For more than a decade the Web has used XMLHttpRequest (XHR) to achieve asynchronous requests in JavaScript. While very useful, XHR is not a very nice API. It suffers from lack of separation of concerns. The input, output and state are all managed by interacting with one object, and state is tracked using events. Also, the event-based model doesn’t play well with JavaScript’s recent focus on Promise- and generator-based asynchronous programming.

The Fetch API intends to fix most of these problems. It does this by introducing the same primitives to JS that are used in the HTTP protocol. In addition, it introduces a utility function fetch() that succinctly captures the intention of retrieving a resource from the network.

The Fetch specification, which defines the API, nails down the semantics of a user agent fetching a resource. This, combined with ServiceWorkers, is an attempt to:

1. Improve the offline experience.
2. Expose the building blocks of the Web to the platform as part of the extensible web movement.

As of this writing, the Fetch API is available in Firefox 39 (currently Nightly) and Chrome 42 (currently dev). Github has a Fetch polyfill.

Feature detection

Fetch API support can be detected by checking for Headers,Request, Response or fetch on the window or worker scope.

Simple fetching

The most useful, high-level part of the Fetch API is the fetch() function. In its simplest form it takes a URL and returns a promise that resolves to the response. The response is captured as a Response object.

fetch("/data.json").then(function(res) {
  // res instanceof Response == true.
  if (res.ok) {
    res.json().then(function(data) {
      console.log(data.entries);
    });
  } else {
    console.log("Looks like the response wasn't perfect, got status", res.status);
  }
}, function(e) {
  console.log("Fetch failed!", e);
});

Submitting some parameters, it would look like this:

fetch("http://www.example.org/submit.php", {
  method: "POST",
  headers: {
    "Content-Type": "application/x-www-form-urlencoded"
  },
  body: "firstName=Nikhil&favColor=blue&password=easytoguess"
}).then(function(res) {
  if (res.ok) {
    alert("Perfect! Your settings are saved.");
  } else if (res.status == 401) {
    alert("Oops! You are not authorized.");
  }
}, function(e) {
  alert("Error submitting form!");
});

The fetch() function’s arguments are the same as those passed to the
Request() constructor, so you may directly pass arbitrarily complex requests to fetch() as discussed below.

Headers

Fetch introduces 3 interfaces. These are Headers, Request and
Response. They map directly to the underlying HTTP concepts, but have
certain visibility filters in place for privacy and security reasons, such as
supporting CORS rules and ensuring cookies aren’t readable by third parties.

The Headers interface is a simple multi-map of names to values:

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

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

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

The contents can be queried and retrieved:

console.log(reqHeaders.has("Content-Type")); // true
console.log(reqHeaders.has("Set-Cookie")); // false
reqHeaders.set("Content-Type", "text/html");
reqHeaders.append("X-Custom-Header", "AnotherValue");

console.log(reqHeaders.get("Content-Length")); // 11
console.log(reqHeaders.getAll("X-Custom-Header")); // ["ProcessThisImmediately", "AnotherValue"]

reqHeaders.delete("X-Custom-Header");
console.log(reqHeaders.getAll("X-Custom-Header")); // []

Some of these operations are only useful in ServiceWorkers, but they provide
a much nicer API to Headers.

Since Headers can be sent in requests, or 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 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 mode “no-cors”.
• “response”: naturally, for Headers obtained from Response (Response.headers).
• “immutable”: Mostly used for ServiceWorkers, renders a Headers object
  read-only.

The details of how each guard affects the behaviors of the Headers object are
in the specification. For example, you may not append or set a “request” guarded Headers’ “Content-Length” header. Similarly, inserting “Set-Cookie” into a Response header is not allowed so that ServiceWorkers may not set cookies via synthesized Responses.

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

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

Request

The Request interface defines a request to fetch a resource over HTTP. URL, method and headers are expected, but the Request also allows specifying a body, a request mode, credentials and cache hints.

The simplest Request is of course, just a URL, as you may do to GET a resource.

var req = new Request("/index.html");
console.log(req.method); // "GET"
console.log(req.url); // "http://example.com/index.html"

You may also pass a Request to the Request() constructor to create a copy.
(This is not the same as calling the clone() method, which is covered in
the “Reading bodies” section.).

var copy = new Request(req);
console.log(copy.method); // "GET"
console.log(copy.url); // "http://example.com/index.html"

Again, this form is probably only useful in ServiceWorkers.

The non-URL attributes of the Request can only be set by passing initial
values as a second argument to the constructor. This argument is a dictionary.

var uploadReq = new Request("/uploadImage", {
  method: "POST",
  headers: {
    "Content-Type": "image/png",
  },
  body: "image data"
});

The Request’s mode is used to determine if cross-origin requests lead to valid responses, and which properties on the response are readable. Legal mode values are "same-origin", "no-cors" (default) and "cors".

The "same-origin" mode is simple, if a request is made to another origin with this mode set, the result is simply an error. You could use this to ensure that
a request is always being made to your origin.

var arbitraryUrl = document.getElementById("url-input").value;
fetch(arbitraryUrl, { mode: "same-origin" }).then(function(res) {
  console.log("Response succeeded?", res.ok);
}, function(e) {
  console.log("Please enter a same-origin URL!");
});

The "no-cors" mode captures what the web platform does by default for scripts you import from CDNs, images hosted on other domains, and so on. First, it prevents the method from being anything other than “HEAD”, “GET” or “POST”. Second, if any ServiceWorkers intercept these requests, they may not add or override any headers except for these. Third, JavaScript may not access any properties of the resulting Response. This ensures that ServiceWorkers do not affect the semantics of the Web and prevents security and privacy issues that could arise from leaking data across domains.

