Using Fetch

This kind of functionality was previously achieved using XMLHttpRequest. Fetch provides a better alternative that can be easily used by other technologies such as Service Workers. Fetch also provides a single logical place to define other HTTP-related concepts such as CORS and extensions to HTTP.

The state of browser support

Fetch support is at a fairly early stage, but progress is being made. It is turned on by default in Firefox 39 and above, and Chrome 42 and above.

If you want to use it now, there is also a Fetch Polyfill available that recreates the functionality for non-supporting browsers. Bear in mind that this is at an experimental stage and not yet feature complete.

Feature detection

Fetch API support can be detected by checking for the existence of Headers, Request, Response or fetch() on the Window or Worker scope. For example, you might do this in your script:

if(self.fetch) {
    // run my fetch request here
} else {
    // do something with XMLHttpRequest?
}

Making fetch requests

A basic fetch request is really simple to set up. Have a look at the following code:

var myImage = document.querySelector('img');

fetch('flowers.jpg')
.then(function(response) {
  return response.blob();
})
.then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

Here we are fetching an image across the network and inserting it into an <img> element. The simplest use of fetch() takes one argument — the path to the resource you want to fetch — and returns a promise containing the response (a Response object).

This is just an HTTP response of course, not the actual image. To extract the image body content from the response, we use the blob() method (defined on the Body mixin, which is implemented by both the Request and Response objects.)

An objectURL is then created from the extracted Blob, which is then inserted into the img.

Fetch requests are controlled by the connect-src directive of Content Security Policy rather than the directive of the resources it's retrieving.

Supplying request options

The fetch() method can optionally accept a second parameter, an init object that allows you to control a number of different settings:

var myHeaders = new Headers();

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default' };

fetch('flowers.jpg',myInit)
.then(function(response) {
  return response.blob();
})
.then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

See fetch() for the full options available, and more descriptions.

Checking that the fetch was successful

A fetch() promise will reject with a TypeError when a network error is encountered, although this usually means permission issues or similar — a 404 does not constitute a network error, for example. An accurate check for a successful fetch() would include checking that the promise resolved, then checking that the Response.ok property has a value of true. The code would look something like this:

fetch('flowers.jpg').then(function(response) {
  if(response.ok) {
    response.blob().then(function(myBlob) {
      var objectURL = URL.createObjectURL(myBlob);
      myImage.src = objectURL;
    });
  } else {
    console.log('Network response was not ok.');
  }
})
.catch(function(error) {
  console.log('There has been a problem with your fetch operation: ' + error.message);
});

Supplying your own request object

Instead of passing a path to the resource you want to request into the fetch() call, you can create a request object using the Request() constructor, and pass that in as a fetch() method argument:

var myHeaders = new Headers();

var myInit = { method: 'GET',
               headers: myHeaders,
               mode: 'cors',
               cache: 'default' };

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

fetch(myRequest)
.then(function(response) {
  return response.blob();
})
.then(function(myBlob) {
  var objectURL = URL.createObjectURL(myBlob);
  myImage.src = objectURL;
});

Request() accepts exactly the same parameters as the fetch() method. You can even pass in an existing request object to create a copy of it:

var anotherRequest = new Request(myRequest,myInit);

This is pretty useful, as request and response bodies are one use only. Making a copy like this allows you to make use of the request/response again, while varying the init options if desired.

Headers

The Headers interface allows you to create your own headers object via the Headers() constructor. A headers object is a simple multi-map of names to values:

var content = "Hello World";
var myHeaders = new Headers();
myHeaders.append("Content-Type", "text/plain");
myHeaders.append("Content-Length", content.length.toString());
myHeaders.append("X-Custom-Header", "ProcessThisImmediately");

The same can be achieved by passing an array of arrays or an object literal to the constructor:

myHeaders = new Headers({
  "Content-Type": "text/plain",
  "Content-Length": content.length.toString(),
  "X-Custom-Header": "ProcessThisImmediately",
});

The contents can be queried and retrieved:

console.log(myHeaders.has("Content-Type")); // true
console.log(myHeaders.has("Set-Cookie")); // false
myHeaders.set("Content-Type", "text/html");
myHeaders.append("X-Custom-Header", "AnotherValue");
 
console.log(myHeaders.get("Content-Length")); // 11
console.log(myHeaders.getAll("X-Custom-Header")); // ["ProcessThisImmediately", "AnotherValue"]
 
myHeaders.delete("X-Custom-Header");
console.log(myHeaders.getAll("X-Custom-Header")); // [ ]

Some of these operations are only useful in ServiceWorkers, but they provide a much nicer API for manipulating headers.

All of the Headers methods throw a TypeError if a header name is used that is not a valid HTTP Header name. The mutation operations will throw a TypeError if there is an immutable guard (see below). Otherwise they fail silently. For example:

var myResponse = Response.error();
try {
  myResponse.headers.set("Origin", "https://mybank.com");
} catch(e) {
  console.log("Cannot pretend to be a bank!");
}

A good use case for headers is checking whether the content type is correct before you process it further. For example:

fetch(myRequest).then(function(response) {
  var contentType = response.headers.get("content-type");
  if(contentType && contentType.indexOf("application/json") !== -1) {
    return response.json().then(function(json) {
      // process your JSON further
    });
  } else {
    console.log("Oops, we haven't got JSON!");
  }
});

Guard

Since headers can be sent in requests and received in responses, and have various limitations about what information can and should be mutable, headers objects have a guard property. This is not exposed to the Web, but it affects which mutation operations are allowed on the headers object.

Possible guard values are:

• none: default.
• request: guard for a headers object obtained from a request (Request.headers).
• request-no-cors: guard for a headers object obtained from a request created with Request.mode no-cors.
• response: guard for a Headers obtained from a response (Response.headers).
• immutable: Mostly used for ServiceWorkers; renders a headers object read-only.

Response objects

As you have seen above, Response instances are returned when fetch() promises are resolved.

They can also be created programmatically via JavaScript, but this is only really useful in ServiceWorkers, when you are providing a custom response to a received request using a respondWith() method:

var myBody = new Blob();

addEventListener('fetch', function(event) {
  event.respondWith(
    new Response(myBody, {
      headers: { "Content-Type" : "text/plain" }
    })
  );
});

The Response() constructor takes two optional arguments — a body for the response, and an init object (similar to the one that Request() accepts.)

The most common response properties you'll use are:

• Response.status — An integer (default value 200) containing the response status code.
• Response.statusText — A string (default value "OK"),which corresponds to the HTTP status code message.
• Response.ok — seen in use above, this is a shorthand for checking that status is in the range 200-299 inclusive. This returns a Boolean.

Body

Both requests and responses may contain body data. A body is an instance of any of the following types.

• ArrayBuffer
• ArrayBufferView (Uint8Array and friends)
• Blob/File
• string
• URLSearchParams
• FormData

The Body mixin defines the following methods to extract a body (implemented by both Request and Response). These all return a promise that is eventually resolved with the actual content.

• arrayBuffer()
• blob()
• json()
• text()
• formData()

This makes usage of non-textual data much easier than it was with XHR.

Request bodies can be set by passing body parameters:

var form = new FormData(document.getElementById('login-form'));
fetch("/login", {
  method: "POST",
  body: form
});

Both request and response (and by extension the fetch() function), will try to intelligently determine the content type. A request will also automatically set a Content-Type header if none is set in the dictionary.

Specifications

Browser compatibility

See also

• ServiceWorker API
• HTTP access control (CORS)
• HTTP
• Fetch polyfill
• Fetch examples on Github