\nvideo { width: 250px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"94\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You can tell React to \",[\"$r\",\"strong\",null,{\"children\":\"skip unnecessarily re-running the Effect\"}],\" by specifying an array of \",[\"$r\",\"em\",null,{\"children\":\"dependencies\"}],\" as the second argument to the \",[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" call. Start by adding an empty \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" array to the above example on line 14:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3}\",\"children\":\" useEffect(() =\u003e {\\n // ...\\n }, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You should see an error saying \",[\"$r\",\"code\",null,{\"children\":\"React Hook useEffect has a missing dependency: 'isPlaying'\"}],\":\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useRef, useEffect } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n console.log('Calling video.play()');\\n ref.current.play();\\n } else {\\n console.log('Calling video.pause()');\\n ref.current.pause();\\n }\\n }, []); // This causes an error\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\\nexport default function App() {\\n const [isPlaying, setIsPlaying] = useState(false);\\n const [text, setText] = useState('');\\n return (\\n \u003c\u003e\\n \u003cinput value={text} onChange={e =\u003e setText(e.target.value)} /\u003e\\n \u003cbutton onClick={() =\u003e setIsPlaying(!isPlaying)}\u003e\\n {isPlaying ? 'Pause' : 'Play'}\\n \u003c/button\u003e\\n \u003cVideoPlayer\\n isPlaying={isPlaying}\\n src=\\\"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4\\\"\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input, button { display: block; margin-bottom: 20px; }\\nvideo { width: 250px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"102\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The problem is that the code inside of your Effect \",[\"$r\",\"em\",null,{\"children\":\"depends on\"}],\" the \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" prop to decide what to do, but this dependency was not explicitly declared. To fix this issue, add \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" to the dependency array:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2,7}\",\"children\":\" useEffect(() =\u003e {\\n if (isPlaying) { // It's used here...\\n // ...\\n } else {\\n // ...\\n }\\n }, [isPlaying]); // ...so it must be declared here!\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now all dependencies are declared, so there is no error. Specifying \",[\"$r\",\"code\",null,{\"children\":\"[isPlaying]\"}],\" as the dependency array tells React that it should skip re-running your Effect if \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" is the same as it was during the previous render. With this change, typing into the input doesn’t cause the Effect to re-run, but pressing Play/Pause does:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useRef, useEffect } from 'react';\\n\\nfunction VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n console.log('Calling video.play()');\\n ref.current.play();\\n } else {\\n console.log('Calling video.pause()');\\n ref.current.pause();\\n }\\n }, [isPlaying]);\\n\\n return \u003cvideo ref={ref} src={src} loop playsInline /\u003e;\\n}\\n\\nexport default function App() {\\n const [isPlaying, setIsPlaying] = useState(false);\\n const [text, setText] = useState('');\\n return (\\n \u003c\u003e\\n \u003cinput value={text} onChange={e =\u003e setText(e.target.value)} /\u003e\\n \u003cbutton onClick={() =\u003e setIsPlaying(!isPlaying)}\u003e\\n {isPlaying ? 'Pause' : 'Play'}\\n \u003c/button\u003e\\n \u003cVideoPlayer\\n isPlaying={isPlaying}\\n src=\\\"https://interactive-examples.mdn.mozilla.net/media/cc0-videos/flower.mp4\\\"\\n /\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input, button { display: block; margin-bottom: 20px; }\\nvideo { width: 250px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"128\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"The dependency array can contain multiple dependencies. React will only skip re-running the Effect if \",[\"$r\",\"em\",null,{\"children\":\"all\"}],\" of the dependencies you specify have exactly the same values as they had during the previous render. React compares the dependency values using the \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"Object.is\"}]}],\" comparison. See the \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useEffect#reference\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" reference\"]}],\" for details.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Notice that you can’t “choose” your dependencies.\"}],\" You will get a lint error if the dependencies you specified don’t match what React expects based on the code inside your Effect. This helps catch many bugs in your code. If you don’t want some code to re-run, \",[\"$r\",\"a\",null,{\"href\":\"/learn/lifecycle-of-reactive-effects#what-to-do-when-you-dont-want-to-re-synchronize\",\"children\":[[\"$r\",\"em\",null,{\"children\":\"edit the Effect code itself\"}],\" to not “need” that dependency.\"]}]]}],\"\\n\",[\"$r\",\"Pitfall\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"The behaviors without the dependency array and with an \",[\"$r\",\"em\",null,{\"children\":\"empty\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" dependency array are different:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{3,7,11}\",\"children\":\"useEffect(() =\u003e {\\n // This runs after every render\\n});\\n\\nuseEffect(() =\u003e {\\n // This runs only on mount (when the component appears)\\n}, []);\\n\\nuseEffect(() =\u003e {\\n // This runs on mount *and also* if either a or b have changed since the last render\\n}, [a, b]);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":\"We’ll take a close look at what “mount” means in the next step.\"}]]}],\"\\n\",[\"$r\",\"DeepDive\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"why-was-the-ref-omitted-from-the-dependency-array\",\"children\":\"Why was the ref omitted from the dependency array? \"}],[\"$r\",\"p\",null,{\"children\":[\"This Effect uses \",[\"$r\",\"em\",null,{\"children\":\"both\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"ref\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\", but only \",[\"$r\",\"code\",null,{\"children\":\"isPlaying\"}],\" is declared as a dependency:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{9}\",\"children\":\"function VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n ref.current.play();\\n } else {\\n ref.current.pause();\\n }\\n }, [isPlaying]);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"This is because the \",[\"$r\",\"code\",null,{\"children\":\"ref\"}],\" object has a \",[\"$r\",\"em\",null,{\"children\":\"stable identity:\"}],\" React guarantees \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useRef#returns\",\"children\":\"you’ll always get the same object\"}],\" from the same \",[\"$r\",\"code\",null,{\"children\":\"useRef\"}],\" call on every render. It never changes, so it will never by itself cause the Effect to re-run. Therefore, it does not matter whether you include it or not. Including it is fine too:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{9}\",\"children\":\"function VideoPlayer({ src, isPlaying }) {\\n const ref = useRef(null);\\n useEffect(() =\u003e {\\n if (isPlaying) {\\n ref.current.play();\\n } else {\\n ref.current.pause();\\n }\\n }, [isPlaying, ref]);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"The \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/useState#setstate\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"set\"}],\" functions\"]}],\" returned by \",[\"$r\",\"code\",null,{\"children\":\"useState\"}],\" also have stable identity, so you will often see them omitted from the dependencies too. If the linter lets you omit a dependency without errors, it is safe to do.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Omitting always-stable dependencies only works when the linter can “see” that the object is stable. For example, if \",[\"$r\",\"code\",null,{\"children\":\"ref\"}],\" was passed from a parent component, you would have to specify it in the dependency array. However, this is good because you can’t know whether the parent component always passes the same ref, or passes one of several refs conditionally. So your Effect \",[\"$r\",\"em\",null,{\"children\":\"would\"}],\" depend on which ref is passed.