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Check cross-browser support before using."> <span class="visually-hidden">Non-standard</span> </abbr><abbr class="icon icon-deprecated" title="Deprecated. Not for use in new websites."> <span class="visually-hidden">Deprecated</span> </abbr></li><li><a href="/en-US/docs/Web/API/FetchEvent/preloadResponse"><code>preloadResponse</code></a></li><li><a href="/en-US/docs/Web/API/FetchEvent/replacesClientId"><code>replacesClientId</code></a></li><li><a href="/en-US/docs/Web/API/FetchEvent/request"><code>request</code></a></li><li><a href="/en-US/docs/Web/API/FetchEvent/resultingClientId"><code>resultingClientId</code></a></li></ol></details></li><li class="toggle"><details open=""><summary>Instance methods</summary><ol><li><em><code>respondWith()</code> </em></li></ol></details></li><li class="toggle"><details open=""><summary>Inheritance</summary><ol><li><a href="/en-US/docs/Web/API/ExtendableEvent"><code>ExtendableEvent</code></a></li><li><a href="/en-US/docs/Web/API/Event"><code>Event</code></a></li></ol></details></li><li class="toggle"><details open=""><summary>Related pages for Service Workers API</summary><ol><li><a href="/en-US/docs/Web/API/Cache"><code>Cache</code></a></li><li><a href="/en-US/docs/Web/API/CacheStorage"><code>CacheStorage</code></a></li><li><a href="/en-US/docs/Web/API/Client"><code>Client</code></a></li><li><a href="/en-US/docs/Web/API/Clients"><code>Clients</code></a></li><li><a href="/en-US/docs/Web/API/ExtendableEvent"><code>ExtendableEvent</code></a></li><li><a href="/en-US/docs/Web/API/ExtendableMessageEvent"><code>ExtendableMessageEvent</code></a></li><li><a href="/en-US/docs/Web/API/InstallEvent"><code>InstallEvent</code></a></li><li><a href="/en-US/docs/Web/API/NavigationPreloadManager"><code>NavigationPreloadManager</code></a></li><li><a href="/en-US/docs/Web/API/Navigator/serviceWorker"><code>Navigator.serviceWorker</code></a></li><li><a href="/en-US/docs/Web/API/ServiceWorker"><code>ServiceWorker</code></a></li><li><a href="/en-US/docs/Web/API/ServiceWorkerContainer"><code>ServiceWorkerContainer</code></a></li><li><a href="/en-US/docs/Web/API/ServiceWorkerGlobalScope"><code>ServiceWorkerGlobalScope</code></a></li><li><a href="/en-US/docs/Web/API/ServiceWorkerRegistration"><code>ServiceWorkerRegistration</code></a></li><li><a href="/en-US/docs/Web/API/Window/caches"><code>Window.caches</code></a></li><li><a href="/en-US/docs/Web/API/WindowClient"><code>WindowClient</code></a></li><li><a href="/en-US/docs/Web/API/WorkerGlobalScope/caches"><code>WorkerGlobalScope.caches</code></a></li><li><a href="/en-US/docs/Web/API/WorkerNavigator/serviceWorker"><code>WorkerNavigator.serviceWorker</code></a></li></ol></details></li></ol></div></div><section class="place side"></section></nav></aside><div class="toc-container"><aside class="toc"><nav><div class="document-toc-container"><section class="document-toc"><header><h2 class="document-toc-heading">In this article</h2></header><ul class="document-toc-list"><li class="document-toc-item "><a class="document-toc-link" href="#syntax">Syntax</a></li><li class="document-toc-item "><a class="document-toc-link" href="#examples">Examples</a></li><li class="document-toc-item "><a class="document-toc-link" href="#specifications">Specifications</a></li><li class="document-toc-item "><a class="document-toc-link" href="#browser_compatibility">Browser compatibility</a></li><li class="document-toc-item "><a class="document-toc-link" href="#see_also">See also</a></li></ul></section></div></nav></aside><section class="place side"></section></div></div><main id="content" class="main-content "><article class="main-page-content" lang="en-US"><header><h1>FetchEvent: respondWith() method</h1></header><div class="section-content"><div class="notecard note"> <p><strong>Note:</strong> This feature is only available in <a href="/en-US/docs/Web/API/Service_Worker_API">Service Workers</a>.