Window: fetch() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2017.

The fetch() method of the Window interface starts the process of fetching a resource from the network, returning a promise that is fulfilled once the response is available.

The promise resolves to the Response object representing the response to your request.

A fetch() promise only rejects when the request fails, for example, because of a badly-formed request URL or a network error. A fetch() promise does not reject if the server responds with HTTP status codes that indicate errors (404, 504, 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

js
fetch(resource)
fetch(resource, options)

Parameters

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. The URL may be relative to the base URL, which is the document's baseURI in a window context, or WorkerGlobalScope.location in a worker context.
  • A Request object.
options Optional

A RequestInit object containing any custom settings that you want to apply to the request.

Return value

A Promise that resolves to a Response object.

Exceptions

AbortError DOMException

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

NotAllowedError DOMException

Thrown if use of the Topics API is specifically disallowed by a browsing-topics Permissions Policy, and a fetch() request was made with browsingTopics: true.

TypeError

Can occur for the following reasons:

Reason Failing examples
Blocked by a permissions policy Use of the Attribution Reporting API is blocked by a attribution-reporting Permissions-Policy, and a fetch() request was made with attributionReporting specified.
Invalid header name. 
// space in "C ontent-Type"
const headers = {
  'C ontent-Type': 'text/xml',
  'Breaking-Bad': '<3',
};
fetch('https://example.com/', { headers });
Invalid header value. The header object must contain exactly two elements. 
const headers = [
  ['Content-Type', 'text/html', 'extra'],
  ['Accept'],
];
fetch('https://example.com/', { headers });
Invalid URL or scheme, or using a scheme that fetch does not support, or using a scheme that is not supported for a particular request mode. 
fetch('blob://example.com/', { mode: 'cors' });
URL includes credentials. 
fetch('https://user:password@example.com/');
Invalid referrer URL. 
fetch('https://example.com/', { referrer: './abc\u0000df' });
Invalid modes (navigate and websocket). 
fetch('https://example.com/', { mode: 'navigate' });
If the request cache mode is "only-if-cached" and the request mode is other than "same-origin". 
fetch('https://example.com/', {
  cache: 'only-if-cached',
  mode: 'no-cors',
});
If the request method is an invalid name token or one of the forbidden headers ('CONNECT', 'TRACE' or 'TRACK'). 
fetch('https://example.com/', { method: 'CONNECT' });
If the request mode is "no-cors" and the request method is not a CORS-safe-listed method ('GET', 'HEAD', or 'POST'). 
fetch('https://example.com/', {
  method: 'CONNECT',
  mode: 'no-cors',
});
If the request method is 'GET' or 'HEAD' and the body is non-null or not undefined. 
fetch('https://example.com/', {
  method: 'GET',
  body: new FormData(),
});
If fetch throws a network error.

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 Response.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.

js
const myImage = document.querySelector("img");

const myRequest = new Request("flowers.jpg");

window
  .fetch(myRequest)
  .then((response) => {
    if (!response.ok) {
      throw new Error(`HTTP error! Status: ${response.status}`);
    }

    return response.blob();
  })
  .then((response) => {
    myImage.src = URL.createObjectURL(response);
  });

In our Fetch Request with init example (see Fetch Request init live) we do the same thing except that we pass in an options object when we invoke fetch(). In this case, we can set a Cache-Control value to indicate what kind of cached responses we're okay with:

js
const myImage = document.querySelector("img");
const reqHeaders = new Headers();

// A cached response is okay unless it's more than a week old
reqHeaders.set("Cache-Control", "max-age=604800");

const options = {
  headers: reqHeaders,
};

// Pass init as an "options" object with our headers.
const req = new Request("flowers.jpg", options);

fetch(req).then((response) => {
  // ...
});

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

js
const req = new Request("flowers.jpg", options);

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

js
const options = {
  headers: {
    "Cache-Control": "max-age=60480",
  },
};

const req = new Request("flowers.jpg", options);

Specifications

Specification
Fetch Standard
# fetch-method

Browser compatibility

BCD tables only load in the browser

See also