WindowOrWorkerGlobalScope.fetch()

The fetch() method of the WindowOrWorkerGlobalScope mixin starts the process of fetching a resource from the network. This returns a promise that resolves to the Response object representing the response to your request.

WorkerOrGlobalScope is implemented by both Window and WorkerGlobalScope, which means that the fetch() method is available in pretty much any context in which you might want to fetch resources.

A fetch() promise rejects with a TypeError when a network error is encountered, although this usually means a permissions issue or similar. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true. An HTTP status of 404 does not constitute a network error.

The fetch() method is controlled by the connect-src directive of Content Security Policy rather than the directive of the resources it's retrieving.

Syntax

Promise<Response> fetch(input[, init]);

Parameters

input

This defines the resource that you wish to fetch. This can either be:

• A USVString containing the direct URL of the resource you want to fetch. Some browsers accept blob: and data: as schemes.
• A Request object.

init Optional

An options object containing any custom settings that you want to apply to the request. The possible options are:

• method: The request method, e.g., GET, POST.
• headers: Any headers you want to add to your request, contained within a Headers object or an object literal with ByteString values.
• body: Any body that you want to add to your request: this can be a Blob, BufferSource, FormData, URLSearchParams, or USVString object. Note that a request using the GET or HEAD method cannot have a body.
• mode: The mode you want to use for the request, e.g., cors, no-cors, or same-origin.
• credentials: The request credentials you want to use for the request: omit, same-origin, or include. To automatically send cookies for the current domain, this option must be provided. Starting with Chrome 50, this property also takes a FederatedCredential instance or a PasswordCredential instance.
• cache: The cache mode you want to use for the request: default, no-store, reload, no-cache, force-cache, or only-if-cached.
• redirect: The redirect mode to use: follow (automatically follow redirects), error (abort with an error if a redirect occurs), or manual (handle redirects manually). In Chrome the default was follow before Chrome 47 and manual starting with Chrome 47.
• referrer: A USVString specifying no-referrer, client, or a URL. The default is client.
• referrerPolicy: Specifies the value of the referer HTTP header. May be one of no-referrer, no-referrer-when-downgrade, origin, origin-when-cross-origin, unsafe-url.
• integrity: Contains the subresource integrity value of the request (e.g., sha256-BpfBw7ivV8q2jLiT13fxDYAe2tJllusRSZ273h2nFSE=).
• signal: A FetchSignal object instance; allows you to communicate with a fetch request and control it via a FetchController.
• observe: An ObserverCallback object — this object's sole purpose is to provide a callback function that runs when the fetch request runs. This returns a FetchObserver object that can be used to retrieve information concerning the status of a fetch request.

Return value

A Promise that resolves to a Response object.

Exceptions

Example

In our Fetch Request example (see Fetch Request live) we create a new Request object using the relevant constructor, then fetch it using a fetch() call. Since we are fetching an image, we run Body.blob() on the response to give it the proper MIME type so it will be handled properly, then create an Object URL of it and display it in an <img> element.

var myImage = document.querySelector('img');

var myRequest = new Request('flowers.jpg');

fetch(myRequest).then(function(response) {
  return response.blob();
}).then(function(response) {
  var objectURL = URL.createObjectURL(response);
  myImage.src = objectURL;
});

In our Fetch with init then Request example (see Fetch Request init live) we do the same thing except that we pass in an init object when we invoke fetch():

var myImage = document.querySelector('img');

var myHeaders = new Headers();
myHeaders.append('Content-Type', 'image/jpeg');

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default' };

var myRequest = new Request('flowers.jpg');

fetch(myRequest,myInit).then(function(response) {
  ... 
});

Note that you could also pass the init object in with the Request constructor to get the same effect, e.g.:

var myRequest = new Request('flowers.jpg', myInit);

You can also use an object literal as headers in init.

var myInit = { method: 'GET',
               headers: {
                   'Content-Type': 'image/jpeg'
               },
               mode: 'cors',
               cache: 'default' };

var myRequest = new Request('flowers.jpg', myInit);

Specifications

Browser compatibility

[1] API is implemented behind a preference.

[2] fetch() now defined on WindowOrWorkerGlobalScope mixin.

[3] Hidden behind a preference in 55+ Nightly. In about:config, you need to create two new boolean prefs — dom.fetchObserver.enabled and dom.fetchController.enabled — and set the values of both to true.

See also

• Fetch API
• ServiceWorker API
• HTTP access control (CORS)
• HTTP