</p> </div> <p> The <strong><code>respondWith()</code></strong> method of <a href="/en-US/docs/Web/API/FetchEvent"><code>FetchEvent</code></a> prevents the browser's default fetch handling, and allows you to provide a promise for a <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> yourself. </p> <p> In most cases you can provide any response that the receiver understands. For example, if an <a href="/en-US/docs/Web/HTML/Element/img"><code><img></code></a> initiates the request, the response body needs to be image data. For security reasons, there are a few global rules: </p> <ul> <li> You can only return <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> objects of <a href="/en-US/docs/Web/API/Response/type" title="type"><code>type</code></a> <code>"opaque"</code> if the <a href="/en-US/docs/Web/API/FetchEvent/request"><code>fetchEvent.request</code></a> object's <a href="/en-US/docs/Web/API/Request/mode" title="mode"><code>mode</code></a> is <code>"no-cors"</code>. This prevents the leaking of private data. </li> <li> You can only return <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> objects of <a href="/en-US/docs/Web/API/Response/type" title="type"><code>type</code></a> <code>"opaqueredirect"</code> if the <a href="/en-US/docs/Web/API/FetchEvent/request"><code>fetchEvent.request</code></a> object's <a href="/en-US/docs/Web/API/Request/mode" title="mode"><code>mode</code></a> is <code>"manual"</code>. </li> <li> You cannot return <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> objects of <a href="/en-US/docs/Web/API/Response/type" title="type"><code>type</code></a> <code>"cors"</code> if the <a href="/en-US/docs/Web/API/FetchEvent/request"><code>fetchEvent.request</code></a> object's <a href="/en-US/docs/Web/API/Request/mode" title="mode"><code>mode</code></a> is <code>"same-origin"</code>. </li> </ul></div><section aria-labelledby="specifying_the_final_url_of_a_resource"><h3 id="specifying_the_final_url_of_a_resource"><a href="#specifying_the_final_url_of_a_resource">Specifying the final URL of a resource</a></h3><div class="section-content"><p> From Firefox 59 onwards, when a service worker provides a <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> to <code>FetchEvent.respondWith()</code>, the <a href="/en-US/docs/Web/API/Response/url"><code>Response.url</code></a> value will be propagated to the intercepted network request as the final resolved URL. If the <a href="/en-US/docs/Web/API/Response/url"><code>Response.url</code></a> value is the empty string, then the <a href="/en-US/docs/Web/API/Request/url" title="FetchEvent.request.url"><code>FetchEvent.request.url</code></a> is used as the final URL. </p> <p> In the past the <a href="/en-US/docs/Web/API/Request/url" title="FetchEvent.request.url"><code>FetchEvent.request.url</code></a> was used as the final URL in all cases. The provided <a href="/en-US/docs/Web/API/Response/url"><code>Response.url</code></a> was effectively ignored. </p> <p> This means, for example, if a service worker intercepts a stylesheet or worker script, then the provided <a href="/en-US/docs/Web/API/Response/url"><code>Response.url</code></a> will be used to resolve any relative <a href="/en-US/docs/Web/CSS/@import"><code>@import</code></a> or <a href="/en-US/docs/Web/API/WorkerGlobalScope/importScripts" title="importScripts()"><code>importScripts()</code></a> subresource loads (<a href="https://bugzil.la/1222008" class="external" target="_blank">Firefox bug 1222008</a>). </p> <p> For most types of network request this change has no impact because you can't observe the final URL. There are a few, though, where it does matter: </p> <ul> <li> If a <a href="/en-US/docs/Web/API/Window/fetch" title="fetch()"><code>fetch()</code></a> is intercepted, then you can observe the final URL on the result's <a href="/en-US/docs/Web/API/Response/url"><code>Response.url</code></a>. </li> <li> If a <a href="/en-US/docs/Web/API/Web_Workers_API">worker</a> script is intercepted, then the final URL is used to set <a href="/en-US/docs/Web/API/WorkerGlobalScope/location"><code>self.location</code></a> and used as the base