fetch()
The global fetch()
method 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.
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.
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.
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 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.
- A string or any other object with a stringifier — including a
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 theOrigin
header is not set on Fetch requests with a method ofHEAD
orGET
. (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 withString
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
, orReadableStream
object. Note that a request using theGET
orHEAD
method cannot have a body. mode
-
The mode you want to use for the request, e.g.,
cors
,no-cors
, orsame-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
, andonly-if-cached
, are documented in the article for thecache
property of theRequest
object. redirect
-
How to handle a
redirect
response:follow
: Automatically follow redirects. Unless otherwise stated the redirect mode is set tofollow
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
, orunsafe-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 thekeepalive
flag is a replacement for theNavigator.sendBeacon()
API. signal
-
An
AbortSignal
object instance; allows you to communicate with a fetch request and abort it if desired via anAbortController
.
Return value
Exceptions
AbortError
-
The request was aborted due to a call to the
AbortController
abort()
method. TypeError
-
Can occur for the following reasons:
Reason | Failing examples |
---|---|
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 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');
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 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');
const myHeaders = new Headers();
myHeaders.append('Accept', 'image/jpeg');
const myInit = {
method: 'GET',
headers: myHeaders,
mode: 'cors',
cache: 'default',
};
const myRequest = new Request('flowers.jpg');
fetch(myRequest, myInit)
.then((response) => {
// ...
});
You could also pass the init
object in with the
Request
constructor to get the same effect:
const 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',
};
const myRequest = new Request('flowers.jpg', myInit);
Specifications
Specification |
---|
Fetch Standard # fetch-method |
Browser compatibility
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