\"]}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"step-3-add-cleanup-if-needed\",\"children\":\"Step 3: Add cleanup if needed \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Consider a different example. You’re writing a \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component that needs to connect to the chat server when it appears. You are given a \",[\"$r\",\"code\",null,{\"children\":\"createConnection()\"}],\" API that returns an object with \",[\"$r\",\"code\",null,{\"children\":\"connect()\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"disconnect()\"}],\" methods. How do you keep the component connected while it is displayed to the user?\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Start by writing the Effect logic:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n});\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"It would be slow to connect to the chat after every re-render, so you add the dependency array:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4}\",\"children\":\"useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n}, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":[\"The code inside the Effect does not use any props or state, so your dependency array is \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" (empty). This tells React to only run this code when the component “mounts”, i.e. appears on the screen for the first time.\"]}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Let’s try running this code:\"}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nexport default function ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n }, []);\\n return \u003ch1\u003eWelcome to the chat!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection() {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected.');\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"144\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This Effect only runs on mount, so you might expect \",[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}],\" to be printed once in the console. \",[\"$r\",\"strong\",null,{\"children\":[\"However, if you check the console, \",[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}],\" gets printed twice. Why does it happen?\"]}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Imagine the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component is a part of a larger app with many different screens. The user starts their journey on the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" page. The component mounts and calls \",[\"$r\",\"code\",null,{\"children\":\"connection.connect()\"}],\". Then imagine the user navigates to another screen—for example, to the Settings page. The \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component unmounts. Finally, the user clicks Back and \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" mounts again. This would set up a second connection—but the first connection was never destroyed! As the user navigates across the app, the connections would keep piling up.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Bugs like this are easy to miss without extensive manual testing. To help you spot them quickly, in development React remounts every component once immediately after its initial mount.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Seeing the \",[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}],\" log twice helps you notice the real issue: your code doesn’t close the connection when the component unmounts.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To fix the issue, return a \",[\"$r\",\"em\",null,{\"children\":\"cleanup function\"}],\" from your Effect:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4-6}\",\"children\":\" useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n return () =\u003e {\\n connection.disconnect();\\n };\\n }, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"React will call your cleanup function each time before the Effect runs again, and one final time when the component unmounts (gets removed). Let’s see what happens when the cleanup function is implemented:\"}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { createConnection } from './chat.js';\\n\\nexport default function ChatRoom() {\\n useEffect(() =\u003e {\\n const connection = createConnection();\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, []);\\n return \u003ch1\u003eWelcome to the chat!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/chat.js\",\"children\":\"export function createConnection() {\\n // A real implementation would actually connect to the server\\n return {\\n connect() {\\n console.log('✅ Connecting...');\\n },\\n disconnect() {\\n console.log('❌ Disconnected.');\\n }\\n };\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"input { display: block; margin-bottom: 20px; }\\n\"}]}]]}],[\"$r\",\"MaxWidth\",\"252\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Now you get three console logs in development:\"}],\"\\n\",[\"$r\",\"ol\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"$r\",\"code\",null,{\"children\":\"\\\"❌ Disconnected.\\\"\"}]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}]}],\"\\n\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"This is the correct behavior in development.\"}],\" By remounting your component, React verifies that navigating away and back would not break your code. Disconnecting and then connecting again is exactly what should happen! When you implement the cleanup well, there should be no user-visible difference between running the Effect once vs running it, cleaning it up, and running it again. There’s an extra connect/disconnect call pair because React is probing your code for bugs in development. This is normal—don’t try to make it go away!\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"In production, you would only see \",[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}],\" printed once.\"]}],\" Remounting components only happens in development to help you find Effects that need cleanup. You can turn off \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/StrictMode\",\"children\":\"Strict Mode\"}],\" to opt out of the development behavior, but we recommend keeping it on. This lets you find many bugs like the one above.\"]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"how-to-handle-the-effect-firing-twice-in-development\",\"children\":\"How to handle the Effect firing twice in development? \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"React intentionally remounts your components in development to find bugs like in the last example. \",[\"$r\",\"strong\",null,{\"children\":\"The right question isn’t “how to run an Effect once”, but “how to fix my Effect so that it works after remounting”.\"}]]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Usually, the answer is to implement the cleanup function. The cleanup function should stop or undo whatever the Effect was doing. The rule of thumb is that the user shouldn’t be able to distinguish between the Effect running once (as in production) and a \",[\"$r\",\"em\",null,{\"children\":\"setup → cleanup → setup\"}],\" sequence (as you’d see in development).\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Most of the Effects you’ll write will fit into one of the common patterns below.\"}],\"\\n\",[\"$r\",\"Pitfall\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"dont-use-refs-to-prevent-effects-from-firing\",\"children\":\"Don’t use refs to prevent Effects from firing \"}],[\"$r\",\"p\",null,{\"children\":[\"A common pitfall for preventing Effects firing twice in development is to use a \",[\"$r\",\"code\",null,{\"children\":\"ref\"}],\" to prevent the Effect from running more than once. For example, you could “fix” the above bug with a \",[\"$r\",\"code\",null,{\"children\":\"useRef\"}],\":\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{1,3-4}\",\"children\":\" const connectionRef = useRef(null);\\n useEffect(() =\u003e {\\n // 🚩 This wont fix the bug!!!\\n if (!connectionRef.current) {\\n connectionRef.current = createConnection();\\n connectionRef.current.connect();\\n }\\n }, []);\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"This makes it so you only see \",[\"$r\",\"code\",null,{\"children\":\"\\\"✅ Connecting...\\\"\"}],\" once in development, but it doesn’t fix the bug.\"]}],[\"$r\",\"p\",null,{\"children\":\"When the user navigates away, the connection still isn’t closed and when they navigate back, a new connection is created. As the user navigates across the app, the connections would keep piling up, the same as it would before the “fix”.\"}],[\"$r\",\"p\",null,{\"children\":\"To fix the bug, it is not enough to just make the Effect run once. The effect needs to work after re-mounting, which means the connection needs to be cleaned up like in the solution above.\"}],[\"$r\",\"p\",null,{\"children\":\"See the examples below for how to handle common patterns.