WindowOrWorkerGlobalScope.fetch()

The fetch() method of the WindowOrWorkerGlobalScope mixin starts the process of fetching a resource from the network, returning a promise which is fulfilled once the response is available. The promise resolves to the Response object representing the response to your request. The promise does not reject on HTTP errors — it only rejects on network errors. You must use then handlers to check for HTTP errors.

WindowOrWorkerGlobalScope 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 only rejects when a network error is encountered (which is usually when there’s a permissions issue or similar). A fetch() promise does not reject on HTTP errors (404, etc.). Instead, a then() handler must check the Response.ok and/or Response.status properties.

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

const fetchResponsePromise = fetch(resource [, init])

resource

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

• A string or any other object with a stringifier — including a URL object — that provides the URL of the resource you want to fetch.
• A Request object.

init Optional

An 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. Note that the Origin header is not set on Fetch requests with a method of HEAD or GET.
(This behavior was corrected in Firefox 65 — see bug 1508661).

headers

Any headers you want to add to your request, contained within a Headers object or an object literal with ByteString values. Note that some names are forbidden.

body

Any body that you want to add to your request: this can be a Blob, BufferSource, FormData, URLSearchParams, USVString, or ReadableStream 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

Controls what browsers do with credentials (cookies, HTTP authentication entries, and TLS client certificates). Must be one of the following strings:

omit

Tells browsers to exclude credentials from the request, and ignore any credentials sent back in the response (e.g., any Set-Cookie header).

same-origin

Tells browsers to include credentials with requests to same-origin URLs, and use any credentials sent back in responses from same-origin URLs.

include

Tells browsers to include credentials in both same- and cross-origin requests, and always use any credentials sent back in responses.

Note

Credentials may be included in simple and "final" cross-origin requests, but should not be included in CORS preflight requests.

cache

A string indicating how the request will interact with the browser’s HTTP cache. The possible values, default, no-store, reload, no-cache, force-cache, and only-if-cached, are documented in the article for the cache property of the Request object.

redirect

How to handle a redirect response:

• follow: Automatically follow redirects. Unless otherwise stated the redirect mode is set to follow
• error: Abort with an error if a redirect occurs.
• manual: Caller intends to process the response in another context.
  See WHATWG fetch standard for more information.

referrer

A USVString specifying the referrer of the request. This can be a same-origin URL, about:client, or an empty string.

referrerPolicy

Specifies the referrer policy to use for the request. May be one of no-referrer, no-referrer-when-downgrade, same-origin, origin, strict-origin, origin-when-cross-origin, strict-origin-when-cross-origin, or unsafe-url.

integrity

Contains the subresource integrity value of the request (e.g., sha256-BpfBw7ivV8q2jLiT13fxDYAe2tJllusRSZ273h2nFSE=).

keepalive

The keepalive option can be used to allow the request to outlive the page. Fetch with the keepalive flag is a replacement for the Navigator.sendBeacon() API.

signal

An AbortSignal object instance; allows you to communicate with a fetch request and abort it if desired via an AbortController.

A Promise that resolves to a Response object.

AbortError

The request was aborted due to a call to the AbortController method abort() method.

TypeError

The specified URL string includes user credentials. This information should instead be provided using an Authorization header.

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.

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

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

fetch(myRequest)
.then(function(response) {
  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  }
  return response.blob();
})
.then(function(response) {
  let objectURL = URL.createObjectURL(response);
  myImage.src = objectURL;
});

In the 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():

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

let myHeaders = new Headers();
myHeaders.append('Accept', 'image/jpeg');

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

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

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

You could also pass the init object in with the Request constructor to get the same effect:

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

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

const myInit = {
  method: 'GET',
  headers: {
    'Accept': 'image/jpeg'
  },
  mode: 'cors',
  cache: 'default'
};

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

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