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You can create a mock function with jest.fn(). If no implementation is given, the mock function will return undefined when invoked."><meta data-rh="true" property="og:description" content="Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than only testing the output. You can create a mock function with jest.fn(). 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padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_Qr34"><div class="theme-doc-version-banner alert alert--warning margin-bottom--md" role="alert"><div>This is documentation for Jest 29.1, which is no longer actively maintained.</div><div class="margin-top--md">For up-to-date documentation, see the <a href="/docs/mock-function-api">latest version</a> (29.6).</div></div><div class="docItemContainer_tjFy"><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_T5ub" aria-label="Breadcrumbs"><ul class="breadcrumbs" itemscope="" itemtype="https://schema.org/BreadcrumbList"><li class="breadcrumbs__item"><a aria-label="Home page" class="breadcrumbs__link" href="/"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_sfvy"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active">Mock Functions<meta itemprop="position" content="1"></li></ul></nav>Version: 29.1<div class="tocCollapsible_wXna theme-doc-toc-mobile tocMobile_Ojys"><button type="button" class="clean-btn tocCollapsibleButton_iI2p">On this page</button></div><div class="theme-doc-markdown markdown"><header><h1>Mock Functions</h1></header>Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than only testing the output. You can create a mock function with <code>jest.fn()</code>. If no implementation is given, the mock function will return <code>undefined</code> when invoked.<div class="theme-admonition theme-admonition-info alert alert--info admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 14 16"><path fill-rule="evenodd" d="M7 2.3c3.14 0 5.7 2.56 5.7 5.7s-2.56 5.7-5.7 5.7A5.71 5.71 0 0 1 1.3 8c0-3.14 2.56-5.7 5.7-5.7zM7 1C3.14 1 0 4.14 0 8s3.14 7 7 7 7-3.14 7-7-3.14-7-7-7zm1 3H6v5h2V4zm0 6H6v2h2v-2z"></path></svg>info</div><div class="admonitionContent_yySL">The TypeScript examples from this page will only work as documented if you explicitly import Jest APIs:<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {expect, jest, test} from '@jest/globals'; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Consult the <a href="/docs/29.1/getting-started#using-typescript">Getting Started</a> guide for details on how to setup Jest with TypeScript.</div></div><h2 class="anchor anchorWithStickyNavbar_JmGV" id="methods">Methods<a href="#methods" class="hash-link" aria-label="Direct link to Methods" title="Direct link to Methods"></a></h2><div class="tableOfContentsInline_pY9K"><ul class="table-of-contents"><li><a href="#reference">Reference</a><ul><li><a href="#mockfngetmockname"><code>mockFn.getMockName()</code></a></li><li><a href="#mockfnmockcalls"><code>mockFn.mock.calls</code></a></li><li><a href="#mockfnmockresults"><code>mockFn.mock.results</code></a></li><li><a href="#mockfnmockinstances"><code>mockFn.mock.instances</code></a></li><li><a href="#mockfnmockcontexts"><code>mockFn.mock.contexts</code></a></li><li><a href="#mockfnmocklastcall"><code>mockFn.mock.lastCall</code></a></li><li><a href="#mockfnmockclear"><code>mockFn.mockClear()</code></a></li><li><a href="#mockfnmockreset"><code>mockFn.mockReset()</code></a></li><li><a href="#mockfnmockrestore"><code>mockFn.mockRestore()</code></a></li><li><a href="#mockfnmockimplementationfn"><code>mockFn.mockImplementation(fn)</code></a></li><li><a href="#mockfnmockimplementationoncefn"><code>mockFn.mockImplementationOnce(fn)</code></a></li><li><a href="#mockfnmocknamename"><code>mockFn.mockName(name)</code></a></li><li><a href="#mockfnmockreturnthis"><code>mockFn.mockReturnThis()</code></a></li><li><a href="#mockfnmockreturnvaluevalue"><code>mockFn.mockReturnValue(value)</code></a></li><li><a href="#mockfnmockreturnvalueoncevalue"><code>mockFn.mockReturnValueOnce(value)</code></a></li><li><a href="#mockfnmockresolvedvaluevalue"><code>mockFn.mockResolvedValue(value)</code></a></li><li><a href="#mockfnmockresolvedvalueoncevalue"><code>mockFn.mockResolvedValueOnce(value)</code></a></li><li><a href="#mockfnmockrejectedvaluevalue"><code>mockFn.mockRejectedValue(value)</code></a></li><li><a href="#mockfnmockrejectedvalueoncevalue"><code>mockFn.mockRejectedValueOnce(value)</code></a></li><li><a href="#mockfnwithimplementationfn-callback"><code>mockFn.withImplementation(fn, callback)</code></a></li></ul></li><li><a href="#typescript-usage">TypeScript Usage</a><ul><li><a href="#jestfnimplementation"><code>jest.fn(implementation?)</code></a></li><li><a href="#jestmockt"><code>jest.Mock<T></code></a></li><li><a href="#jestmockedsource"><code>jest.Mocked<Source></code></a></li><li><a href="#jestmockedsource-options"><code>jest.mocked(source, options?)