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"controlling-non-react-widgets\",\"children\":\"Controlling non-React widgets \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Sometimes you need to add UI widgets that aren’t written in React. For example, let’s say you’re adding a map component to your page. It has a \",[\"$r\",\"code\",null,{\"children\":\"setZoomLevel()\"}],\" method, and you’d like to keep the zoom level in sync with a \",[\"$r\",\"code\",null,{\"children\":\"zoomLevel\"}],\" state variable in your React code. Your Effect would look similar to this:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"useEffect(() =\u003e {\\n const map = mapRef.current;\\n map.setZoomLevel(zoomLevel);\\n}, [zoomLevel]);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Note that there is no cleanup needed in this case. In development, React will call the Effect twice, but this is not a problem because calling \",[\"$r\",\"code\",null,{\"children\":\"setZoomLevel\"}],\" twice with the same value does not do anything. It may be slightly slower, but this doesn’t matter because it won’t remount needlessly in production.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Some APIs may not allow you to call them twice in a row. For example, the \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement/showModal\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"showModal\"}]}],\" method of the built-in \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/HTMLDialogElement\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"\u003cdialog\u003e\"}]}],\" element throws if you call it twice. Implement the cleanup function and make it close the dialog:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4}\",\"children\":\"useEffect(() =\u003e {\\n const dialog = dialogRef.current;\\n dialog.showModal();\\n return () =\u003e dialog.close();\\n}, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In development, your Effect will call \",[\"$r\",\"code\",null,{\"children\":\"showModal()\"}],\", then immediately \",[\"$r\",\"code\",null,{\"children\":\"close()\"}],\", and then \",[\"$r\",\"code\",null,{\"children\":\"showModal()\"}],\" again. This has the same user-visible behavior as calling \",[\"$r\",\"code\",null,{\"children\":\"showModal()\"}],\" once, as you would see in production.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"subscribing-to-events\",\"children\":\"Subscribing to events \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"If your Effect subscribes to something, the cleanup function should unsubscribe:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{6}\",\"children\":\"useEffect(() =\u003e {\\n function handleScroll(e) {\\n console.log(window.scrollX, window.scrollY);\\n }\\n window.addEventListener('scroll', handleScroll);\\n return () =\u003e window.removeEventListener('scroll', handleScroll);\\n}, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In development, your Effect will call \",[\"$r\",\"code\",null,{\"children\":\"addEventListener()\"}],\", then immediately \",[\"$r\",\"code\",null,{\"children\":\"removeEventListener()\"}],\", and then \",[\"$r\",\"code\",null,{\"children\":\"addEventListener()\"}],\" again with the same handler. So there would be only one active subscription at a time. This has the same user-visible behavior as calling \",[\"$r\",\"code\",null,{\"children\":\"addEventListener()\"}],\" once, as in production.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"triggering-animations\",\"children\":\"Triggering animations \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"If your Effect animates something in, the cleanup function should reset the animation to the initial values:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{4-6}\",\"children\":\"useEffect(() =\u003e {\\n const node = ref.current;\\n node.style.opacity = 1; // Trigger the animation\\n return () =\u003e {\\n node.style.opacity = 0; // Reset to the initial value\\n };\\n}, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In development, opacity will be set to \",[\"$r\",\"code\",null,{\"children\":\"1\"}],\", then to \",[\"$r\",\"code\",null,{\"children\":\"0\"}],\", and then to \",[\"$r\",\"code\",null,{\"children\":\"1\"}],\" again. This should have the same user-visible behavior as setting it to \",[\"$r\",\"code\",null,{\"children\":\"1\"}],\" directly, which is what would happen in production. If you use a third-party animation library with support for tweening, your cleanup function should reset the timeline to its initial state.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"fetching-data\",\"children\":\"Fetching data \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"If your Effect fetches something, the cleanup function should either \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/AbortController\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"abort the fetch\"}],\" or ignore its result:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2,6,13-15}\",\"children\":\"useEffect(() =\u003e {\\n let ignore = false;\\n\\n async function startFetching() {\\n const json = await fetchTodos(userId);\\n if (!ignore) {\\n setTodos(json);\\n }\\n }\\n\\n startFetching();\\n\\n return () =\u003e {\\n ignore = true;\\n };\\n}, [userId]);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You can’t “undo” a network request that already happened, but your cleanup function should ensure that the fetch that’s \",[\"$r\",\"em\",null,{\"children\":\"not relevant anymore\"}],\" does not keep affecting your application. If the \",[\"$r\",\"code\",null,{\"children\":\"userId\"}],\" changes from \",[\"$r\",\"code\",null,{\"children\":\"'Alice'\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\", cleanup ensures that the \",[\"$r\",\"code\",null,{\"children\":\"'Alice'\"}],\" response is ignored even if it arrives after \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\".\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"In development, you will see two fetches in the Network tab.\"}],\" There is nothing wrong with that. With the approach above, the first Effect will immediately get cleaned up so its copy of the \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" variable will be set to \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\". So even though there is an extra request, it won’t affect the state thanks to the \",[\"$r\",\"code\",null,{\"children\":\"if (!ignore)\"}],\" check.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"In production, there will only be one request.\"}],\" If the second request in development is bothering you, the best approach is to use a solution that deduplicates requests and caches their responses between components:\"]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"function TodoList() {\\n const todos = useSomeDataLibrary(`/api/user/${userId}/todos`);\\n // ...\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This will not only improve the development experience, but also make your application feel faster. For example, the user pressing the Back button won’t have to wait for some data to load again because it will be cached. You can either build such a cache yourself or use one of the many alternatives to manual fetching in Effects.\"}],\"\\n\",[\"$r\",\"DeepDive\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"what-are-good-alternatives-to-data-fetching-in-effects\",\"children\":\"What are good alternatives to data fetching in Effects? \"}],[\"$r\",\"p\",null,{\"children\":[\"Writing \",[\"$r\",\"code\",null,{\"children\":\"fetch\"}],\" calls inside Effects is a \",[\"$r\",\"a\",null,{\"href\":\"https://www.robinwieruch.de/react-hooks-fetch-data/\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"popular way to fetch data\"}],\", especially in fully client-side apps. This is, however, a very manual approach and it has significant downsides:\"]}],[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Effects don’t run on the server.\"}],\" This means that the initial server-rendered HTML will only include a loading state with no data. The client computer will have to download all JavaScript and render your app only to discover that now it needs to load the data. This is not very efficient.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Fetching directly in Effects makes it easy to create “network waterfalls”.\"}],\" You render the parent component, it fetches some data, renders the child components, and then they start fetching their data. If the network is not very fast, this is significantly slower than fetching all data in parallel.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Fetching directly in Effects usually means you don’t preload or cache data.\"}],\" For example, if the component unmounts and then mounts again, it would have to fetch the data again.