"cors" mode is what you’ll usually use to make known cross-origin requests to access various APIs offered by other vendors. These are expected to adhere to
the CORS protocol. Only a limited set of headers is exposed in the Response, but the body is readable. For example, you could get a list of Flickr’s most interesting photos today like this:

var u = new URLSearchParams();
u.append('method', 'flickr.interestingness.getList');
u.append('api_key', '');
u.append('format', 'json');
u.append('nojsoncallback', '1');

var apiCall = fetch('https://api.flickr.com/services/rest?' + u);

apiCall.then(function(response) {
  return response.json().then(function(json) {
    // photo is a list of photos.
    return json.photos.photo;
  });
}).then(function(photos) {
  photos.forEach(function(photo) {
    console.log(photo.title);
  });
});

You may not read out the “Date” header since Flickr does not allow it via
Access-Control-Expose-Headers.

response.headers.get("Date"); // null

The credentials enumeration determines if cookies for the other domain are
sent to cross-origin requests. This is similar to XHR’s withCredentials
flag, but tri-valued as "omit" (default), "same-origin" and "include".

The Request object will also give the ability to offer caching hints to the user-agent. This is currently undergoing some security review. Firefox exposes the attribute, but it has no effect.

Requests have two read-only attributes that are relevant to ServiceWorkers
intercepting them. There is the string referrer, which is set by the UA to be
the referrer of the Request. This may be an empty string. The other is
context which is a rather large enumeration defining what sort of resource is being fetched. This could be “image” if the request is from an tag in the controlled document, “worker” if it is an attempt to load a worker script, and so on. When used with the fetch() function, it is “fetch”.

Response

Response instances are returned by calls to fetch(). They can also be created by JS, but this is only useful in ServiceWorkers.

We have already seen some attributes of Response when we looked at fetch(). The most obvious candidates are status, an integer (default value 200) and statusText (default value “OK”), which correspond to the HTTP status code and reason. The ok attribute is just a shorthand for checking that status is in the range 200-299 inclusive.

headers is the Response’s Headers object, with guard “response”. The url attribute reflects the URL of the corresponding request.

Response also has a type, which is “basic”, “cors”, “default”, “error” or
“opaque”.

• "basic": normal, same origin response, with all headers exposed except
  “Set-Cookie” and “Set-Cookie2”.
• "cors": response was received from a valid cross-origin request. Certain headers and the body may be accessed.
• "error": network error. No useful information describing the error is available. The Response’s status is 0, headers are empty and immutable. This is the type for a Response obtained from Response.error().
• "opaque": response for “no-cors” request to cross-origin resource. Severely
  restricted

The “error” type results in the fetch() Promise rejecting with TypeError.

There are certain attributes that are useful only in a ServiceWorker scope. The
idiomatic way to return a Response to an intercepted request in ServiceWorkers is:

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

As you can see, Response has a two argument constructor, where both arguments are optional. The first argument is a body initializer, and the second is a dictionary to set the status, statusText and headers.

The static method Response.error() simply returns an error response. Similarly, Response.redirect(url, status) returns a Response resulting in
a redirect to url.

Dealing with bodies

Both Requests and Responses may contain body data. We’ve been glossing over it because of the various data types body may contain, but we will cover it in detail now.

A body is an instance of any of the following types.

• ArrayBuffer
• ArrayBufferView (Uint8Array and friends)
• Blob/File
• string
• URLSearchParams
• FormData – currently not supported by either Gecko or Blink. Firefox expects to ship this in version 39 along with the rest of Fetch.

In addition, Request and Response both offer the following methods to extract their body. These all return a Promise that is eventually resolved with the actual content.

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

This is a significant improvement over XHR in terms of ease of use of non-text data!

Request bodies can be set by passing body parameters:

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

Responses take the first argument as the body.

var res = new Response(new File(["chunk", "chunk"], "archive.zip",
                       { type: "application/zip" }));

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

Streams and cloning

It is important to realise that Request and Response bodies can only be read once! Both interfaces have a boolean attribute bodyUsed to determine if it is safe to read or not.

var res = new Response("one time use");
console.log(res.bodyUsed); // false
res.text().then(function(v) {
  console.log(res.bodyUsed); // true
});
console.log(res.bodyUsed); // true

res.text().catch(function(e) {
  console.log("Tried to read already consumed Response");
});

This decision allows easing the transition to an eventual stream-based Fetch API. The intention is to let applications consume data as it arrives, allowing for JavaScript to deal with larger files like videos, and perform things like compression and editing on the fly.

Often, you’ll want access to the body multiple times. For example, you can use the upcoming Cache API to store Requests and Responses for offline use, and Cache requires bodies to be available for reading.

So how do you read out the body multiple times within such constraints? The API provides a clone() method on the two interfaces. This will return a clone of the object, with a ‘new’ body. clone() MUST be called before the body of the corresponding object has been used. That is, clone() first, read later.

addEventListener('fetch', function(evt) {
  var sheep = new Response("Dolly");
  console.log(sheep.bodyUsed); // false
  var clone = sheep.clone();
  console.log(clone.bodyUsed); // false

  clone.text();
  console.log(sheep.bodyUsed); // false
  console.log(clone.bodyUsed); // true

  evt.respondWith(cache.add(sheep.clone()).then(function(e) {
    return sheep;
  });
});

Future improvements

Along with the transition to streams, Fetch will eventually have the ability to abort running fetch()es and some way to report the progress of a fetch. These are provided by XHR, but are a little tricky to fit in the Promise-based nature of the Fetch API.

You can contribute to the evolution of this API by participating in discussions on the WHATWG mailing list and in the issues in the Fetch and ServiceWorker specifications.

For a better web!

The author would like to thank Andrea Marchesini, Anne van Kesteren and Ben
Kelly for helping with the specification and implementation.