</code></a></li></ul></li></ul></div><hr><h2 class="anchor anchorWithStickyNavbar_JmGV" id="reference">Reference<a href="#reference" class="hash-link" aria-label="Direct link to Reference" title="Direct link to Reference"></a></h2><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfngetmockname"><code>mockFn.getMockName()</code><a href="#mockfngetmockname" class="hash-link" aria-label="Direct link to mockfngetmockname" title="Direct link to mockfngetmockname"></a></h3>Returns the mock name string set by calling <a href="#mockfnmocknamename"><code>.mockName()</code></a>.<h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockcalls"><code>mockFn.mock.calls</code><a href="#mockfnmockcalls" class="hash-link" aria-label="Direct link to mockfnmockcalls" title="Direct link to mockfnmockcalls"></a></h3>An array containing the call arguments of all calls that have been made to this mock function. Each item in the array is an array of arguments that were passed during the call.For example: A mock function <code>f</code> that has been called twice, with the arguments <code>f('arg1', 'arg2')</code>, and then with the arguments <code>f('arg3', 'arg4')</code>, would have a <code>mock.calls</code> array that looks like this:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">[ ['arg1', 'arg2'], ['arg3', 'arg4'], ]; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockresults"><code>mockFn.mock.results</code><a href="#mockfnmockresults" class="hash-link" aria-label="Direct link to mockfnmockresults" title="Direct link to mockfnmockresults"></a></h3>An array containing the results of all calls that have been made to this mock function. Each entry in this array is an object containing a <code>type</code> property, and a <code>value</code> property. <code>type</code> will be one of the following:<ul><li><code>'return'</code> - Indicates that the call completed by returning normally.</li><li><code>'throw'</code> - Indicates that the call completed by throwing a value.</li><li><code>'incomplete'</code> - Indicates that the call has not yet completed. This occurs if you test the result from within the mock function itself, or from within a function that was called by the mock.</li></ul>The <code>value</code> property contains the value that was thrown or returned. <code>value</code> is undefined when <code>type === 'incomplete'</code>.For example: A mock function <code>f</code> that has been called three times, returning <code>'result1'</code>, throwing an error, and then returning <code>'result2'</code>, would have a <code>mock.results</code> array that looks like this:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">[ { type: 'return', value: 'result1', }, { type: 'throw', value: { /* Error instance */ }, }, { type: 'return', value: 'result2', }, ]; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockinstances"><code>mockFn.mock.instances</code><a href="#mockfnmockinstances" class="hash-link" aria-label="Direct link to mockfnmockinstances" title="Direct link to mockfnmockinstances"></a></h3>An array that contains all the object instances that have been instantiated from this mock function using <code>new</code>.For example: A mock function that has been instantiated twice would have the following <code>mock.instances</code> array:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest.fn(); const a = new mockFn(); const b = new mockFn(); mockFn.mock.instances[0] === a; // true mockFn.mock.instances[1] === b; // true </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockcontexts"><code>mockFn.mock.contexts</code><a href="#mockfnmockcontexts" class="hash-link" aria-label="Direct link to mockfnmockcontexts" title="Direct link to mockfnmockcontexts"></a></h3>An array that contains the contexts for all calls of the mock function.A context is the <code>this</code> value that a function receives when called. The context can be set using <code>Function.prototype.bind</code>, <code>Function.prototype.call</code> or <code>Function.prototype.apply</code>.For example:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest.fn(); const boundMockFn = mockFn.bind(thisContext0); boundMockFn('a', 'b'); mockFn.call(thisContext1, 'a', 'b'); mockFn.apply(thisContext2, ['a', 'b']); mockFn.mock.contexts[0] === thisContext0; // true mockFn.mock.contexts[1] === thisContext1; // true mockFn.mock.contexts[2] === thisContext2; // true </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmocklastcall"><code>mockFn.mock.lastCall</code><a href="#mockfnmocklastcall" class="hash-link" aria-label="Direct link to mockfnmocklastcall" title="Direct link to mockfnmocklastcall"></a></h3>An array containing the call arguments of the last call that was made to this mock function. If the function was not called, it will return <code>undefined</code>.For example: A mock function <code>f</code> that has been called twice, with the arguments <code>f('arg1', 'arg2')</code>, and