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"It’s not very ergonomic.\"}],\" There’s quite a bit of boilerplate code involved when writing \",[\"$r\",\"code\",null,{\"children\":\"fetch\"}],\" calls in a way that doesn’t suffer from bugs like \",[\"$r\",\"a\",null,{\"href\":\"https://maxrozen.com/race-conditions-fetching-data-react-with-useeffect\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"race conditions.\"}]]}],\"\\n\"]}],[\"$r\",\"p\",null,{\"children\":\"This list of downsides is not specific to React. It applies to fetching data on mount with any library. Like with routing, data fetching is not trivial to do well, so we recommend the following approaches:\"}],[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"If you use a \",[\"$r\",\"a\",null,{\"href\":\"/learn/start-a-new-react-project#production-grade-react-frameworks\",\"children\":\"framework\"}],\", use its built-in data fetching mechanism.\"]}],\" Modern React frameworks have integrated data fetching mechanisms that are efficient and don’t suffer from the above pitfalls.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"Otherwise, consider using or building a client-side cache.\"}],\" Popular open source solutions include \",[\"$r\",\"a\",null,{\"href\":\"https://tanstack.com/query/latest\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"React Query\"}],\", \",[\"$r\",\"a\",null,{\"href\":\"https://swr.vercel.app/\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"useSWR\"}],\", and \",[\"$r\",\"a\",null,{\"href\":\"https://beta.reactrouter.com/en/main/start/overview\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"React Router 6.4+.\"}],\" You can build your own solution too, in which case you would use Effects under the hood, but add logic for deduplicating requests, caching responses, and avoiding network waterfalls (by preloading data or hoisting data requirements to routes).\"]}],\"\\n\"]}],[\"$r\",\"p\",null,{\"children\":\"You can continue fetching data directly in Effects if neither of these approaches suit you.\"}]]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"sending-analytics\",\"children\":\"Sending analytics \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Consider this code that sends an analytics event on the page visit:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"useEffect(() =\u003e {\\n logVisit(url); // Sends a POST request\\n}, [url]);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"In development, \",[\"$r\",\"code\",null,{\"children\":\"logVisit\"}],\" will be called twice for every URL, so you might be tempted to try to fix that. \",[\"$r\",\"strong\",null,{\"children\":\"We recommend keeping this code as is.\"}],\" Like with earlier examples, there is no \",[\"$r\",\"em\",null,{\"children\":\"user-visible\"}],\" behavior difference between running it once and running it twice. From a practical point of view, \",[\"$r\",\"code\",null,{\"children\":\"logVisit\"}],\" should not do anything in development because you don’t want the logs from the development machines to skew the production metrics. Your component remounts every time you save its file, so it logs extra visits in development anyway.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":\"In production, there will be no duplicate visit logs.\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"To debug the analytics events you’re sending, you can deploy your app to a staging environment (which runs in production mode) or temporarily opt out of \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/StrictMode\",\"children\":\"Strict Mode\"}],\" and its development-only remounting checks. You may also send analytics from the route change event handlers instead of Effects. For more precise analytics, \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"intersection observers\"}],\" can help track which components are in the viewport and how long they remain visible.\"]}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"not-an-effect-initializing-the-application\",\"children\":\"Not an Effect: Initializing the application \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Some logic should only run once when the application starts. You can put it outside your components:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-3}\",\"children\":\"if (typeof window !== 'undefined') { // Check if we're running in the browser.\\n checkAuthToken();\\n loadDataFromLocalStorage();\\n}\\n\\nfunction App() {\\n // ...\\n}\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This guarantees that such logic only runs once after the browser loads the page.\"}],\"\\n\",[\"$r\",\"h3\",null,{\"id\":\"not-an-effect-buying-a-product\",\"children\":\"Not an Effect: Buying a product \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Sometimes, even if you write a cleanup function, there’s no way to prevent user-visible consequences of running the Effect twice. For example, maybe your Effect sends a POST request like buying a product:\"}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-3}\",\"children\":\"useEffect(() =\u003e {\\n // 🔴 Wrong: This Effect fires twice in development, exposing a problem in the code.\\n fetch('/api/buy', { method: 'POST' });\\n}, []);\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You wouldn’t want to buy the product twice. However, this is also why you shouldn’t put this logic in an Effect. What if the user goes to another page and then presses Back? Your Effect would run again. You don’t want to buy the product when the user \",[\"$r\",\"em\",null,{\"children\":\"visits\"}],\" a page; you want to buy it when the user \",[\"$r\",\"em\",null,{\"children\":\"clicks\"}],\" the Buy button.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Buying is not caused by rendering; it’s caused by a specific interaction. It should run only when the user presses the button. \",[\"$r\",\"strong\",null,{\"children\":[\"Delete the Effect and move your \",[\"$r\",\"code\",null,{\"children\":\"/api/buy\"}],\" request into the Buy button event handler:\"]}]]}],\"\\n\",[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"{2-3}\",\"children\":\" function handleClick() {\\n // ✅ Buying is an event because it is caused by a particular interaction.\\n fetch('/api/buy', { method: 'POST' });\\n }\\n\"}]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":\"This illustrates that if remounting breaks the logic of your application, this usually uncovers existing bugs.\"}],\" From a user’s perspective, visiting a page shouldn’t be different from visiting it, clicking a link, then pressing Back to view the page again. React verifies that your components abide by this principle by remounting them once in development.\"]}],\"\\n\",[\"$r\",\"h2\",null,{\"id\":\"putting-it-all-together\",\"children\":\"Putting it all together \"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"This playground can help you “get a feel” for how Effects work in practice.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"This example uses \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/setTimeout\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"setTimeout\"}]}],\" to schedule a console log with the input text to appear three seconds after the Effect runs. The cleanup function cancels the pending timeout. Start by pressing “Mount the component”:\"]}],\"\\n\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nfunction Playground() {\\n const [text, setText] = useState('a');\\n\\n useEffect(() =\u003e {\\n function onTimeout() {\\n console.log('⏰ ' + text);\\n }\\n\\n console.log('🔵 Schedule \\\"' + text + '\\\" log');\\n const timeoutId = setTimeout(onTimeout, 3000);\\n\\n return () =\u003e {\\n console.log('🟡 Cancel \\\"' + text + '\\\" log');\\n clearTimeout(timeoutId);\\n };\\n }, [text]);\\n\\n return (\\n \u003c\u003e\\n \u003clabel\u003e\\n What to log:{' '}\\n \u003cinput\\n value={text}\\n onChange={e =\u003e setText(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003ch1\u003e{text}\u003c/h1\u003e\\n \u003c/\u003e\\n );\\n}\\n\\nexport default function App() {\\n const [show, setShow] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(!show)}\u003e\\n {show ? 'Unmount' : 'Mount'} the component\\n \u003c/button\u003e\\n {show \u0026\u0026 \u003chr /\u003e}\\n {show \u0026\u0026 \u003cPlayground /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}]}],[\"$r\",\"MaxWidth\",\"268\",{\"children\":[\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"You will see three logs at first: \",[\"$r\",\"code\",null,{\"children\":\"Schedule \\\"a\\\" log\"}],\", \",[\"$r\",\"code\",null,{\"children\":\"Cancel \\\"a\\\" log\"}],\", and \",[\"$r\",\"code\",null,{\"children\":\"Schedule \\\"a\\\" log\"}],\" again. Three second later there will also be a log saying \",[\"$r\",\"code\",null,{\"children\":\"a\"}],\". As you learned earlier, the extra schedule/cancel pair is because React remounts the component once in development to verify that you’ve implemented cleanup well.