Fetch API

The Fetch API provides an interface for fetching resources (including across the network). It will seem familiar to anyone who has used XMLHttpRequest, but the new API provides a more powerful and flexible feature set.

Concepts and usage

Fetch provides a generic definition of Request and Response objects (and other things involved with network requests). This will allow them to be used wherever they are needed in the future, whether it’s for service workers, Cache API and other similar things that handle or modify requests and responses, or any kind of use case that might require you to generate your own responses programmatically.

It also provides a definition for related concepts such as CORS and the HTTP origin header semantics, supplanting their separate definitions elsewhere.

For making a request and fetching a resource, use the GlobalFetch.fetch method. It is implemented in multiple interfaces, specifically Window and WorkerGlobalScope. This makes it available in pretty much any context you might want to fetch resources in.

The fetch() method takes one mandatory argument, the path to the resource you want to fetch. It returns a promise that resolves to the Response to that request, whether it is successful or not. You can also optionally pass in an init options object as the second argument (see Request).

Once a Response is retrieved, there are a number of methods available to define what the body content is and how it should be handled (see Body).

You can create a request and response directly using the Request() and Response() constructors, but you are unlikely to do this directly. Instead, these are more likely to be created as results of other API actions (for example, FetchEvent.respondWith from service workers).

Fetch Interfaces

GlobalFetch

Contains the fetch() method used to fetch a resource.

Headers

Represents response/request headers, allowing you to query them and take different actions depending on the results.

Request

Represents a resource request.

Response

Represents the response to a request.

Experimental fetch controller/signal/observer interfaces

Also known as "cancellable fetch", the following interfaces contain functionality to allow fetch request operations to be observed and controlled during their lifetimes.

FetchController

Represents a controller object that allows you to control one or more fetch requests.

FetchSignal

Represents a signal object that allows you to communicate with a fetch request and control it via a FetchController.

FetchObserver

Represents an observer object that allows you to retrieve information concerning the status of a fetch request.

ObserverCallback

Provides a callback function that runs when the fetch request it is assocated with runs. This provides a FetchObserver object that can be used to retrieve information concerning the status of a fetch request.

Fetch mixin

Body

Provides methods relating to the body of the response/request, allowing you to declare what its content type is and how it should be handled.

Specifications

Browser compatibility

[1] This API is implemented behind a preference.

[2] Prior to Firefox 52, get() only returned the first value in the specified header, with getAll() returning all values. From 52 onwards, get() now returns all values and getAll() has been removed.

[3] Hidden behind a preference in 55+ Nightly. In about:config, you need to create two new boolean prefs — dom.fetchObserver.enabled and dom.fetchController.enabled — and set the values of both to true.

See also

• Using Fetch
• ServiceWorker API
• HTTP access control (CORS)
• HTTP
• Fetch polyfill
• Fetch basic concepts