then with the arguments <code>f('arg3', 'arg4')</code>, would have a <code>mock.lastCall</code> array that looks like this:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">['arg3', 'arg4']; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockclear"><code>mockFn.mockClear()</code><a href="#mockfnmockclear" class="hash-link" aria-label="Direct link to mockfnmockclear" title="Direct link to mockfnmockclear"></a></h3>Clears all information stored in the <a href="#mockfnmockcalls"><code>mockFn.mock.calls</code></a>, <a href="#mockfnmockinstances"><code>mockFn.mock.instances</code></a>, <a href="#mockfnmockcontexts"><code>mockFn.mock.contexts</code></a> and <a href="#mockfnmockresults"><code>mockFn.mock.results</code></a> arrays. Often this is useful when you want to clean up a mocks usage data between two assertions.The <a href="/docs/29.1/configuration#clearmocks-boolean"><code>clearMocks</code></a> configuration option is available to clear mocks automatically before each tests.<div class="theme-admonition theme-admonition-warning alert alert--danger admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M5.05.31c.81 2.17.41 3.38-.52 4.31C3.55 5.67 1.98 6.45.9 7.98c-1.45 2.05-1.7 6.53 3.53 7.7-2.2-1.16-2.67-4.52-.3-6.61-.61 2.03.53 3.33 1.94 2.86 1.39-.47 2.3.53 2.27 1.67-.02.78-.31 1.44-1.13 1.81 3.42-.59 4.78-3.42 4.78-5.56 0-2.84-2.53-3.22-1.25-5.61-1.52.13-2.03 1.13-1.89 2.75.09 1.08-1.02 1.8-1.86 1.33-.67-.41-.66-1.19-.06-1.78C8.18 5.31 8.68 2.45 5.05.32L5.03.3l.02.01z"></path></svg>danger</div><div class="admonitionContent_yySL">Beware that <code>mockFn.mockClear()</code> will replace <code>mockFn.mock</code>, not just reset the values of its properties! You should, therefore, avoid assigning <code>mockFn.mock</code> to other variables, temporary or not, to make sure you don't access stale data.</div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockreset"><code>mockFn.mockReset()</code><a href="#mockfnmockreset" class="hash-link" aria-label="Direct link to mockfnmockreset" title="Direct link to mockfnmockreset"></a></h3>Does everything that <a href="#mockfnmockclear"><code>mockFn.mockClear()</code></a> does, and also removes any mocked return values or implementations.This is useful when you want to completely reset a mock back to its initial state.The <a href="/docs/29.1/configuration#resetmocks-boolean"><code>resetMocks</code></a> configuration option is available to reset mocks automatically before each test.<div class="theme-admonition theme-admonition-info alert alert--info admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 14 16"><path fill-rule="evenodd" d="M7 2.3c3.14 0 5.7 2.56 5.7 5.7s-2.56 5.7-5.7 5.7A5.71 5.71 0 0 1 1.3 8c0-3.14 2.56-5.7 5.7-5.7zM7 1C3.14 1 0 4.14 0 8s3.14 7 7 7 7-3.14 7-7-3.14-7-7-7zm1 3H6v5h2V4zm0 6H6v2h2v-2z"></path></svg>info</div><div class="admonitionContent_yySL">Resetting a mock created with <code>jest.spyOn()</code> will result in a function with no return value.</div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockrestore"><code>mockFn.mockRestore()</code><a href="#mockfnmockrestore" class="hash-link" aria-label="Direct link to mockfnmockrestore" title="Direct link to mockfnmockrestore"></a></h3>Does everything that <a href="#mockfnmockreset"><code>mockFn.mockReset()</code></a> does, and also restores the original (non-mocked) implementation.This is useful when you want to mock functions in certain test cases and restore the original implementation in others.The <a href="/docs/29.1/configuration#restoremocks-boolean"><code>restoreMocks</code></a> configuration option is available to restore mocks automatically before each test.<div class="theme-admonition theme-admonition-info alert alert--info admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 14 16"><path fill-rule="evenodd" d="M7 2.3c3.14 0 5.7 2.56 5.7 5.7s-2.56 5.7-5.7 5.7A5.71 5.71 0 0 1 1.3 8c0-3.14 2.56-5.7 5.7-5.7zM7 1C3.14 1 0 4.14 0 8s3.14 7 7 7 7-3.14 7-7-3.14-7-7-7zm1 3H6v5h2V4zm0 6H6v2h2v-2z"></path></svg>info</div><div class="admonitionContent_yySL"><code>mockFn.mockRestore()</code> only works when the mock was created with <code>jest.spyOn()</code>. Thus you have to take care of restoration yourself when manually assigning <code>jest.fn()</code>.</div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockimplementationfn"><code>mockFn.mockImplementation(fn)</code><a href="#mockfnmockimplementationfn" class="hash-link" aria-label="Direct link to mockfnmockimplementationfn" title="Direct link to mockfnmockimplementationfn"></a></h3>Accepts a function that should be used as the implementation of the mock. The mock itself will still record all calls that go into and instances that come from itself – the only difference is that the implementation will also be executed when the mock is called.