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Now edit the input to say \",[\"$r\",\"code\",null,{\"children\":\"abc\"}],\". If you do it fast enough, you’ll see \",[\"$r\",\"code\",null,{\"children\":\"Schedule \\\"ab\\\" log\"}],\" immediately followed by \",[\"$r\",\"code\",null,{\"children\":\"Cancel \\\"ab\\\" log\"}],\" and \",[\"$r\",\"code\",null,{\"children\":\"Schedule \\\"abc\\\" log\"}],\". \",[\"$r\",\"strong\",null,{\"children\":\"React always cleans up the previous render’s Effect before the next render’s Effect.\"}],\" This is why even if you type into the input fast, there is at most one timeout scheduled at a time. Edit the input a few times and watch the console to get a feel for how Effects get cleaned up.\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":\"Type something into the input and then immediately press “Unmount the component”. Notice how unmounting cleans up the last render’s Effect. Here, it clears the last timeout before it has a chance to fire.\"}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Finally, edit the component above and comment out the cleanup function so that the timeouts don’t get cancelled. Try typing \",[\"$r\",\"code\",null,{\"children\":\"abcde\"}],\" fast. What do you expect to happen in three seconds? Will \",[\"$r\",\"code\",null,{\"children\":\"console.log(text)\"}],\" inside the timeout print the \",[\"$r\",\"em\",null,{\"children\":\"latest\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"text\"}],\" and produce five \",[\"$r\",\"code\",null,{\"children\":\"abcde\"}],\" logs? Give it a try to check your intuition!\"]}],\"\\n\",[\"$r\",\"p\",null,{\"children\":[\"Three seconds later, you should see a sequence of logs (\",[\"$r\",\"code\",null,{\"children\":\"a\"}],\", \",[\"$r\",\"code\",null,{\"children\":\"ab\"}],\", \",[\"$r\",\"code\",null,{\"children\":\"abc\"}],\", \",[\"$r\",\"code\",null,{\"children\":\"abcd\"}],\", and \",[\"$r\",\"code\",null,{\"children\":\"abcde\"}],\") rather than five \",[\"$r\",\"code\",null,{\"children\":\"abcde\"}],\" logs. \",[\"$r\",\"strong\",null,{\"children\":[\"Each Effect “captures” the \",[\"$r\",\"code\",null,{\"children\":\"text\"}],\" value from its corresponding render.\"]}],\" It doesn’t matter that the \",[\"$r\",\"code\",null,{\"children\":\"text\"}],\" state changed: an Effect from the render with \",[\"$r\",\"code\",null,{\"children\":\"text = 'ab'\"}],\" will always see \",[\"$r\",\"code\",null,{\"children\":\"'ab'\"}],\". In other words, Effects from each render are isolated from each other. If you’re curious how this works, you can read about \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"closures\"}],\".\"]}],\"\\n\",[\"$r\",\"DeepDive\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"each-render-has-its-own-effects\",\"children\":\"Each render has its own Effects \"}],[\"$r\",\"p\",null,{\"children\":[\"You can think of \",[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\" as “attaching” a piece of behavior to the render output. Consider this Effect:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\"export default function ChatRoom({ roomId }) {\\n useEffect(() =\u003e {\\n const connection = createConnection(roomId);\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n }, [roomId]);\\n\\n return \u003ch1\u003eWelcome to {roomId}!\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"p\",null,{\"children\":\"Let’s see what exactly happens as the user navigates around the app.\"}],[\"$r\",\"h4\",null,{\"id\":\"initial-render\",\"children\":\"Initial render \"}],[\"$r\",\"p\",null,{\"children\":[\"The user visits \",[\"$r\",\"code\",null,{\"children\":\"\u003cChatRoom roomId=\\\"general\\\" /\u003e\"}],\". Let’s \",[\"$r\",\"a\",null,{\"href\":\"/learn/state-as-a-snapshot#rendering-takes-a-snapshot-in-time\",\"children\":\"mentally substitute\"}],\" \",[\"$r\",\"code\",null,{\"children\":\"roomId\"}],\" with \",[\"$r\",\"code\",null,{\"children\":\"'general'\"}],\":\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // JSX for the first render (roomId = \\\"general\\\")\\n return \u003ch1\u003eWelcome to general!\u003c/h1\u003e;\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"The Effect is \",[\"$r\",\"em\",null,{\"children\":\"also\"}],\" a part of the rendering output.\"]}],\" The first render’s Effect becomes:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // Effect for the first render (roomId = \\\"general\\\")\\n () =\u003e {\\n const connection = createConnection('general');\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n },\\n // Dependencies for the first render (roomId = \\\"general\\\")\\n ['general']\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"React runs this Effect, which connects to the \",[\"$r\",\"code\",null,{\"children\":\"'general'\"}],\" chat room.\"]}],[\"$r\",\"h4\",null,{\"id\":\"re-render-with-same-dependencies\",\"children\":\"Re-render with same dependencies \"}],[\"$r\",\"p\",null,{\"children\":[\"Let’s say \",[\"$r\",\"code\",null,{\"children\":\"\u003cChatRoom roomId=\\\"general\\\" /\u003e\"}],\" re-renders. The JSX output is the same:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // JSX for the second render (roomId = \\\"general\\\")\\n return \u003ch1\u003eWelcome to general!\u003c/h1\u003e;\\n\"}]}],[\"$r\",\"p\",null,{\"children\":\"React sees that the rendering output has not changed, so it doesn’t update the DOM.\"}],[\"$r\",\"p\",null,{\"children\":\"The Effect from the second render looks like this:\"}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // Effect for the second render (roomId = \\\"general\\\")\\n () =\u003e {\\n const connection = createConnection('general');\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n },\\n // Dependencies for the second render (roomId = \\\"general\\\")\\n ['general']\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"React compares \",[\"$r\",\"code\",null,{\"children\":\"['general']\"}],\" from the second render with \",[\"$r\",\"code\",null,{\"children\":\"['general']\"}],\" from the first render. \",[\"$r\",\"strong\",null,{\"children\":[\"Because all dependencies are the same, React \",[\"$r\",\"em\",null,{\"children\":\"ignores\"}],\" the Effect from the second render.\"]}],\" It never gets called.\"]}],[\"$r\",\"h4\",null,{\"id\":\"re-render-with-different-dependencies\",\"children\":\"Re-render with different dependencies \"}],[\"$r\",\"p\",null,{\"children\":[\"Then, the user visits \",[\"$r\",\"code\",null,{\"children\":\"\u003cChatRoom roomId=\\\"travel\\\" /\u003e\"}],\". This time, the component returns different JSX:\"]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // JSX for the third render (roomId = \\\"travel\\\")\\n return \u003ch1\u003eWelcome to travel!\u003c/h1\u003e;\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"React updates the DOM to change \",[\"$r\",\"code\",null,{\"children\":\"\\\"Welcome to general\\\"\"}],\" into \",[\"$r\",\"code\",null,{\"children\":\"\\\"Welcome to travel\\\"\"}],\".\"]}],[\"$r\",\"p\",null,{\"children\":\"The Effect from the third render looks like this:\"}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"children\":\" // Effect for the third render (roomId = \\\"travel\\\")\\n () =\u003e {\\n const connection = createConnection('travel');\\n connection.connect();\\n return () =\u003e connection.disconnect();\\n },\\n // Dependencies for the third render (roomId = \\\"travel\\\")\\n ['travel']\\n\"}]}],[\"$r\",\"p\",null,{\"children\":[\"React compares \",[\"$r\",\"code\",null,{\"children\":\"['travel']\"}],\" from the third render with \",[\"$r\",\"code\",null,{\"children\":\"['general']\"}],\" from the second render. One dependency is different: \",[\"$r\",\"code\",null,{\"children\":\"Object.is('travel', 'general')\"}],\" is \",[\"$r\",\"code\",null,{\"children\":\"false\"}],\". The Effect can’t be skipped.\"]}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"strong\",null,{\"children\":[\"Before React can apply the Effect from the third render, it needs to clean up the last Effect that \",[\"$r\",\"em\",null,{\"children\":\"did\"}],\" run.\"]}],\" The second render’s Effect was skipped, so React needs to clean up the first render’s Effect. If you scroll up to the first render, you’ll see that its cleanup calls \",[\"$r\",\"code\",null,{\"children\":\"disconnect()\"}],\" on the connection that was created with \",[\"$r\",\"code\",null,{\"children\":\"createConnection('general')\"}],\". This disconnects the app from the \",[\"$r\",\"code\",null,{\"children\":\"'general'\"}],\" chat room.