XMLHttpRequest

XMLHttpRequest is a built-in browser object that allows to make HTTP requests in JavaScript.

Despite of having the word “XML” in its name, it can operate on any data, not only in XML format. We can upload/download files, track progress and much more.

Right now, there’s another, more modern method fetch, that somewhat deprecates XMLHttpRequest.

In modern web-development XMLHttpRequest is used for three reasons:

1. Historical reasons: we need to support existing scripts with XMLHttpRequest.
2. We need to support old browsers, and don’t want polyfills (e.g. to keep scripts tiny).
3. We need something that fetch can’t do yet, e.g. to track upload progress.

Does that sound familiar? If yes, then all right, go on with XMLHttpRequest. Otherwise, please head on to fetch.

The basics

XMLHttpRequest has two modes of operation: synchronous and asynchronous.

Let’s see the asynchronous first, as it’s used in the majority of cases.

To do the request, we need 3 steps:

1. Create XMLHttpRequest:

   let xhr = new XMLHttpRequest();

   The constructor has no arguments.

2. Initialize it, usually right after new XMLHttpRequest:

   xhr.open(method, URL, [async, user, password])

   This method specifies the main parameters of the request:

   • method – HTTP-method. Usually "GET" or "POST".
   • URL – the URL to request, a string, can be URL object.
   • async – if explicitly set to false, then the request is synchronous, we’ll cover that a bit later.
   • user, password – login and password for basic HTTP auth (if required).

   Please note that open call, contrary to its name, does not open the connection. It only configures the request, but the network activity only starts with the call of send.

3. Send it out.

   xhr.send([body])

   This method opens the connection and sends the request to server. The optional body parameter contains the request body.

   Some request methods like GET do not have a body. And some of them like POST use body to send the data to the server. We’ll see examples of that later.

4. Listen to xhr events for response.

   These three events are the most widely used:

   • load – when the request is complete (even if HTTP status is like 400 or 500), and the response is fully downloaded.
   • error – when the request couldn’t be made, e.g. network down or invalid URL.
   • progress – triggers periodically while the response is being downloaded, reports how much has been downloaded.

   xhr.onload = function() {
     alert(`Loaded: ${xhr.status} ${xhr.response}`);
   };
   
   xhr.onerror = function() { // only triggers if the request couldn't be made at all
     alert(`Network Error`);
   };
   
   xhr.onprogress = function(event) { // triggers periodically
     // event.loaded - how many bytes downloaded
     // event.lengthComputable = true if the server sent Content-Length header
     // event.total - total number of bytes (if lengthComputable)
     alert(`Received ${event.loaded} of ${event.total}`);
   };

Here’s a full example. The code below loads the URL at /article/xmlhttprequest/example/load from the server and prints the progress:

// 1. Create a new XMLHttpRequest object
let xhr = new XMLHttpRequest();

// 2. Configure it: GET-request for the URL /article/.../load
xhr.open('GET', '/article/xmlhttprequest/example/load');

// 3. Send the request over the network
xhr.send();

// 4. This will be called after the response is received
xhr.onload = function() {
  if (xhr.status != 200) { // analyze HTTP status of the response
    alert(`Error ${xhr.status}: ${xhr.statusText}`); // e.g. 404: Not Found
  } else { // show the result
    alert(`Done, got ${xhr.response.length} bytes`); // response is the server
  }
};

xhr.onprogress = function(event) {
  if (event.lengthComputable) {
    alert(`Received ${event.loaded} of ${event.total} bytes`);
  } else {
    alert(`Received ${event.loaded} bytes`); // no Content-Length
  }

};

xhr.onerror = function() {
  alert("Request failed");
};

Once the server has responded, we can receive the result in the following xhr properties:

status

HTTP status code (a number): 200, 404, 403 and so on, can be 0 in case of a non-HTTP failure.

statusText

HTTP status message (a string): usually OK for 200, Not Found for 404, Forbidden for 403 and so on.

response (old scripts may use responseText)

The server response body.

We can also specify a timeout using the corresponding property:

xhr.timeout = 10000; // timeout in ms, 10 seconds

If the request does not succeed within the given time, it gets canceled and timeout event triggers.

let url = new URL('https://google.com/search');
url.searchParams.set('q', 'test me!');

// the parameter 'q' is encoded
xhr.open('GET', url); // https://google.com/search?q=test+me%21

Response Type

We can use xhr.responseType property to set the response format:

• "" (default) – get as string,
• "text" – get as string,
• "arraybuffer" – get as ArrayBuffer (for binary data, see chapter ArrayBuffer, binary arrays),
• "blob" – get as Blob (for binary data, see chapter Blob),
• "document" – get as XML document (can use XPath and other XML methods),
• "json" – get as JSON (parsed automatically).

