AJAX and XMLHttpRequest

This article is part of my personal wiki where I write personal notes while I am learning new technologies. You are welcome to use it for your own learning!


The XMLHttpRequest is a web api that allows you to send/receive HTTP requests from the browser to the server. Sending an HTTP request with XMLHttpRequest consists on these steps:

  1. Instantiate an XMLHttpRequest object
  2. Open a URL
  3. Optionally configure the XMLHttpRequest object and add event handlers for asynchronous HTTP communication. It is better to configure after the XHR is open because some options only work when the XHR is open.
  4. Send the request
var result;
// 1. Instantiate object
var xhr = new XMLHttpRequest();
// 2. open url"get", "", /* async */ true);
// 3. configure and add event handlers
xhr.onreadystatechange = function(e){
    if (xhr.readyState === 4){ // DONE
       if (xhr.status === 200){
           result = xhr.response;
       } else {
           console.error("Error response: ", xhr.status);
xhr.ontimeout = function(e){
    console.error("Request timed-out");
// 4. send request

The open method allows you to provide some basic configuration for the XHR request:

open(method, url, async, user, password);

The send method lets you send the request with or without data (depending on the type of request):


Example: Using XHR to get some JSON

See this jsFiddle:

<button id="get-repos">Get Vintharas Repos! in JSON!!</button>
<pre id="response"></pre>
    console.log('loading event handlers');
    var $code = document.getElementById("response");
    var $getReposBtn = document.getElementById("get-repos");
    $getReposBtn.onclick = function(){
        var xhr = new XMLHttpRequest();
        xhr.timeout = 2000;
        xhr.onreadystatechange = function(e){
            if (xhr.readyState === 4){
                if (xhr.status === 200){
                    $code.innerHTML = xhr.response;
                } else {
                    console.error("XHR didn't work: ", xhr.status);
        xhr.ontimeout = function (){
            console.error("request timedout: ", xhr);
        }"get", "", /*async*/ true);

XMLHttpRequest properties

readyState get current state of the XHR object
response get the response returned from the server according to responseType
responseBody gets the response body as an array of bytes
responseText gets the response body as a string
responseType gets/sets the data type associated with the response, such as blob, text, array-buffer, json, document, etc. By default it is an empty string that denotes that the response type is string. This is actually only used when deserializing the body of a response, it doesn't affect the HTTP accept headers sent to the server. The way it works is, for instance, that if you set "json", you can get javascript objects directly from the "xhr.response" property, whereas if you leave it empty or use "text" you get a text version of the json response in the "xhr.response" property.
responseXML gets the response body as an XML DOM object
status gets the HTTP Status code of the response
statusText Gets the friendly HTTP status code text
timeout Sets the timeout threshold for the requests
withCredentials specifies whether the request should include user credentials

XMLHttpRequest methods

abort cancel current request
getAllResponseHeaders get a complete list of the response headers
getResponseHeader get specific response header
send make the http request and receive the response
setRequestHeader add a HTTP header to the request
open set the properties for the request such as URL, username and password

XMLHttpRequest events

  • ontimeout: lets you handle a timeout (having configured the timeout in the XMLTHttpRequest object)
  • onreadystatechange: lets you handle the event of the state of the XMLHttpRequest changing within these states: [UNSENT, OPENED, HEADERS_RECEIVED, LOADING, DONE]. It is used in async calls.
  • upload: helps you track an ongoing upload

AJAX and jQuery

jQuery provides simplified wrappers around the XMLHttpRequest object: $.ajax(), $.get(), $.post(), $.getJSON(), etc.

$.ajax method

The $.ajax method allows you to make async HTTP requests:

$.ajax(url [, settings])
$.ajax([settings]) // all settings are optional, you can set defaults via $.ajaxSetup()

A common GET request with $.ajax could be:

var $response = document.getElementById("response");
            $response.innerHTML = JSON.stringify(data,null,2);

As you can see the $.ajax method returns a jqXHR object with the Promise interface: done (success), fail (error), always (complete) and then method usually available to promises.


The $.get method is a simplification of the $.ajax method to handle purely get requests:

$.get(url [, data] [, success] [,dataType])

// it is equivalent to

    url: url,
    data: data,
    success: sucess,
    dataType: dataType


  • url is the url to which the request is sent
  • data is an object or string to send to the server
  • success is a success callback
  • dataType is the type of data expected from the server (xml, json, script or html)

This method also returns a jqXHR object that acts as a promise.


The $.getJSON method is a shorthand for:

    dataType: "json",
    url: url,
    data: data,
    success: success

// it looks like this

$.getJSON(url [, data] [, success])


Sending Data to a server using a Form

Serializing a Form Data With jQuery