<div class="theme-admonition theme-admonition-tip alert alert--success admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M6.5 0C3.48 0 1 2.19 1 5c0 .92.55 2.25 1 3 1.34 2.25 1.78 2.78 2 4v1h5v-1c.22-1.22.66-1.75 2-4 .45-.75 1-2.08 1-3 0-2.81-2.48-5-5.5-5zm3.64 7.48c-.25.44-.47.8-.67 1.11-.86 1.41-1.25 2.06-1.45 3.23-.02.05-.02.11-.02.17H5c0-.06 0-.13-.02-.17-.2-1.17-.59-1.83-1.45-3.23-.2-.31-.42-.67-.67-1.11C2.44 6.78 2 5.65 2 5c0-2.2 2.02-4 4.5-4 1.22 0 2.36.42 3.22 1.19C10.55 2.94 11 3.94 11 5c0 .66-.44 1.78-.86 2.48zM4 14h5c-.23 1.14-1.3 2-2.5 2s-2.27-.86-2.5-2z"></path></svg>tip</div><div class="admonitionContent_yySL"><code>jest.fn(implementation)</code> is a shorthand for <code>jest.fn().mockImplementation(implementation)</code>.</div></div><div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest.fn(scalar => 42 + scalar); mockFn(0); // 42 mockFn(1); // 43 mockFn.mockImplementation(scalar => 36 + scalar); mockFn(2); // 38 mockFn(3); // 39 </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; const mockFn = jest.fn((scalar: number) => 42 + scalar); mockFn(0); // 42 mockFn(1); // 43 mockFn.mockImplementation(scalar => 36 + scalar); mockFn(2); // 38 mockFn(3); // 39 </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><code>.mockImplementation()</code> can also be used to mock class constructors:<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">SomeClass.js</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">module.exports = class SomeClass { method(a, b) {} }; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">SomeClass.test.js</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const SomeClass = require('./SomeClass'); jest.mock('./SomeClass'); // this happens automatically with automocking const mockMethod = jest.fn(); SomeClass.mockImplementation(() => { return { method: mockMethod, }; }); const some = new SomeClass(); some.method('a', 'b'); console.log('Calls to method: ', mockMethod.mock.calls); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">SomeClass.ts</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">export class SomeClass { method(a: string, b: string): void {} } </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">SomeClass.test.ts</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; import {SomeClass} from './SomeClass'; jest.mock('./SomeClass'); // this happens automatically with automocking const mockMethod = jest.fn<(a: string, b: string) => void>(); jest.mocked(SomeClass).mockImplementation(() => { return { method: mockMethod, }; }); const some = new SomeClass(); some.method('a', 'b'); console.log('Calls to method: ', mockMethod.mock.calls); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockimplementationoncefn"><code>mockFn.mockImplementationOnce(fn)</code><a href="#mockfnmockimplementationoncefn" class="hash-link" aria-label="Direct link to mockfnmockimplementationoncefn" title="Direct link to mockfnmockimplementationoncefn"></a></h3>Accepts a function that will be used as an implementation of the mock for one call to the mocked function. Can be chained so that multiple function calls produce different results.<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest .fn() .mockImplementationOnce(cb => cb(null, true)) .mockImplementationOnce(cb => cb(null, false)); mockFn((err, val) => console.log(val)); // true mockFn((err, val) => console.log(val)); // false </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; const mockFn = jest .fn<(cb: (a: null, b: boolean) => void) => void>() .mockImplementationOnce(cb => cb(null, true)) .mockImplementationOnce(cb => cb(null, false)); mockFn((err, val) => console.log(val)); // true mockFn((err, val) => console.log(val)); // false </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div>When the mocked function runs out of implementations defined with <code>.mockImplementationOnce()</code>, it will execute the default implementation set with <code>jest.fn(() => defaultValue)</code> or <code>.mockImplementation(() => defaultValue)</code> if they were called:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest .fn(() => 'default') .mockImplementationOnce(() => 'first call') .mockImplementationOnce(() => 'second call'); mockFn(); // 'first call' mockFn(); // 'second call' mockFn(); // 'default' mockFn(); // 'default' </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmocknamename"><code>mockFn.mockName(name)</code><a href="#mockfnmocknamename" class="hash-link" aria-label="Direct link to mockfnmocknamename" title="Direct link to mockfnmocknamename"></a></h3>Accepts a string to use in test result output in place of <code>'jest.fn()'</code> to indicate which mock function is being referenced.For example:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest.fn().mockName('mockedFunction'); // mockFn(); expect(mockFn).toHaveBeenCalled(); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Will result in this error:<div class="language-bash codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-bash codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">expect(mockedFunction).toHaveBeenCalled() Expected