JavaScript: The Definitive Guide, 6th Edition

Using XMLHttpRequest

Browsers define their HTTP API on an XMLHttpRequest class. Each instance of this class represents a single request/response pair, and the properties and methods of the object allow you to specify request details and extract response data. XMLHttpRequest has been supported by web browsers for many years, and the API is in the final stages of standardization through the W3C. At the same time, the W3C is working on a draft “XMLHttpRequest Level 2” standard. This section covers the core XMLHttpRequest API and also those portions of the Level 2 draft (which I’ll call XHR2) that are currently implemented by at least two browsers.

The first thing you must do to use this HTTP API, of course, is to instantiate an XMLHttpRequest object:

var request = new XMLHttpRequest();

You can also reuse an existing XMLHttpRequest object, but note that doing so will abort any request pending through that object.

XMLHttpRequest in IE6

Microsoft introduced the XMLHttpRequest object to the world in IE5, and in IE5 and IE6 it is available only as an ActiveX object. The now-standard XMLHttpRequest() constructor is not supported before IE7, but it can be emulated like this:

// Emulate the XMLHttpRequest() constructor in IE5 and IE6
if (window.XMLHttpRequest === undefined) {
    window.XMLHttpRequest = function() {
        try {
            // Use the latest version of the ActiveX object if available
            return new ActiveXObject("Msxml2.XMLHTTP.6.0");
        }
        catch (e1) {
            try {
                // Otherwise fall back on an older version
                return ...