URL for relative URLs in the worker script. </li> <li> If a stylesheet is intercepted, then the final URL is used as the base URL for resolving relative <a href="/en-US/docs/Web/CSS/@import"><code>@import</code></a> loads. </li> </ul> <p> Note that navigation requests for <a href="/en-US/docs/Web/API/Window" title="Windows"><code>Windows</code></a> and <a href="/en-US/docs/Web/API/HTMLIFrameElement" title="iframes"><code>iframes</code></a> do NOT use the final URL. The way the HTML specification handles redirects for navigations ends up using the request URL for the resulting <a href="/en-US/docs/Web/API/Window/location"><code>Window.location</code></a>. This means sites can still provide an "alternate" view of a web page when offline without changing the user-visible URL. </p></div></section><section aria-labelledby="syntax"><h2 id="syntax"><a href="#syntax">Syntax</a></h2><div class="section-content"><div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>respondWith(response) </code></pre></div></div></section><section aria-labelledby="parameters"><h3 id="parameters"><a href="#parameters">Parameters</a></h3><div class="section-content"><dl> <dt id="response"><a href="#response"><code>response</code></a></dt> <dd> <p> A <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> or a <a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise"><code>Promise</code></a> that resolves to a <code>Response</code>. Otherwise, a network error is returned to Fetch. </p> </dd> </dl></div></section><section aria-labelledby="return_value"><h3 id="return_value"><a href="#return_value">Return value</a></h3><div class="section-content"><p>None (<a href="/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined"><code>undefined</code></a>).</p></div></section><section aria-labelledby="exceptions"><h3 id="exceptions"><a href="#exceptions">Exceptions</a></h3><div class="section-content"><dl> <dt id="networkerror"><a href="#networkerror"><code>NetworkError</code></a> <a href="/en-US/docs/Web/API/DOMException"><code>DOMException</code></a></dt> <dd> <p> Returned if a network error is triggered on certain combinations of <a href="/en-US/docs/Web/API/Request/mode" title="FetchEvent.request.mode"><code>FetchEvent.request.mode</code></a> and <a href="/en-US/docs/Web/API/Response/type"><code>Response.type</code></a> values, as hinted at in the "global rules" listed above. </p> </dd> <dt id="invalidstateerror"><a href="#invalidstateerror"><code>InvalidStateError</code></a> <a href="/en-US/docs/Web/API/DOMException"><code>DOMException</code></a></dt> <dd> <p> Returned if the event has not been dispatched or <code>respondWith()</code> has already been invoked. </p> </dd> </dl></div></section><section aria-labelledby="examples"><h2 id="examples"><a href="#examples">Examples</a></h2><div class="section-content"><p> This fetch event tries to return a response from the cache API, falling back to the network otherwise. </p> <div class="code-example"><div class="example-header"><span class="language-name">js</span></div><pre class="brush: js notranslate"><code>addEventListener("fetch", (event) => { // Prevent the default, and handle the request ourselves. event.respondWith( (async () => { // Try to get the response from a cache. const cachedResponse = await caches.match(event.request); // Return it if we found one. if (cachedResponse) return cachedResponse; // If we didn't find a match in the cache, use the network. return fetch(event.request); })(), ); }); </code></pre></div> <div class="notecard note"> <p> <strong>Note:</strong> <a href="/en-US/docs/Web/API/CacheStorage/match" title="caches.match()"><code>caches.match()</code></a> is a convenience method. Equivalent functionality is to call <a href="/en-US/docs/Web/API/Cache/match"><code>cache.match()</code></a> on each cache (in the order returned by <a href="/en-US/docs/Web/API/CacheStorage/keys" title="caches.keys()"><code>caches.keys()</code></a>) until a <a href="/en-US/docs/Web/API/Response"><code>Response</code></a> is returned. </p> </div></div></section><h2 id="specifications"><a href="#specifications">Specifications</a></h2><table class="standard-table"><thead><tr><th scope="col">Specification</th></tr></thead><tbody><tr><td><a href="https://w3c.github.io/ServiceWorker/#fetch-event-respondwith">Service Workers <br/><small># fetch-event-respondwith</small></a></td></tr></tbody></table><h2 id="browser_compatibility"><a href="#browser_compatibility">Browser compatibility</a></h2><p>BCD tables only load in the browser<noscript> with JavaScript enabled. Enable JavaScript to view data.