WindowOrWorkerGlobalScope.fetch() - Web APIs 编辑

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.

Note: The fetch() method's parameters are identical to those of the Request() constructor.

Syntax

const fetchResponsePromise = fetch(resource [, init])

Parameters

resource

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 the blob: and data: schemes.
• 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

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.

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 is follow (before Chrome 47 it defaulted to manual).

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.

Return value

A Promise that resolves to a Response object.

Exceptions

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.

Examples

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('Content-Type', '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: {
    'Content-Type': 'image/jpeg'
  },
  mode: 'cors',
  cache: 'default'
};

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

Specifications

Browser compatibility

BCD tables only load in the browser

See also

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