Learning JavaScript

The Ajax Object: XMLHttpRequest and IE’s ActiveX Objects

Microsoft was the first company to implement XMLHttpRequest as an ActiveX object. Mozilla followed with a direct implementation of XMLHttpRequest, and other companies have responded with their own browsers: Apple and Safari, Netscape and Navigator, and Opera. Though the constructor for the objects differs between the two formats, each shares the same functionality and methods. Once the initial object is created and assigned a variable, the one cross-browser issue is resolved. But taking care of this issue isn’t as simple as it first looks.

Object, Object, Who Has the Object?

Example 13-1 demonstrates one way to create an XMLHttpRequest object: using a conditional statement and testing for its existence. If it doesn’t exist, the object is created as an ActiveXObject; it passes in the progID (program ID) of the ActiveX object—in this case, Microsoft.XMLHTTP. However, a possible problem with this is that the object used in the ActiveXObject method call may differ from machine to machine. Among the various versions of the object could be MSXML2.XMLHttp, MSXML2.XMLHttp.3.0, MSXML2.XMLHttp.4.0, etc.

You can try to resolve every version of the XMLHttp object, but most Ajax libraries and applications focus on just two: the older Microsoft.XMLHttp, and the base version of the newer MSXML2.XMLHttp. In addition, since Microsoft throws errors if it attempts to create an ActiveX object that doesn’t exist, developers use this to ...