</noscript></p><section aria-labelledby="see_also"><h2 id="see_also"><a href="#see_also">See also</a></h2><div class="section-content"><ul> <li><a href="/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers">Using Service Workers</a></li> <li><a href="/en-US/docs/Web/API/Fetch_API">Fetch API</a></li> </ul></div></section></article><aside class="article-footer"><div class="article-footer-inner"><div class="svg-container"><svg xmlns="http://www.w3.org/2000/svg" width="162" height="162" viewBox="0 0 162 162" fill="none" role="none"><mask id="b" fill="#fff"><path d="M97.203 47.04c8.113-7.886 18.004-13.871 28.906-17.492a78 78 0 0 1 33.969-3.39c11.443 1.39 22.401 5.295 32.024 11.411s17.656 14.28 23.476 23.86c5.819 9.579 9.269 20.318 10.083 31.385a69.85 69.85 0 0 1-5.387 32.44c-4.358 10.272-11.115 19.443-19.747 26.801-8.632 7.359-18.908 12.709-30.034 15.637l-6.17-21.698c7.666-2.017 14.746-5.703 20.694-10.773 5.948-5.071 10.603-11.389 13.606-18.467a48.14 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Content available under <a href="/en-US/docs/MDN/Writing_guidelines/Attrib_copyright_license">a Creative Commons license</a>.</p></div></div></footer></div><script type="application/json" id="hydration">{"url":"/en-US/docs/Web/API/FetchEvent/respondWith","doc":{"isMarkdown":true,"isTranslated":false,"isActive":true,"flaws":{},"title":"FetchEvent: respondWith() method","mdn_url":"/en-US/docs/Web/API/FetchEvent/respondWith","locale":"en-US","native":"English (US)","browserCompat":["api.FetchEvent.respondWith"],"sidebarHTML":"<ol><li class=\"section\"><a href=\"/en-US/docs/Web/API/Service_Worker_API\">Service Worker API</a></li><li class=\"section\"><a href=\"/en-US/docs/Web/API/FetchEvent\"><code>FetchEvent</code></a></li><li class=\"toggle\"><details open=\"\"><summary>Constructor</summary><ol><li><a href=\"/en-US/docs/Web/API/FetchEvent/FetchEvent\"><code>FetchEvent()</code></a></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary>Instance properties</summary><ol><li><a href=\"/en-US/docs/Web/API/FetchEvent/clientId\"><code>clientId</code></a></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/handled\"><code>handled</code></a></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/isReload\"><code>isReload</code></a><abbr class=\"icon icon-nonstandard\" title=\"Non-standard. Check cross-browser support before using.\">\n <span class=\"visually-hidden\">Non-standard</span>\n</abbr><abbr class=\"icon icon-deprecated\" title=\"Deprecated. Not for use in new websites.\">\n <span class=\"visually-hidden\">Deprecated</span>\n</abbr></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/preloadResponse\"><code>preloadResponse</code></a></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/replacesClientId\"><code>replacesClientId</code></a></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/request\"><code>request</code></a></li><li><a href=\"/en-US/docs/Web/API/FetchEvent/resultingClientId\"><code>resultingClientId</code></a></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary>Instance methods</summary><ol><li><em><code>respondWith()</code> </em></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary>Inheritance</summary><ol><li><a href=\"/en-US/docs/Web/API/ExtendableEvent\"><code>ExtendableEvent</code></a></li><li><a href=\"/en-US/docs/Web/API/Event\"><code>Event</code></a></li></ol></details></li><li class=\"toggle\"><details open=\"\"><summary>Related pages for Service Workers API</summary><ol><li><a