\"]}],[\"$r\",\"p\",null,{\"children\":[\"After that, React runs the third render’s Effect. It connects to the \",[\"$r\",\"code\",null,{\"children\":\"'travel'\"}],\" chat room.\"]}],[\"$r\",\"h4\",null,{\"id\":\"unmount\",\"children\":\"Unmount \"}],[\"$r\",\"p\",null,{\"children\":[\"Finally, let’s say the user navigates away, and the \",[\"$r\",\"code\",null,{\"children\":\"ChatRoom\"}],\" component unmounts. React runs the last Effect’s cleanup function. The last Effect was from the third render. The third render’s cleanup destroys the \",[\"$r\",\"code\",null,{\"children\":\"createConnection('travel')\"}],\" connection. So the app disconnects from the \",[\"$r\",\"code\",null,{\"children\":\"'travel'\"}],\" room.\"]}],[\"$r\",\"h4\",null,{\"id\":\"development-only-behaviors\",\"children\":\"Development-only behaviors \"}],[\"$r\",\"p\",null,{\"children\":[\"When \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/StrictMode\",\"children\":\"Strict Mode\"}],\" is on, React remounts every component once after mount (state and DOM are preserved). This \",[\"$r\",\"a\",null,{\"href\":\"#step-3-add-cleanup-if-needed\",\"children\":\"helps you find Effects that need cleanup\"}],\" and exposes bugs like race conditions early. Additionally, React will remount the Effects whenever you save a file in development. Both of these behaviors are development-only.\"]}]]}],\"\\n\",[\"$r\",\"Recap\",null,{\"children\":[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Unlike events, Effects are caused by rendering itself rather than a particular interaction.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"Effects let you synchronize a component with some external system (third-party API, network, etc).\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"By default, Effects run after every render (including the initial one).\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"React will skip the Effect if all of its dependencies have the same values as during the last render.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"You can’t “choose” your dependencies. They are determined by the code inside the Effect.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Empty dependency array (\",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\") corresponds to the component “mounting”, i.e. being added to the screen.\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"In Strict Mode, React mounts components twice (in development only!) to stress-test your Effects.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"If your Effect breaks because of remounting, you need to implement a cleanup function.\"}],\"\\n\",[\"$r\",\"li\",null,{\"children\":\"React will call your cleanup function before the Effect runs next time, and during the unmount.\"}],\"\\n\"]}]}],\"\\n\"]}],[\"$r\",\"Challenges\",null,{\"children\":[[\"$r\",\"h4\",null,{\"id\":\"focus-a-field-on-mount\",\"children\":\"Focus a field on mount \"}],[\"$r\",\"p\",null,{\"children\":[\"In this example, the form renders a \",[\"$r\",\"code\",null,{\"children\":\"\u003cMyInput /\u003e\"}],\" component.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Use the input’s \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/focus\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"focus()\"}]}],\" method to make \",[\"$r\",\"code\",null,{\"children\":\"MyInput\"}],\" automatically focus when it appears on the screen. There is already a commented out implementation, but it doesn’t quite work. Figure out why it doesn’t work, and fix it. (If you’re familiar with the \",[\"$r\",\"code\",null,{\"children\":\"autoFocus\"}],\" attribute, pretend that it does not exist: we are reimplementing the same functionality from scratch.)\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/MyInput.js active\",\"children\":\"import { useEffect, useRef } from 'react';\\n\\nexport default function MyInput({ value, onChange }) {\\n const ref = useRef(null);\\n\\n // TODO: This doesn't quite work. Fix it.\\n // ref.current.focus() \\n\\n return (\\n \u003cinput\\n ref={ref}\\n value={value}\\n onChange={onChange}\\n /\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport MyInput from './MyInput.js';\\n\\nexport default function Form() {\\n const [show, setShow] = useState(false);\\n const [name, setName] = useState('Taylor');\\n const [upper, setUpper] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} form\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 (\\n \u003c\u003e\\n \u003clabel\u003e\\n Enter your name:\\n \u003cMyInput\\n value={name}\\n onChange={e =\u003e setName(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={upper}\\n onChange={e =\u003e setUpper(e.target.checked)}\\n /\u003e\\n Make it uppercase\\n \u003c/label\u003e\\n \u003cp\u003eHello, \u003cb\u003e{upper ? name.toUpperCase() : name}\u003c/b\u003e\u003c/p\u003e\\n \u003c/\u003e\\n )}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":\"To verify that your solution works, press “Show form” and verify that the input receives focus (becomes highlighted and the cursor is placed inside). Press “Hide form” and “Show form” again. Verify the input is highlighted again.\"}],[\"$r\",\"p\",null,{\"children\":[[\"$r\",\"code\",null,{\"children\":\"MyInput\"}],\" should only focus \",[\"$r\",\"em\",null,{\"children\":\"on mount\"}],\" rather than after every render. To verify that the behavior is right, press “Show form” and then repeatedly press the “Make it uppercase” checkbox. Clicking the checkbox should \",[\"$r\",\"em\",null,{\"children\":\"not\"}],\" focus the input above it.\"]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"Calling \",[\"$r\",\"code\",null,{\"children\":\"ref.current.focus()\"}],\" during render is wrong because it is a \",[\"$r\",\"em\",null,{\"children\":\"side effect\"}],\". Side effects should either be placed inside an event handler or be declared with \",[\"$r\",\"code\",null,{\"children\":\"useEffect\"}],\". In this case, the side effect is \",[\"$r\",\"em\",null,{\"children\":\"caused\"}],\" by the component appearing rather than by any specific interaction, so it makes sense to put it in an Effect.\"]}],[\"$r\",\"p\",null,{\"children\":[\"To fix the mistake, wrap the \",[\"$r\",\"code\",null,{\"children\":\"ref.current.focus()\"}],\" call into an Effect declaration. Then, to ensure that this Effect runs only on mount rather than after every render, add the empty \",[\"$r\",\"code\",null,{\"children\":\"[]\"}],\" dependencies to it.\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/MyInput.js active\",\"children\":\"import { useEffect, useRef } from 'react';\\n\\nexport default function MyInput({ value, onChange }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n ref.current.focus();\\n }, []);\\n\\n return (\\n \u003cinput\\n ref={ref}\\n value={value}\\n onChange={onChange}\\n /\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport MyInput from './MyInput.js';\\n\\nexport default function Form() {\\n const [show, setShow] = useState(false);\\n const [name, setName] = useState('Taylor');\\n const [upper, setUpper] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} form\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 (\\n \u003c\u003e\\n \u003clabel\u003e\\n Enter your name:\\n \u003cMyInput\\n value={name}\\n onChange={e =\u003e setName(e.target.value)}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n \u003cinput\\n type=\\\"checkbox\\\"\\n checked={upper}\\n onChange={e =\u003e setUpper(e.target.checked)}\\n /\u003e\\n Make it uppercase\\n \u003c/label\u003e\\n \u003cp\u003eHello, \u003cb\u003e{upper ? name.toUpperCase() : name}\u003c/b\u003e\u003c/p\u003e\\n \u003c/\u003e\\n )}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}]]}],[\"$r\",\"h4\",null,{\"id\":\"focus-a-field-conditionally\",\"children\":\"Focus a field conditionally \"}],[\"$r\",\"p\",null,{\"children\":[\"This form renders two \",[\"$r\",\"code\",null,{\"children\":\"\u003cMyInput /\u003e\"}],\" components.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Press “Show form” and notice that the second field automatically gets focused. This is because both of the \",[\"$r\",\"code\",null,{\"children\":\"\u003cMyInput /\u003e\"}],\" components try to focus the field inside. When you call \",[\"$r\",\"code\",null,{\"children\":\"focus()\"}],\" for two input fields in a row, the last one always “wins”.\"]}],[\"$r\",\"p\",null,{\"children\":[\"Let’s say you want to focus the first field. The first \",[\"$r\",\"code\",null,{\"children\":\"MyInput\"}],\" component now receives a boolean \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus\"}],\" prop set to \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\". Change the logic so that \",[\"$r\",\"code\",null,{\"children\":\"focus()\"}],\" is only called if the \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus\"}],\" prop received by \",[\"$r\",\"code\",null,{\"children\":\"MyInput\"}],\" is \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\".