Web/API/Fetchevent/respondWith-2

The respondWith() method of FetchEvent prevents the browser's default fetch handling, and allows you to provide a promise for a Response yourself.

In most cases you can provide any response that the receiver understands. For example, if an <img> initiates the request, the response body needs to be image data. For security reasons, there are a few global rules:

You can only return Response objects of type "opaque" if the fetchEvent.request object's mode is "no-cors". This prevents the leaking of private data.
You can only return Response objects of type "opaqueredirect" if the fetchEvent.request object's mode is "manual".
You cannot return Response objects of type "cors" if the fetchEvent.request object's mode is "same-origin".

Specifying the final URL of a resource

From Firefox 59 onwards, when a service worker provides a Response to FetchEvent.respondWith(), the Response.url value will be propagated to the intercepted network request as the final resolved URL. If the Response.url value is the empty string, then the FetchEvent.request.url is used as the final URL.

In the past the FetchEvent.request.url was used as the final URL in all cases. The provided Response.url was effectively ignored.

This means, for example, if a service worker intercepts a stylesheet or worker script, then the provided Response.url will be used to resolve any relative @import or importScripts() subresource loads (bug 1222008).

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:

If a fetch() is intercepted, then you can observe the final URL on the result's Response.url.
If a worker script is intercepted, then the final URL is used to set self.location and used as the base URL for relative URLs in the worker script.
If a stylesheet is intercepted, then the final URL is used as the base URL for resolving relative @import loads.

Note that navigation requests for Windows and iframes do NOT use the final URL. The way the HTML specification handles redirects for navigations ends up using the request URL for the resulting Window.location. This means sites can still provide an "alternate" view of a web page when offline without changing the user-visible URL.

Syntax

fetchEvent.respondWith(
  // Promise that resolves to a Response.
);

Parameters

A Response or a Promise that resolves to a Response. Otherwise, a network error is returned to Fetch.

Return value

undefined.

Exceptions

Exception	Notes
`NetworkError`	A network error is triggered on certain combinations of `FetchEvent.request.mode` and `Response.type` values, as hinted at in the "global rules" listed above.
`InvalidStateError`	The event has not been dispatched or `respondWith()` has already been invoked.

Examples

This fetch event tries to return a response from the cache API, falling back to the network otherwise.

addEventListener('fetch', event => {
  // Prevent the default, and handle the request ourselves.
  event.respondWith(async function() {
    // 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);
  }());
});

Note: caches.match() is a convenience method. Equivalent functionality is to call cache.match() on each cache (in the order returned by caches.keys()) until a Response is returned.

Specifications

Specification	Status	Comment
Service WorkersThe definition of 'respondWith()' in that specification.	Working Draft	Initial definition.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`respondWith` Experimental'	Chrome Full support 42 Notes' Full support 42 Notes' Notes' NetworkError thrown if request mode is same-origin and response type is cors (see bug 1222008). This is being worked on - see https://www.chromestatus.com/feature/5694278818856960.	Edge Full support ≤79 Notes' Full support ≤79 Notes' Notes' NetworkError thrown if request mode is same-origin and response type is cors (see bug 1222008). This is being worked on - see https://www.chromestatus.com/feature/5694278818856960.	Firefox Full support 59 Notes' Full support 59 Notes' Notes' NetworkError thrown if request mode is same-origin and response type is cors (see bug 1222008). Full support 44 Notes' Notes' Extended Support Releases (ESR) before Firefox 78 ESR do not support service workers and the Push API.	IE No support No	Opera Full support 29	Safari No support No	WebView Android Full support 42 Notes' Full support 42 Notes' Notes' NetworkError thrown if request mode is same-origin and response type is cors (see bug 1222008). This is being worked on - see https://www.chromestatus.com/feature/5694278818856960.	Chrome Android Full support 42 Notes' Full support 42 Notes' Notes' NetworkError thrown if request mode is same-origin and response type is cors (see bug 1222008). This is being worked on - see https://www.chromestatus.com/feature/5694278818856960.	Firefox Android ?	Opera Android Full support 29	Safari iOS No support No	Samsung Internet Android Full support 4.0
Change in behavior when specifying the final URL of a resource. Experimental'	Chrome No support No	Edge No support No	Firefox Full support 59	IE No support No	Opera ?	Safari No support No	WebView Android No support No	Chrome Android No support No	Firefox Android ?	Opera Android ?	Safari iOS No support No	Samsung Internet Android No support No

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
Experimental. Expect behavior to change in the future.': Experimental. Expect behavior to change in the future.
See implementation notes.': See implementation notes.