href=\"/en-US/docs/Web/API/Cache\"><code>Cache</code></a></li><li><a href=\"/en-US/docs/Web/API/CacheStorage\"><code>CacheStorage</code></a></li><li><a href=\"/en-US/docs/Web/API/Client\"><code>Client</code></a></li><li><a href=\"/en-US/docs/Web/API/Clients\"><code>Clients</code></a></li><li><a href=\"/en-US/docs/Web/API/ExtendableEvent\"><code>ExtendableEvent</code></a></li><li><a href=\"/en-US/docs/Web/API/ExtendableMessageEvent\"><code>ExtendableMessageEvent</code></a></li><li><a href=\"/en-US/docs/Web/API/InstallEvent\"><code>InstallEvent</code></a></li><li><a href=\"/en-US/docs/Web/API/NavigationPreloadManager\"><code>NavigationPreloadManager</code></a></li><li><a href=\"/en-US/docs/Web/API/Navigator/serviceWorker\"><code>Navigator.serviceWorker</code></a></li><li><a href=\"/en-US/docs/Web/API/ServiceWorker\"><code>ServiceWorker</code></a></li><li><a href=\"/en-US/docs/Web/API/ServiceWorkerContainer\"><code>ServiceWorkerContainer</code></a></li><li><a href=\"/en-US/docs/Web/API/ServiceWorkerGlobalScope\"><code>ServiceWorkerGlobalScope</code></a></li><li><a href=\"/en-US/docs/Web/API/ServiceWorkerRegistration\"><code>ServiceWorkerRegistration</code></a></li><li><a href=\"/en-US/docs/Web/API/Window/caches\"><code>Window.caches</code></a></li><li><a href=\"/en-US/docs/Web/API/WindowClient\"><code>WindowClient</code></a></li><li><a href=\"/en-US/docs/Web/API/WorkerGlobalScope/caches\"><code>WorkerGlobalScope.caches</code></a></li><li><a href=\"/en-US/docs/Web/API/WorkerNavigator/serviceWorker\"><code>WorkerNavigator.serviceWorker</code></a></li></ol></details></li></ol>","sidebarMacro":"APIRef","body":[{"type":"prose","value":{"id":null,"title":null,"isH3":false,"content":"<div class=\"notecard note\">\n <p><strong>Note:</strong> This feature is only available in <a href=\"/en-US/docs/Web/API/Service_Worker_API\">Service Workers</a>.</p>\n</div>\n<p>\n The <strong><code>respondWith()</code></strong> method of\n <a href=\"/en-US/docs/Web/API/FetchEvent\"><code>FetchEvent</code></a> prevents the browser's default fetch handling, and\n allows you to provide a promise for a <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> yourself.\n</p>\n<p>\n In most cases you can provide any response that the receiver understands. For example,\n if an <a href=\"/en-US/docs/Web/HTML/Element/img\"><code><img></code></a> initiates the request, the response body needs to be\n image data. For security reasons, there are a few global rules:\n</p>\n<ul>\n <li>\n You can only return <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> objects of <a href=\"/en-US/docs/Web/API/Response/type\" title=\"type\"><code>type</code></a>\n <code>\"opaque\"</code> if the <a href=\"/en-US/docs/Web/API/FetchEvent/request\"><code>fetchEvent.request</code></a> object's\n <a href=\"/en-US/docs/Web/API/Request/mode\" title=\"mode\"><code>mode</code></a> is <code>\"no-cors\"</code>. This prevents the\n leaking of private data.\n </li>\n <li>\n You can only return <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> objects of <a href=\"/en-US/docs/Web/API/Response/type\" title=\"type\"><code>type</code></a>\n <code>\"opaqueredirect\"</code> if the <a href=\"/en-US/docs/Web/API/FetchEvent/request\"><code>fetchEvent.request</code></a>\n object's <a href=\"/en-US/docs/Web/API/Request/mode\" title=\"mode\"><code>mode</code></a> is <code>\"manual\"</code>.\n </li>\n <li>\n You cannot return <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> objects of <a href=\"/en-US/docs/Web/API/Response/type\" title=\"type\"><code>type</code></a>\n <code>\"cors\"</code> if the <a href=\"/en-US/docs/Web/API/FetchEvent/request\"><code>fetchEvent.request</code></a> object's\n <a href=\"/en-US/docs/Web/API/Request/mode\" title=\"mode\"><code>mode</code></a> is <code>\"same-origin\"</code>.