\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/MyInput.js active\",\"children\":\"import { useEffect, useRef } from 'react';\\n\\nexport default function MyInput({ shouldFocus, value, onChange }) {\\n const ref = useRef(null);\\n\\n // TODO: call focus() only if shouldFocus is true.\\n useEffect(() =\u003e {\\n ref.current.focus();\\n }, []);\\n\\n return (\\n \u003cinput\\n ref={ref}\\n value={value}\\n onChange={onChange}\\n /\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport MyInput from './MyInput.js';\\n\\nexport default function Form() {\\n const [show, setShow] = useState(false);\\n const [firstName, setFirstName] = useState('Taylor');\\n const [lastName, setLastName] = useState('Swift');\\n const [upper, setUpper] = useState(false);\\n const name = firstName + ' ' + lastName;\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} form\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 (\\n \u003c\u003e\\n \u003clabel\u003e\\n Enter your first name:\\n \u003cMyInput\\n value={firstName}\\n onChange={e =\u003e setFirstName(e.target.value)}\\n shouldFocus={true}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Enter your last name:\\n \u003cMyInput\\n value={lastName}\\n onChange={e =\u003e setLastName(e.target.value)}\\n shouldFocus={false}\\n /\u003e\\n \u003c/label\u003e\\n \u003cp\u003eHello, \u003cb\u003e{upper ? name.toUpperCase() : name}\u003c/b\u003e\u003c/p\u003e\\n \u003c/\u003e\\n )}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"To verify your solution, press “Show form” and “Hide form” repeatedly. When the form appears, only the \",[\"$r\",\"em\",null,{\"children\":\"first\"}],\" input should get focused. This is because the parent component renders the first input with \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus={true}\"}],\" and the second input with \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus={false}\"}],\". Also check that both inputs still work and you can type into both of them.\"]}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"You can’t declare an Effect conditionally, but your Effect can include conditional logic.\"}]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"Put the conditional logic inside the Effect. You will need to specify \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus\"}],\" as a dependency because you are using it inside the Effect. (This means that if some input’s \",[\"$r\",\"code\",null,{\"children\":\"shouldFocus\"}],\" changes from \",[\"$r\",\"code\",null,{\"children\":\"false\"}],\" to \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\", it will focus after mount.)\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/MyInput.js active\",\"children\":\"import { useEffect, useRef } from 'react';\\n\\nexport default function MyInput({ shouldFocus, value, onChange }) {\\n const ref = useRef(null);\\n\\n useEffect(() =\u003e {\\n if (shouldFocus) {\\n ref.current.focus();\\n }\\n }, [shouldFocus]);\\n\\n return (\\n \u003cinput\\n ref={ref}\\n value={value}\\n onChange={onChange}\\n /\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport MyInput from './MyInput.js';\\n\\nexport default function Form() {\\n const [show, setShow] = useState(false);\\n const [firstName, setFirstName] = useState('Taylor');\\n const [lastName, setLastName] = useState('Swift');\\n const [upper, setUpper] = useState(false);\\n const name = firstName + ' ' + lastName;\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} form\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 (\\n \u003c\u003e\\n \u003clabel\u003e\\n Enter your first name:\\n \u003cMyInput\\n value={firstName}\\n onChange={e =\u003e setFirstName(e.target.value)}\\n shouldFocus={true}\\n /\u003e\\n \u003c/label\u003e\\n \u003clabel\u003e\\n Enter your last name:\\n \u003cMyInput\\n value={lastName}\\n onChange={e =\u003e setLastName(e.target.value)}\\n shouldFocus={false}\\n /\u003e\\n \u003c/label\u003e\\n \u003cp\u003eHello, \u003cb\u003e{upper ? name.toUpperCase() : name}\u003c/b\u003e\u003c/p\u003e\\n \u003c/\u003e\\n )}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-an-interval-that-fires-twice\",\"children\":\"Fix an interval that fires twice \"}],[\"$r\",\"p\",null,{\"children\":[\"This \",[\"$r\",\"code\",null,{\"children\":\"Counter\"}],\" component displays a counter that should increment every second. On mount, it calls \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/setInterval\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\".\"]}],\" This causes \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" to run every second. The \",[\"$r\",\"code\",null,{\"children\":\"onTick\"}],\" function increments the counter.\"]}],[\"$r\",\"p\",null,{\"children\":\"However, instead of incrementing once per second, it increments twice. Why is that? Find the cause of the bug and fix it.\"}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":[\"Keep in mind that \",[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\" returns an interval ID, which you can pass to \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/clearInterval\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"clearInterval\"}]}],\" to stop the interval.\"]}]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/Counter.js active\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function Counter() {\\n const [count, setCount] = useState(0);\\n\\n useEffect(() =\u003e {\\n function onTick() {\\n setCount(c =\u003e c + 1);\\n }\\n\\n setInterval(onTick, 1000);\\n }, []);\\n\\n return \u003ch1\u003e{count}\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport Counter from './Counter.js';\\n\\nexport default function Form() {\\n const [show, setShow] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} counter\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 \u003cCounter /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":[\"When \",[\"$r\",\"a\",null,{\"href\":\"/reference/react/StrictMode\",\"children\":\"Strict Mode\"}],\" is on (like in the sandboxes on this site), React remounts each component once in development. This causes the interval to be set up twice, and this is why each second the counter increments twice.\"]}],[\"$r\",\"p\",null,{\"children\":[\"However, React’s behavior is not the \",[\"$r\",\"em\",null,{\"children\":\"cause\"}],\" of the bug: the bug already exists in the code. React’s behavior makes the bug more noticeable. The real cause is that this Effect starts a process but doesn’t provide a way to clean it up.\"]}],[\"$r\",\"p\",null,{\"children\":[\"To fix this code, save the interval ID returned by \",[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\", and implement a cleanup function with \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/clearInterval\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"clearInterval\"}]}],\":\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/Counter.js active\",\"children\":\"import { useState, useEffect } from 'react';\\n\\nexport default function Counter() {\\n const [count, setCount] = useState(0);\\n\\n useEffect(() =\u003e {\\n function onTick() {\\n setCount(c =\u003e c + 1);\\n }\\n\\n const intervalId = setInterval(onTick, 1000);\\n return () =\u003e clearInterval(intervalId);\\n }, []);\\n\\n return \u003ch1\u003e{count}\u003c/h1\u003e;\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js hidden\",\"children\":\"import { useState } from 'react';\\nimport Counter from './Counter.js';\\n\\nexport default function App() {\\n const [show, setShow] = useState(false);\\n return (\\n \u003c\u003e\\n \u003cbutton onClick={() =\u003e setShow(s =\u003e !s)}\u003e{show ? 'Hide' : 'Show'} counter\u003c/button\u003e\\n \u003cbr /\u003e\\n \u003chr /\u003e\\n {show \u0026\u0026 \u003cCounter /\u003e}\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-css\",\"children\":\"label {\\n display: block;\\n margin-top: 20px;\\n margin-bottom: 20px;\\n}\\n\\nbody {\\n min-height: 150px;\\n}\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"In development, React will still remount your component once to verify that you’ve implemented cleanup well. So there will be a \",[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\" call, immediately followed by \",[\"$r\",\"code\",null,{\"children\":\"clearInterval\"}],\", and \",[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\" again. In production, there will be only one \",[\"$r\",\"code\",null,{\"children\":\"setInterval\"}],\" call. The user-visible behavior in both cases is the same: the counter increments once per second.