number of calls: >= 1 Received number of calls: 0 </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockreturnthis"><code>mockFn.mockReturnThis()</code><a href="#mockfnmockreturnthis" class="hash-link" aria-label="Direct link to mockfnmockreturnthis" title="Direct link to mockfnmockreturnthis"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn(function () { return this; }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockreturnvaluevalue"><code>mockFn.mockReturnValue(value)</code><a href="#mockfnmockreturnvaluevalue" class="hash-link" aria-label="Direct link to mockfnmockreturnvaluevalue" title="Direct link to mockfnmockreturnvaluevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementation(() => value); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Accepts a value that will be returned whenever the mock function is called.<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mock = jest.fn(); mock.mockReturnValue(42); mock(); // 42 mock.mockReturnValue(43); mock(); // 43 </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; const mock = jest.fn<() => number>(); mock.mockReturnValue(42); mock(); // 42 mock.mockReturnValue(43); mock(); // 43 </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockreturnvalueoncevalue"><code>mockFn.mockReturnValueOnce(value)</code><a href="#mockfnmockreturnvalueoncevalue" class="hash-link" aria-label="Direct link to mockfnmockreturnvalueoncevalue" title="Direct link to mockfnmockreturnvalueoncevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementationOnce(() => value); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Accepts a value that will be returned for one call to the mock function. Can be chained so that successive calls to the mock function return different values. When there are no more <code>mockReturnValueOnce</code> values to use, calls will return a value specified by <code>mockReturnValue</code>.<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">const mockFn = jest .fn() .mockReturnValue('default') .mockReturnValueOnce('first call') .mockReturnValueOnce('second call'); mockFn(); // 'first call' mockFn(); // 'second call' mockFn(); // 'default' mockFn(); // 'default' </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; const mockFn = jest .fn<() => string>() .mockReturnValue('default') .mockReturnValueOnce('first call') .mockReturnValueOnce('second call'); mockFn(); // 'first call' mockFn(); // 'second call' mockFn(); // 'default' mockFn(); // 'default' </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockresolvedvaluevalue"><code>mockFn.mockResolvedValue(value)</code><a href="#mockfnmockresolvedvaluevalue" class="hash-link" aria-label="Direct link to mockfnmockresolvedvaluevalue" title="Direct link to mockfnmockresolvedvaluevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementation(() => Promise.resolve(value)); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Useful to mock async functions in async tests:<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('async test', async () => { const asyncMock = jest.fn().mockResolvedValue(43); await asyncMock(); // 43 }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest, test} from '@jest/globals'; test('async test', async () => { const asyncMock = jest.fn<() => Promise<number>>().mockResolvedValue(43); await asyncMock(); // 43 }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockresolvedvalueoncevalue"><code>mockFn.mockResolvedValueOnce(value)</code><a href="#mockfnmockresolvedvalueoncevalue" class="hash-link" aria-label="Direct link to mockfnmockresolvedvalueoncevalue" title="Direct link to mockfnmockresolvedvalueoncevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementationOnce(() => Promise.resolve(value)); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Useful to resolve different values over multiple async calls:<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('async test', async () => { const asyncMock = jest .fn() .mockResolvedValue('default') .mockResolvedValueOnce('first call') .mockResolvedValueOnce('second call'); await asyncMock(); // 'first call' await asyncMock(); // 'second call' await asyncMock(); // 'default' await asyncMock(); // 'default' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest, test} from '@jest/globals'; test('async test', async () => { const asyncMock = jest .fn<() => Promise<string>>() .mockResolvedValue('default') .mockResolvedValueOnce('first call') .mockResolvedValueOnce('second call'); await asyncMock(); // 'first call' await asyncMock(); // 'second call' await asyncMock(); // 'default' await asyncMock(); // 'default' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockrejectedvaluevalue"><code>mockFn.mockRejectedValue(value)</code><a href="#mockfnmockrejectedvaluevalue" class="hash-link" aria-label="Direct link to mockfnmockrejectedvaluevalue" title="Direct link to mockfnmockrejectedvaluevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementation(() => Promise.reject(value)); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Useful to create async mock functions that will always reject:<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('async test', async () => { const asyncMock = jest .fn() .mockRejectedValue(new Error('Async error message')); await asyncMock(); // throws 'Async error message' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest, test} from '@jest/globals'; test('async test', async () => { const asyncMock = jest .fn<() => Promise<never>>() .mockRejectedValue(new Error('Async error message')); await asyncMock(); // throws 'Async error message' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnmockrejectedvalueoncevalue"><code>mockFn.mockRejectedValueOnce(value)</code><a href="#mockfnmockrejectedvalueoncevalue" class="hash-link" aria-label="Direct link to mockfnmockrejectedvalueoncevalue" title="Direct link to mockfnmockrejectedvalueoncevalue"></a></h3>Shorthand for:<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">jest.fn().mockImplementationOnce(() => Promise.reject(value)); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Useful together with <code>.mockResolvedValueOnce()</code> or to reject with different exceptions over multiple async calls:<div class="tabs-container tabList_J5MA"><ul role="tablist" aria-orientation="horizontal" class="tabs"><li role="tab" tabindex="0" aria-selected="true" class="tabs__item tabItem_l0OV tabs__item--active">JavaScript</li><li role="tab" tabindex="-1" aria-selected="false" class="tabs__item tabItem_l0OV">TypeScript</li></ul><div class="margin-top--md"><div role="tabpanel" class="tabItem_wHwb"><div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('async test', async () => { const asyncMock = jest .fn() .mockResolvedValueOnce('first call') .mockRejectedValueOnce(new Error('Async error message')); await asyncMock(); // 'first call' await asyncMock(); // throws 'Async error message' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><div role="tabpanel" class="tabItem_wHwb" hidden=""><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest, test} from '@jest/globals'; test('async test', async () => { const asyncMock = jest .fn<() => Promise<string>>() .mockResolvedValueOnce('first call') .mockRejectedValueOnce(new Error('Async error message')); await asyncMock(); // 'first call' await asyncMock(); // throws 'Async error message' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="mockfnwithimplementationfn-callback"><code>mockFn.withImplementation(fn, callback)</code><a href="#mockfnwithimplementationfn-callback" class="hash-link" aria-label="Direct link to mockfnwithimplementationfn-callback" title="Direct link to mockfnwithimplementationfn-callback"></a></h3>Accepts a function which should be temporarily used as the implementation of the mock while the callback is being executed.<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('test', () => { const mock = jest.fn(() => 'outside callback'); mock.withImplementation( () => 'inside callback', () => { mock(); // 'inside callback' }, ); mock(); // 'outside callback' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><code>mockFn.withImplementation</code> can be used regardless of whether or not the callback is asynchronous (returns a <code>thenable</code>). If the callback is asynchronous a promise will be returned. Awaiting the promise will await the callback and reset the implementation.<div class="language-js codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-js codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">test('async test', async () => { const mock = jest.fn(() => 'outside callback'); // We await this call since the callback is async await mock.withImplementation( () => 'inside callback', async () => { mock(); // 'inside callback' }, ); mock(); // 'outside callback' }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h2 class="anchor anchorWithStickyNavbar_JmGV" id="typescript-usage">TypeScript Usage<a href="#typescript-usage" class="hash-link" aria-label="Direct link to TypeScript Usage" title="Direct link to TypeScript Usage"></a></h2><div class="theme-admonition theme-admonition-info alert alert--info admonition_uH4V"><div class="admonitionHeading_P5_N"><svg viewBox="0 0 14 16"><path fill-rule="evenodd" d="M7 2.3c3.14 0 5.7 2.56 5.7 5.7s-2.56 5.7-5.7 5.7A5.71 5.71 0 0 1 1.3 8c0-3.14 2.56-5.7 5.7-5.7zM7 1C3.14 1 0 4.14 0 8s3.14 7 7 7 7-3.14 7-7-3.14-7-7-7zm1 3H6v5h2V4zm0 6H6v2h2v-2z"></path></svg>info</div><div class="admonitionContent_yySL">The TypeScript examples from this page will only work as documented if you explicitly import Jest APIs:<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {expect, jest, test} from '@jest/globals'; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Consult the <a href="/docs/29.1/getting-started#using-typescript">Getting Started</a> guide for details on how to setup Jest with TypeScript.