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XMLHttpRequest is a Javascript object that was created by Microsoft and adopted by Mozilla. You can use it to easily retrieve data via HTTP. Despite its name, it can be used for more than just XML documents.


Basic Usage

Using XMLHttpRequest is very simple. You create an instance of the object, open a URL, and send the request. The HTTP status code of the result, as well as the result document are available in the request object afterwards.


req = new XMLHttpRequest();'GET', '', false); 
if(req.status == 200)

Note that this example works synchronously, so it will block the user interface if you call this from your javascript. You should not use this in practice.

Asynchronous Usage

If you intend to use XMLHttpRequest from an extension, you should let it load asynchronously. In asynchronous usage, you get a callback when the data has been received, which lets the browser continue to work as normal while your request is happening.


req = new XMLHttpRequest();'GET', '', true);
req.onreadystatechange = function (aEvt) {
  if (req.readyState == 4) {
     if(req.status == 200)
      dump("Error loading page\n");

Other Properties and Methods

In addition to the properties and methods shown above, there are other useful properties and methods on the request object.


If you load an XML document, the responseXML property will contain the document as an XmlDocument object that you can manipulate using DOM methods.


This method can be used to set a HTTP header on the request before you send it.

req = new XMLHttpRequest();'GET', '', true);
req.setRequestHeader("X-Foo", "Bar");


This method can be used to get a HTTP header from the server response.

req = new XMLHttpRequest();'GET', '', true);
dump("Content-Type: " + req.getResponseHeader("Content-Type") + "\n");


  1. XULPlanet documentation <li>Microsoft documentation <li>"Using the XMLHttpRequest Object" ( </ol>