AJAX and PHP

Chapter 2
Figure 2.6: The Four HTTP Request Status Codes
Don't worry if your browser doesn't display exactly the same message. Some
XMLHttpRequest implementations simply ignore some status codes. Opera, for example,
will only fire the event for status codes 3 and 4. Internet Explorer will report status codes
2, 3, and 4 when using a more recent XMLHttp version.
What Just Happened?
To understand the exact flow of execution, let's start from where the processing begins—the
async.html file:
AJAX Foundations: Using XMLHttpRequest
This bit of code hides some interesting functionality. First, it references the async.js file, the
moment at which the code in that file is parsed. Note that the code residing in JavaScript functions
does not execute automatically, but the rest of the code does. All the code in our JavaScript file is
packaged as functions, except one line:
// holds an instance of XMLHttpRequest
var xmlHttp = createXmlHttpRequestObject();
This way we ensure that the xmlHttp variable contains an XMLHttpRequest instance right from the
start. The XMLHttpRequest instance is created by calling the createXmlHttpRequestObject
function that you encountered a bit earlier.
The process() method gets executed when the onload event fires. The process() method can
rely on the xmlHttp object being already initialized, so it only focuses on initializing a server
request. The proper error-handling sequence is used to guard against potential problems. The code
that initiates the server request is:
// initiate reading the async.txt file from the server
xmlHttp.open("GET", "async.txt", true);
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