For example, let’s get the response as JSON:

let xhr = new XMLHttpRequest();

xhr.open('GET', '/article/xmlhttprequest/example/json');

xhr.responseType = 'json';

xhr.send();

// the response is {"message": "Hello, world!"}
xhr.onload = function() {
  let responseObj = xhr.response;
  alert(responseObj.message); // Hello, world!
};

Ready states

XMLHttpRequest changes between states as it progresses. The current state is accessible as xhr.readyState.

All states, as in the specification:

UNSENT = 0; // initial state
OPENED = 1; // open called
HEADERS_RECEIVED = 2; // response headers received
LOADING = 3; // response is loading (a data packed is received)
DONE = 4; // request complete

An XMLHttpRequest object travels them in the order 0 → 1 → 2 → 3 → … → 3 → 4. State 3 repeats every time a data packet is received over the network.

We can track them using readystatechange event:

xhr.onreadystatechange = function() {
  if (xhr.readyState == 3) {
    // loading
  }
  if (xhr.readyState == 4) {
    // request finished
  }
};

You can find readystatechange listeners in really old code, it’s there for historical reasons, as there was a time when there were no load and other events. Nowadays, load/error/progress handlers deprecate it.

Aborting request

We can terminate the request at any time. The call to xhr.abort() does that:

xhr.abort(); // terminate the request

That triggers abort event, and xhr.status becomes 0.

Synchronous requests

If in the open method the third parameter async is set to false, the request is made synchronously.

In other words, JavaScript execution pauses at send() and resumes when the response is received. Somewhat like alert or prompt commands.

Here’s the rewritten example, the 3rd parameter of open is false:

let xhr = new XMLHttpRequest();

xhr.open('GET', '/article/xmlhttprequest/hello.txt', false);

try {
  xhr.send();
  if (xhr.status != 200) {
    alert(`Error ${xhr.status}: ${xhr.statusText}`);
  } else {
    alert(xhr.response);
  }
} catch(err) { // instead of onerror
  alert("Request failed");
}

It might look good, but synchronous calls are used rarely, because they block in-page JavaScript till the loading is complete. In some browsers it becomes impossible to scroll. If a synchronous call takes too much time, the browser may suggest to close the “hanging” webpage.

Many advanced capabilities of XMLHttpRequest, like requesting from another domain or specifying a timeout, are unavailable for synchronous requests. Also, as you can see, no progress indication.

Because of all that, synchronous requests are used very sparingly, almost never. We won’t talk about them any more.

HTTP-headers

XMLHttpRequest allows both to send custom headers and read headers from the response.

There are 3 methods for HTTP-headers:

setRequestHeader(name, value)

Sets the request header with the given name and value.

For instance:

xhr.setRequestHeader('Content-Type', 'application/json');

Headers limitations

Several headers are managed exclusively by the browser, e.g. Referer and Host. The full list is in the specification.

XMLHttpRequest is not allowed to change them, for the sake of user safety and correctness of the request.

Can’t remove a header

Another peculiarity of XMLHttpRequest is that one can’t undo setRequestHeader.

Once the header is set, it’s set. Additional calls add information to the header, don’t overwrite it.

For instance:

xhr.setRequestHeader('X-Auth', '123');
xhr.setRequestHeader('X-Auth', '456');

// the header will be:
// X-Auth: 123, 456

getResponseHeader(name)

Gets the response header with the given name (except Set-Cookie and Set-Cookie2).

For instance:

xhr.getResponseHeader('Content-Type')

getAllResponseHeaders()

Returns all response headers, except Set-Cookie and Set-Cookie2.

Headers are returned as a single line, e.g.:

Cache-Control: max-age=31536000
Content-Length: 4260
Content-Type: image/png
Date: Sat, 08 Sep 2012 16:53:16 GMT

The line break between headers is always "\r\n" (doesn’t depend on OS), so we can easily split it into individual headers. The separator between the name and the value is always a colon followed by a space ": ". That’s fixed in the specification.

So, if we want to get an object with name/value pairs, we need to throw in a bit JS.

Like this (assuming that if two headers have the same name, then the latter one overwrites the former one):

let headers = xhr
  .getAllResponseHeaders()
  .split('\r\n')
  .reduce((result, current) => {
    let [name, value] = current.split(': ');
    result[name] = value;
    return result;
  }, {});

// headers['Content-Type'] = 'image/png'

POST, FormData

To make a POST request, we can use the built-in FormData object.

The syntax:

let formData = new FormData([form]); // creates an object, optionally fill from <form>
formData.append(name, value); // appends a field

We create it, optionally fill from a form, append more fields if needed, and then:

1. xhr.open('POST', ...) – use POST method.
2. xhr.send(formData) to submit the form to the server.

