It’s necessary to write a function for creating new XMLHTTPRequest objects because Microsoft
browsers use two different methods of creating them, while all other
major browsers use a third. By writing a function to test the browser
in use, you can ensure that your code will work on all major
browsers.
The purpose of the try...catch construct is to set an error
trap for the code inside the try
statement. If the code causes an error, the catch section will be executed instead of a
general error being issued.
An XMLHTTPRequest object has
six properties and six methods (see Tables 17-1 and 17-2).
You can tell that an Ajax call has completed when the readyState property of an object has a value
of 4.
When an Ajax call successfully completes, the object’s status property will have a value of
200.
The responseText property of
an XMLHTTPRequest object contains
the value returned by a successful Ajax call.
The responseXML property of
an XMLHTTPRequest object contains a
DOM tree created from the XML returned by a successful Ajax
call.
To specify a callback function to handle Ajax responses, assign
the function name to the XMLHTTPRequest object’s onreadystatechange property. You can also
use an unnamed, inline function.
To initiate an Ajax request, an XMLHTTPRequest object’s send method is called.
The main differences between Ajax GET and POST requests are that GET requests append the data to the URL,
while POST requests instead pass
the data as a parameter of the send
method and require the correct form headers to be sent first.