\n </li>\n</ul>"}},{"type":"prose","value":{"id":"specifying_the_final_url_of_a_resource","title":"Specifying the final URL of a resource","isH3":true,"content":"<p>\n From Firefox 59 onwards, when a service worker provides a <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> to\n <code>FetchEvent.respondWith()</code>, the <a href=\"/en-US/docs/Web/API/Response/url\"><code>Response.url</code></a> value will be\n propagated to the intercepted network request as the final resolved URL. If the\n <a href=\"/en-US/docs/Web/API/Response/url\"><code>Response.url</code></a> value is the empty string, then the\n <a href=\"/en-US/docs/Web/API/Request/url\" title=\"FetchEvent.request.url\"><code>FetchEvent.request.url</code></a> is used as the final URL.\n</p>\n<p>\n In the past the <a href=\"/en-US/docs/Web/API/Request/url\" title=\"FetchEvent.request.url\"><code>FetchEvent.request.url</code></a> was used as the\n final URL in all cases. The provided <a href=\"/en-US/docs/Web/API/Response/url\"><code>Response.url</code></a> was effectively\n ignored.\n</p>\n<p>\n This means, for example, if a service worker intercepts a stylesheet or worker script,\n then the provided <a href=\"/en-US/docs/Web/API/Response/url\"><code>Response.url</code></a> will be used to resolve any relative\n <a href=\"/en-US/docs/Web/CSS/@import\"><code>@import</code></a> or\n <a href=\"/en-US/docs/Web/API/WorkerGlobalScope/importScripts\" title=\"importScripts()\"><code>importScripts()</code></a> subresource loads\n (<a href=\"https://bugzil.la/1222008\" class=\"external\" target=\"_blank\">Firefox bug 1222008</a>).\n</p>\n<p>\n For most types of network request this change has no impact because you can't observe\n the final URL. There are a few, though, where it does matter:\n</p>\n<ul>\n <li>\n If a <a href=\"/en-US/docs/Web/API/Window/fetch\" title=\"fetch()\"><code>fetch()</code></a> is intercepted,\n then you can observe the final URL on the result's <a href=\"/en-US/docs/Web/API/Response/url\"><code>Response.url</code></a>.\n </li>\n <li>\n If a <a href=\"/en-US/docs/Web/API/Web_Workers_API\">worker</a> script is\n intercepted, then the final URL is used to set\n <a href=\"/en-US/docs/Web/API/WorkerGlobalScope/location\"><code>self.location</code></a>\n and used as the base URL for relative URLs in the worker script.\n </li>\n <li>\n If a stylesheet is intercepted, then the final URL is used as the base URL for\n resolving relative <a href=\"/en-US/docs/Web/CSS/@import\"><code>@import</code></a> loads.\n </li>\n</ul>\n<p>\n Note that navigation requests for <a href=\"/en-US/docs/Web/API/Window\" title=\"Windows\"><code>Windows</code></a> and\n <a href=\"/en-US/docs/Web/API/HTMLIFrameElement\" title=\"iframes\"><code>iframes</code></a> do NOT use the final URL. The way the HTML\n specification handles redirects for navigations ends up using the request URL for the\n resulting <a href=\"/en-US/docs/Web/API/Window/location\"><code>Window.location</code></a>. This means sites can still provide an\n \"alternate\" view of a web page when offline without changing the user-visible URL.\n</p>"}},{"type":"prose","value":{"id":"syntax","title":"Syntax","isH3":false,"content":"<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>respondWith(response)\n</code></pre></div>"}},{"type":"prose","value":{"id":"parameters","title":"Parameters","isH3":true,"content":"<dl>\n <dt id=\"response\"><a href=\"#response\"><code>response</code></a></dt>\n <dd>\n <p>\n A <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> or a <a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise\"><code>Promise</code></a> that resolves to a\n <code>Response</code>. Otherwise, a network error is returned to Fetch.\n </p>\n </dd>\n</dl>"}},{"type":"prose","value":{"id":"return_value","title":"Return value","isH3":true,"content":"<p>None (<a href=\"/en-US/docs/Web/JavaScript/Reference/Global_Objects/undefined\"><code>undefined</code></a>).</p>"}},{"type":"prose","value":{"id":"exceptions","title":"Exceptions","isH3":true,"content":"<dl>\n <dt id=\"networkerror\"><a href=\"#networkerror\"><code>NetworkError</code></a> <a href=\"/en-US/docs/Web/API/DOMException\"><code>DOMException</code></a></dt>\n <dd>\n <p>\n Returned if a network error is triggered on certain combinations of\n <a href=\"/en-US/docs/Web/API/Request/mode\" title=\"FetchEvent.request.mode\"><code>FetchEvent.request.mode</code></a> and\n <a href=\"/en-US/docs/Web/API/Response/type\"><code>Response.type</code></a> values, as hinted at in the \"global rules\"\n listed above.