\"]}]]}],[\"$r\",\"h4\",null,{\"id\":\"fix-fetching-inside-an-effect\",\"children\":\"Fix fetching inside an Effect \"}],[\"$r\",\"p\",null,{\"children\":[\"This component shows the biography for the selected person. It loads the biography by calling an asynchronous function \",[\"$r\",\"code\",null,{\"children\":\"fetchBio(person)\"}],\" on mount and whenever \",[\"$r\",\"code\",null,{\"children\":\"person\"}],\" changes. That asynchronous function returns a \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"Promise\"}],\" which eventually resolves to a string. When fetching is done, it calls \",[\"$r\",\"code\",null,{\"children\":\"setBio\"}],\" to display that string under the select box.\"]}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { fetchBio } from './api.js';\\n\\nexport default function Page() {\\n const [person, setPerson] = useState('Alice');\\n const [bio, setBio] = useState(null);\\n\\n useEffect(() =\u003e {\\n setBio(null);\\n fetchBio(person).then(result =\u003e {\\n setBio(result);\\n });\\n }, [person]);\\n\\n return (\\n \u003c\u003e\\n \u003cselect value={person} onChange={e =\u003e {\\n setPerson(e.target.value);\\n }}\u003e\\n \u003coption value=\\\"Alice\\\"\u003eAlice\u003c/option\u003e\\n \u003coption value=\\\"Bob\\\"\u003eBob\u003c/option\u003e\\n \u003coption value=\\\"Taylor\\\"\u003eTaylor\u003c/option\u003e\\n \u003c/select\u003e\\n \u003chr /\u003e\\n \u003cp\u003e\u003ci\u003e{bio ?? 'Loading...'}\u003c/i\u003e\u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/api.js hidden\",\"children\":\"export async function fetchBio(person) {\\n const delay = person === 'Bob' ? 2000 : 200;\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n resolve('This is ' + person + '’s bio.');\\n }, delay);\\n })\\n}\\n\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":\"There is a bug in this code. Start by selecting “Alice”. Then select “Bob” and then immediately after that select “Taylor”. If you do this fast enough, you will notice that bug: Taylor is selected, but the paragraph below says “This is Bob’s bio.”\"}],[\"$r\",\"p\",null,{\"children\":\"Why does this happen? Fix the bug inside this Effect.\"}],[\"$r\",\"Hint\",null,{\"children\":[\"$r\",\"p\",null,{\"children\":\"If an Effect fetches something asynchronously, it usually needs cleanup.\"}]}],[\"$r\",\"Solution\",null,{\"children\":[[\"$r\",\"p\",null,{\"children\":\"To trigger the bug, things need to happen in this order:\"}],[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Selecting \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" triggers \",[\"$r\",\"code\",null,{\"children\":\"fetchBio('Bob')\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Selecting \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" triggers \",[\"$r\",\"code\",null,{\"children\":\"fetchBio('Taylor')\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"$r\",\"strong\",null,{\"children\":[\"Fetching \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" completes \",[\"$r\",\"em\",null,{\"children\":\"before\"}],\" fetching \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}]]}]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"The Effect from the \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" render calls \",[\"$r\",\"code\",null,{\"children\":\"setBio('This is Taylor’s bio')\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Fetching \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" completes\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"The Effect from the \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" render calls \",[\"$r\",\"code\",null,{\"children\":\"setBio('This is Bob’s bio')\"}]]}],\"\\n\"]}],[\"$r\",\"p\",null,{\"children\":[\"This is why you see Bob’s bio even though Taylor is selected. Bugs like this are called \",[\"$r\",\"a\",null,{\"href\":\"https://en.wikipedia.org/wiki/Race_condition\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":\"race conditions\"}],\" because two asynchronous operations are “racing” with each other, and they might arrive in an unexpected order.\"]}],[\"$r\",\"p\",null,{\"children\":\"To fix this race condition, add a cleanup function:\"}],[\"$r\",\"Sandpack\",null,{\"children\":[[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/App.js\",\"children\":\"import { useState, useEffect } from 'react';\\nimport { fetchBio } from './api.js';\\n\\nexport default function Page() {\\n const [person, setPerson] = useState('Alice');\\n const [bio, setBio] = useState(null);\\n useEffect(() =\u003e {\\n let ignore = false;\\n setBio(null);\\n fetchBio(person).then(result =\u003e {\\n if (!ignore) {\\n setBio(result);\\n }\\n });\\n return () =\u003e {\\n ignore = true;\\n }\\n }, [person]);\\n\\n return (\\n \u003c\u003e\\n \u003cselect value={person} onChange={e =\u003e {\\n setPerson(e.target.value);\\n }}\u003e\\n \u003coption value=\\\"Alice\\\"\u003eAlice\u003c/option\u003e\\n \u003coption value=\\\"Bob\\\"\u003eBob\u003c/option\u003e\\n \u003coption value=\\\"Taylor\\\"\u003eTaylor\u003c/option\u003e\\n \u003c/select\u003e\\n \u003chr /\u003e\\n \u003cp\u003e\u003ci\u003e{bio ?? 'Loading...'}\u003c/i\u003e\u003c/p\u003e\\n \u003c/\u003e\\n );\\n}\\n\"}]}],[\"$r\",\"pre\",null,{\"children\":[\"$r\",\"code\",null,{\"className\":\"language-js\",\"meta\":\"src/api.js hidden\",\"children\":\"export async function fetchBio(person) {\\n const delay = person === 'Bob' ? 2000 : 200;\\n return new Promise(resolve =\u003e {\\n setTimeout(() =\u003e {\\n resolve('This is ' + person + '’s bio.');\\n }, delay);\\n })\\n}\\n\\n\"}]}]]}],[\"$r\",\"p\",null,{\"children\":[\"Each render’s Effect has its own \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" variable. Initially, the \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" variable is set to \",[\"$r\",\"code\",null,{\"children\":\"false\"}],\". However, if an Effect gets cleaned up (such as when you select a different person), its \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" variable becomes \",[\"$r\",\"code\",null,{\"children\":\"true\"}],\". So now it doesn’t matter in which order the requests complete. Only the last person’s Effect will have \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" set to \",[\"$r\",\"code\",null,{\"children\":\"false\"}],\", so it will call \",[\"$r\",\"code\",null,{\"children\":\"setBio(result)\"}],\". Past Effects have been cleaned up, so the \",[\"$r\",\"code\",null,{\"children\":\"if (!ignore)\"}],\" check will prevent them from calling \",[\"$r\",\"code\",null,{\"children\":\"setBio\"}],\":\"]}],[\"$r\",\"ul\",null,{\"children\":[\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Selecting \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" triggers \",[\"$r\",\"code\",null,{\"children\":\"fetchBio('Bob')\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Selecting \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" triggers \",[\"$r\",\"code\",null,{\"children\":\"fetchBio('Taylor')\"}],\" \",[\"$r\",\"strong\",null,{\"children\":\"and cleans up the previous (Bob’s) Effect\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Fetching \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" completes \",[\"$r\",\"em\",null,{\"children\":\"before\"}],\" fetching \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"The Effect from the \",[\"$r\",\"code\",null,{\"children\":\"'Taylor'\"}],\" render calls \",[\"$r\",\"code\",null,{\"children\":\"setBio('This is Taylor’s bio')\"}]]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"Fetching \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" completes\"]}],\"\\n\",[\"$r\",\"li\",null,{\"children\":[\"The Effect from the \",[\"$r\",\"code\",null,{\"children\":\"'Bob'\"}],\" render \",[\"$r\",\"strong\",null,{\"children\":[\"does not do anything because its \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" flag was set to \",[\"$r\",\"code\",null,{\"children\":\"true\"}]]}]]}],\"\\n\"]}],[\"$r\",\"p\",null,{\"children\":[\"In addition to ignoring the result of an outdated API call, you can also use \",[\"$r\",\"a\",null,{\"href\":\"https://developer.mozilla.org/en-US/docs/Web/API/AbortController\",\"target\":\"_blank\",\"rel\":\"nofollow noopener noreferrer\",\"children\":[\"$r\",\"code\",null,{\"children\":\"AbortController\"}]}],\" to cancel the requests that are no longer needed. However, by itself this is not enough to protect against race conditions. More asynchronous steps could be chained after the fetch, so using an explicit flag like \",[\"$r\",\"code\",null,{\"children\":\"ignore\"}],\" is the most reliable way to fix this type of problem.\"]}]]}]]}]]","meta":{"title":"Synchronizing with Effects"},"languages":null},"__N_SSG":true},"page":"/[[...markdownPath]]","query":{"markdownPath":["learn","synchronizing-with-effects"]},"buildId":"_Fz6u7K2TvuR-_M7SFBnH","isFallback":false,"gsp":true,"scriptLoader":[]}</script></body></html>