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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 may be used for three reasons:

Historical reasons: we need to support existing scripts with


          XMLHttpRequest

We need to support old browsers, and don’t want polyfills (e.g. to keep scripts tiny).

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: Temel Bilgiler .

Basic flow

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:

Create XMLHttpRequest .

This method is usually called first after new XMLHttpRequest . It specifies the main parameters of the request:


         method

– HTTP-method. Usually


         "GET"


         "POST"

URL

– the URL to request.


         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 .

Send it out.

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 later.

Listen to events for response.

These three are the most widely used:


       load

– when the result is ready, that includes HTTP errors like 404.


       error

– when the request couldn’t be made, e.g. network down or invalid URL.


       progress

– triggers periodically during the download, reports how much downloaded.

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}`); 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`); // responseText 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 properties of the request object:


     status

HTTP status code (a number):

and so on, can be

in case of a non-HTTP failure.


     statusText

HTTP status message (a string): usually

OK

for


     Not Found

for


     Forbidden

for

and so on.


     response

(old scripts may use


     responseText

)

The server response.

If we changed our mind, we can terminate the request at any time. The call to xhr.abort() does that:

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

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:

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

In the old scripts you may also find xhr.responseText and even xhr.responseXML properties.

They exist for historical reasons, to get either a string or XML document. Nowadays, we should set the format in xhr.responseType and get xhr.response as demonstrated above.

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:

You can find readystatechange listeners in really old code, for historical reasons.

Nowadays, load/error/progress handlers deprecate it.

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:
            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:
getResponseHeader(name)
Gets the response header with the given name (except Set-Cookie and Set-Cookie2).
For instance:
      
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):
  .reduce((result, current) => {
    let [name, value] = current.split(': ');
    result[name] = value;
    return result;
  }, {});

Create it, optionally from a form, append more fields if needed, and then:


    xhr.open('POST', ...)

– use


    POST

method.


    xhr.send(formData)

to submit the form to the server.

For instance:

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:

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 everything, including Blob and BufferSource objects.

Upload progress

The progress event only works on the downloading stage.

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

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

There’s another object xhr.upload , without methods, exclusively for upload events.

Here’s the list:


    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:
  // 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-domain 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 :

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


    responseText


    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).