</div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="jestfnimplementation"><code>jest.fn(implementation?)</code><a href="#jestfnimplementation" class="hash-link" aria-label="Direct link to jestfnimplementation" title="Direct link to jestfnimplementation"></a></h3>Correct mock typings will be inferred if implementation is passed to <a href="/docs/29.1/jest-object#jestfnimplementation"><code>jest.fn()</code></a>. There are many use cases where the implementation is omitted. To ensure type safety you may pass a generic type argument (also see the examples above for more reference):<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {expect, jest, test} from '@jest/globals'; import type add from './add'; import calculate from './calc'; test('calculate calls add', () => { // Create a new mock that can be used in place of `add`. const mockAdd = jest.fn<typeof add>(); // `.mockImplementation()` now can infer that `a` and `b` are `number` // and that the returned value is a `number`. mockAdd.mockImplementation((a, b) => { // Yes, this mock is still adding two numbers but imagine this // was a complex function we are mocking. return a + b; }); // `mockAdd` is properly typed and therefore accepted by anything // requiring `add`. calculate(mockAdd, 1, 2); expect(mockAdd).toHaveBeenCalledTimes(1); expect(mockAdd).toHaveBeenCalledWith(1, 2); }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="jestmockt"><code>jest.Mock<T></code><a href="#jestmockt" class="hash-link" aria-label="Direct link to jestmockt" title="Direct link to jestmockt"></a></h3>Constructs the type of a mock function, e.g. the return type of <code>jest.fn()</code>. It can be useful if you have to defined a recursive mock function:<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {jest} from '@jest/globals'; const sumRecursively: jest.Mock<(value: number) => number> = jest.fn(value => { if (value === 0) { return 0; } else { return value + fn(value - 1); } }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><h3 class="anchor anchorWithStickyNavbar_JmGV" id="jestmockedsource"><code>jest.Mocked<Source></code><a href="#jestmockedsource" class="hash-link" aria-label="Direct link to jestmockedsource" title="Direct link to jestmockedsource"></a></h3>The <code>jest.Mocked<Source></code> utility type returns the <code>Source</code> type wrapped with type definitions of Jest mock function.<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {expect, jest, test} from '@jest/globals'; import type {fetch} from 'node-fetch'; jest.mock('node-fetch'); let mockedFetch: jest.Mocked<typeof fetch>; afterEach(() => { mockedFetch.mockClear(); }); test('makes correct call', () => { mockedFetch = getMockedFetch(); // ... }); test('returns correct data', () => { mockedFetch = getMockedFetch(); // ... }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div>Types of classes, functions or objects can be passed as type argument to <code>jest.Mocked<Source></code>. If you prefer to constrain the input type, use: <code>jest.MockedClass<Source></code>, <code>jest.MockedFunction<Source></code> or <code>jest.MockedObject<Source></code>.<h3 class="anchor anchorWithStickyNavbar_JmGV" id="jestmockedsource-options"><code>jest.mocked(source, options?)</code><a href="#jestmockedsource-options" class="hash-link" aria-label="Direct link to jestmockedsource-options" title="Direct link to jestmockedsource-options"></a></h3>The <code>mocked()</code> helper method wraps types of the <code>source</code> object and its deep nested members with type definitions of Jest mock function. You can pass <code>{shallow: true}</code> as the <code>options</code> argument to disable the deeply mocked behavior.Returns the <code>source</code> object.<div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">song.ts</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">export const song = { one: { more: { time: (t: number) => { return t; }, }, }, }; </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div><div class="language-ts codeBlockContainer_mQmQ theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f6f6"><div class="codeBlockTitle_x_ju">song.test.ts</div><div class="codeBlockContent_D5yF"><pre tabindex="0" class="prism-code language-ts codeBlock_RMoD thin-scrollbar"><code class="codeBlockLines_AclH">import {expect, jest, test} from '@jest/globals'; import {song} from './song'; jest.mock('./song'); jest.spyOn(console, 'log'); const mockedSong = jest.mocked(song); // or through `jest.Mocked<Source>` // const mockedSong = song as jest.Mocked<typeof song>; test('deep method is typed correctly', () => { mockedSong.one.more.time.mockReturnValue(12); expect(mockedSong.one.more.time(10)).toBe(12); expect(mockedSong.one.more.time.mock.calls).toHaveLength(1); }); test('direct usage', () => { jest.mocked(console.log).mockImplementation(() => { return; }); console.log('one more