This article guides you through the AJAX basics and gives you two simple hands-on examples to get you started.
Tutorial Details:
What's AJAX?
AJAX (Asynchronous JavaScript and XML) is a newly coined term for two powerful browser features that have been around for years, but were overlooked by many web developers until recently when applications such as Gmail, Google suggest, and Google Maps hit the streets.
The two features in question are that you can:
Make requests to the server without reloading the page
Parse and work with XML documents
Step 1 – say "Please!" or How to Make an HTTP Request
In order to make an HTTP request to the server using JavaScript, you need an instance of a class that provides you this functionality. Such a class was originally introduced in Internet Explorer as an ActiveX object, called XMLHTTP. Then Mozilla, Safari and other browsers followed, implementing an XMLHttpRequest class that supports the methods and properties of Microsoft's original ActiveX object.
As a result, in order to create a cross-browser instance (object) of the required class, you can do:
if (window.XMLHttpRequest) { // Mozilla, Safari, ...
http_request = new XMLHttpRequest();
} else if (window.ActiveXObject) { // IE
http_request = new ActiveXObject("Microsoft.XMLHTTP");
}
(For illustration purposes, the above is a bit simplified version of the code to be used for creating an XMLHTTP instance. For a more real-life example, see step 3 of this article.)
Step 2 – "There you go!" or Handling the Server ResponseRemember that when you were sending the request, you provided the name of a JavaScript function that is designed to handle the response.
http_request.onreadystatechange = nameOfTheFunction;
Let's see what this function should do. First, the function needs to check for the state of the request. If the state has the value of 4, that means that the full server response is received and it's OK for you to continue processing it.
if (http_request.readyState == 4) {
// everything is good, the response is received
} else {
// still not ready
}
Step 3 – "All together now!" - A Simple ExampleLet's put it all together and do a simple HTTP request. Our JavaScript will request an HTML document, test.html, which contains the text "I'm a test." and then we'll alert() the contents of the test.html file.
Step 4 – "The X-Files" or Working with the XML ResponseIn the previous example, after the response to the HTTP request was received, we used the reponseText property of the request object and it contained the contents of the test.html file. Now let's try the responseXML property.
First off, let's create a valid XML document that we'll request later on. The document (test.xml) contains the following:
I'm a test.
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