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
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'sbaseURI
in a window context, orWorkerGlobalScope.location
in a worker context. - A
Request
object.
- A string or any other object with a stringifier — including a
options
Optional-
A
RequestInit
object containing any custom settings that you want to apply to the request.
Return value
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 afetch()
request was made withbrowsingTopics: 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.
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:
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:
const req = new Request("flowers.jpg", options);
You can also use an object literal as headers
in init
:
const options = {
headers: {
"Cache-Control": "max-age=60480",
},
};
const req = new Request("flowers.jpg", options);
Specifications
Specification |
---|
Fetch Standard # fetch-method |
Browser compatibility
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