time'); expect(jest.mocked(console.log).mock.calls).toHaveLength(1); }); </code></pre><div class="buttonGroup_aaMX"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><svg viewBox="0 0 24 24" class="copyButtonIcon_FoOz"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_L0B6"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></button></div></div></div></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/jestjs/jest/edit/main/website/versioned_docs/version-29.1/MockFunctionAPI.md" target="_blank" rel="noreferrer noopener" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_bHB7" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_pbO5">Last updated on <time datetime="2023-02-07T22:20:39.000Z">Feb 7, 2023</time> by Tom Mrazauskas</div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/docs/29.1/expect"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Expect</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/docs/29.1/jest-object"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">The Jest Object</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_XG6w thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#methods" class="table-of-contents__link toc-highlight">Methods</a></li><li><a href="#reference" class="table-of-contents__link toc-highlight">Reference</a><ul><li><a href="#mockfngetmockname" class="table-of-contents__link toc-highlight"><code>mockFn.getMockName()</code></a></li><li><a href="#mockfnmockcalls" class="table-of-contents__link toc-highlight"><code>mockFn.mock.calls</code></a></li><li><a href="#mockfnmockresults" class="table-of-contents__link toc-highlight"><code>mockFn.mock.results</code></a></li><li><a href="#mockfnmockinstances" class="table-of-contents__link toc-highlight"><code>mockFn.mock.instances</code></a></li><li><a href="#mockfnmockcontexts" class="table-of-contents__link toc-highlight"><code>mockFn.mock.contexts</code></a></li><li><a href="#mockfnmocklastcall" class="table-of-contents__link toc-highlight"><code>mockFn.mock.lastCall</code></a></li><li><a href="#mockfnmockclear" class="table-of-contents__link toc-highlight"><code>mockFn.mockClear()</code></a></li><li><a href="#mockfnmockreset" class="table-of-contents__link toc-highlight"><code>mockFn.mockReset()</code></a></li><li><a href="#mockfnmockrestore" class="table-of-contents__link toc-highlight"><code>mockFn.mockRestore()</code></a></li><li><a href="#mockfnmockimplementationfn" class="table-of-contents__link toc-highlight"><code>mockFn.mockImplementation(fn)</code></a></li><li><a href="#mockfnmockimplementationoncefn" class="table-of-contents__link toc-highlight"><code>mockFn.mockImplementationOnce(fn)</code></a></li><li><a href="#mockfnmocknamename" class="table-of-contents__link toc-highlight"><code>mockFn.mockName(name)</code></a></li><li><a href="#mockfnmockreturnthis" class="table-of-contents__link toc-highlight"><code>mockFn.mockReturnThis()</code></a></li><li><a href="#mockfnmockreturnvaluevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockReturnValue(value)</code></a></li><li><a href="#mockfnmockreturnvalueoncevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockReturnValueOnce(value)</code></a></li><li><a href="#mockfnmockresolvedvaluevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockResolvedValue(value)</code></a></li><li><a href="#mockfnmockresolvedvalueoncevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockResolvedValueOnce(value)</code></a></li><li><a href="#mockfnmockrejectedvaluevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockRejectedValue(value)</code></a></li><li><a href="#mockfnmockrejectedvalueoncevalue" class="table-of-contents__link toc-highlight"><code>mockFn.mockRejectedValueOnce(value)</code></a></li><li><a href="#mockfnwithimplementationfn-callback" class="table-of-contents__link toc-highlight"><code>mockFn.withImplementation(fn, callback)</code></a></li></ul></li><li><a href="#typescript-usage" class="table-of-contents__link toc-highlight">TypeScript Usage</a><ul><li><a href="#jestfnimplementation" class="table-of-contents__link toc-highlight"><code>jest.fn(implementation?)</code></a></li><li><a href="#jestmockt" class="table-of-contents__link toc-highlight"><code>jest.Mock<T></code></a></li><li><a href="#jestmockedsource" class="table-of-contents__link toc-highlight"><code>jest.Mocked<Source></code></a></li><li><a href="#jestmockedsource-options" class="table-of-contents__link toc-highlight"><code>jest.mocked(source, options?)</code></a></li></ul></li></ul></div></div></div></div></main></div></div><footer class="footer footer--dark"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">Docs</div><ul class="footer__items clean-list"><li class="footer__item"><a class="footer__link-item" href="/docs/getting-started">Getting Started</a></li><li class="footer__item"><a class="footer__link-item" href="/docs/snapshot-testing">Guides</a></li><li 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