\n </p>\n </dd>\n <dt id=\"invalidstateerror\"><a href=\"#invalidstateerror\"><code>InvalidStateError</code></a> <a href=\"/en-US/docs/Web/API/DOMException\"><code>DOMException</code></a></dt>\n <dd>\n <p>\n Returned if the event has not been dispatched or <code>respondWith()</code> has\n already been invoked.\n </p>\n </dd>\n</dl>"}},{"type":"prose","value":{"id":"examples","title":"Examples","isH3":false,"content":"<p>\n This fetch event tries to return a response from the cache API, falling back to the\n network otherwise.\n</p>\n<div class=\"code-example\"><div class=\"example-header\"><span class=\"language-name\">js</span></div><pre class=\"brush: js notranslate\"><code>addEventListener(\"fetch\", (event) => {\n // Prevent the default, and handle the request ourselves.\n event.respondWith(\n (async () => {\n // Try to get the response from a cache.\n const cachedResponse = await caches.match(event.request);\n // Return it if we found one.\n if (cachedResponse) return cachedResponse;\n // If we didn't find a match in the cache, use the network.\n return fetch(event.request);\n })(),\n );\n});\n</code></pre></div>\n<div class=\"notecard note\">\n <p>\n <strong>Note:</strong> <a href=\"/en-US/docs/Web/API/CacheStorage/match\" title=\"caches.match()\"><code>caches.match()</code></a> is a\n convenience method. Equivalent functionality is to call\n <a href=\"/en-US/docs/Web/API/Cache/match\"><code>cache.match()</code></a> on each cache (in the order returned by\n <a href=\"/en-US/docs/Web/API/CacheStorage/keys\" title=\"caches.keys()\"><code>caches.keys()</code></a>) until a\n <a href=\"/en-US/docs/Web/API/Response\"><code>Response</code></a> is returned.\n </p>\n</div>"}},{"type":"specifications","value":{"title":"Specifications","id":"specifications","isH3":false,"specifications":[{"bcdSpecificationURL":"https://w3c.github.io/ServiceWorker/#fetch-event-respondwith","title":"Service Workers"}],"query":"api.FetchEvent.respondWith"}},{"type":"browser_compatibility","value":{"title":"Browser compatibility","id":"browser_compatibility","isH3":false,"query":"api.FetchEvent.respondWith"}},{"type":"prose","value":{"id":"see_also","title":"See also","isH3":false,"content":"<ul>\n <li><a href=\"/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers\">Using Service Workers</a></li>\n <li><a href=\"/en-US/docs/Web/API/Fetch_API\">Fetch API</a></li>\n</ul>"}}],"toc":[{"text":"Syntax","id":"syntax"},{"text":"Examples","id":"examples"},{"text":"Specifications","id":"specifications"},{"text":"Browser compatibility","id":"browser_compatibility"},{"text":"See also","id":"see_also"}],"summary":"The respondWith() method of\n FetchEvent prevents the browser's default fetch handling, and\n allows you to provide a promise for a Response yourself.","popularity":0,"modified":"2024-09-24T13:57:44.000Z","other_translations":[{"locale":"de","title":"FetchEvent: respondWith() Methode","native":"Deutsch"},{"locale":"ja","title":"FetchEvent.respondWith()","native":"日本語"},{"locale":"zh-CN","title":"FetchEvent.respondWith()","native":"中文 (简体)"}],"pageType":"web-api-instance-method","source":{"folder":"en-us/web/api/fetchevent/respondwith","github_url":"https://github.com/mdn/content/blob/main/files/en-us/web/api/fetchevent/respondwith/index.md","last_commit_url":"https://github.com/mdn/content/commit/dcbb1d99185118360cc84b3a0e935e77fe0a03e3","filename":"index.md"},"short_title":"respondWith()","parents":[{"uri":"/en-US/docs/Web","title":"References"},{"uri":"/en-US/docs/Web/API","title":"Web APIs"},{"uri":"/en-US/docs/Web/API/FetchEvent","title":"FetchEvent"},{"uri":"/en-US/docs/Web/API/FetchEvent/respondWith","title":"respondWith()"}],"pageTitle":"FetchEvent: respondWith() method - Web APIs | MDN","noIndexing":false}}</script></body></html>