For instance:

<form name="person">
  <input name="name" value="John">
  <input name="surname" value="Smith">
</form>

<script>
  // pre-fill FormData from the form
  let formData = new FormData(document.forms.person);

  // add one more field
  formData.append("middle", "Lee");

  // send it out
  let xhr = new XMLHttpRequest();
  xhr.open("POST", "/article/xmlhttprequest/post/user");
  xhr.send(formData);

  xhr.onload = () => alert(xhr.response);
</script>

The form is sent with multipart/form-data encoding.

Or, if we like JSON more, then JSON.stringify and send as a string.

Just don’t forget to set the header Content-Type: application/json, many server-side frameworks automatically decode JSON with it:

let xhr = new XMLHttpRequest();

let json = JSON.stringify({
  name: "John",
  surname: "Smith"
});

xhr.open("POST", '/submit')
xhr.setRequestHeader('Content-type', 'application/json; charset=utf-8');

xhr.send(json);

The .send(body) method is pretty omnivore. It can send almost any body, including Blob and BufferSource objects.

Upload progress

The progress event triggers only on the downloading stage.

That is: if we POST something, XMLHttpRequest first uploads our data (the request body), then downloads the response.

If we’re uploading something big, then we’re surely more interested in tracking the upload progress. But xhr.onprogress doesn’t help here.

There’s another object, without methods, exclusively to track upload events: xhr.upload.

It generates events, similar to xhr, but xhr.upload triggers them solely on uploading:

• loadstart – upload started.
• progress – triggers periodically during the upload.
• abort – upload aborted.
• error – non-HTTP error.
• load – upload finished successfully.
• timeout – upload timed out (if timeout property is set).
• loadend – upload finished with either success or error.

Example of handlers:

xhr.upload.onprogress = function(event) {
  alert(`Uploaded ${event.loaded} of ${event.total} bytes`);
};

xhr.upload.onload = function() {
  alert(`Upload finished successfully.`);
};

xhr.upload.onerror = function() {
  alert(`Error during the upload: ${xhr.status}`);
};

Here’s a real-life example: file upload with progress indication:

<input type="file" onchange="upload(this.files[0])">

<script>
function upload(file) {
  let xhr = new XMLHttpRequest();

  // track upload progress
  xhr.upload.onprogress = function(event) {
    console.log(`Uploaded ${event.loaded} of ${event.total}`);
  };

  // track completion: both successful or not
  xhr.onloadend = function() {
    if (xhr.status == 200) {
      console.log("success");
    } else {
      console.log("error " + this.status);
    }
  };

  xhr.open("POST", "/article/xmlhttprequest/post/upload");
  xhr.send(file);
}
</script>

Cross-origin requests

XMLHttpRequest can make cross-origin requests, using the same CORS policy as fetch.

Just like fetch, it doesn’t send cookies and HTTP-authorization to another origin by default. To enable them, set xhr.withCredentials to true:

let xhr = new XMLHttpRequest();
xhr.withCredentials = true;

xhr.open('POST', 'http://anywhere.com/request');
...

See the chapter CORS for details about cross-origin headers.

Summary

Typical code of the GET-request with XMLHttpRequest:

let xhr = new XMLHttpRequest();

xhr.open('GET', '/my/url');

xhr.send();

xhr.onload = function() {
  if (xhr.status != 200) { // HTTP error?
    // handle error
    alert( 'Error: ' + xhr.status);
    return;
  }

  // get the response from xhr.response
};

xhr.onprogress = function(event) {
  // report progress
  alert(`Loaded ${event.loaded} of ${event.total}`);
};

xhr.onerror = function() {
  // handle non-HTTP error (e.g. network down)
};

There are actually more events, the modern specification lists them (in the lifecycle order):

• loadstart – the request has started.
• progress – a data packet of the response has arrived, the whole response body at the moment is in response.
• abort – the request was canceled by the call xhr.abort().
• error – connection error has occurred, e.g. wrong domain name. Doesn’t happen for HTTP-errors like 404.
• load – the request has finished successfully.
• timeout – the request was canceled due to timeout (only happens if it was set).
• loadend – triggers after load, error, timeout or abort.

The error, abort, timeout, and load events are mutually exclusive. Only one of them may happen.

The most used events are load completion (load), load failure (error), or we can use a single loadend handler and check the properties of the request object xhr to see what happened.

We’ve already seen another event: readystatechange. Historically, it appeared long ago, before the specification settled. Nowadays, there’s no need to use it, we can replace it with newer events, but it can often be found in older scripts.

If we need to track uploading specifically, then we should listen to same events on xhr.upload object.