Detect Browser Compatibility with the Request Object

Wrap the compatibility check inside a JavaScript function, then call this function before you make any HTTP requests using the object. For example, in Mozilla-based browsers such as Netscape 7.1 and Firefox 1.5 (as well as in Safari 2.0 and Opera 8.5), the request object is available as a property of the top-levelwindowobject. The reference to this object in JavaScript code iswindow.XMLHttpRequest. The compatibility check for these browser types looks like this:

  if(window.XMLHttpRequest){
      request = new XMLHttpRequest();
  request.onreadystatechange=handleResponse;
      request.open("GET",theURL,true);
      request.send(null);
  }

The JavaScript variablerequest is to a top-level variable that will refer to the request object.

As an alternative model, the open-source library Prototype uses object-oriented JavaScript to wrap the request object into its own object, as in the object Ajax.Request (see Chapter 6).

If the browser supportsXMLHttpRequest, then:

1. if(window.XMLHttpRequest)returnstruebecause theXMLHttpRequestis notnullorundefined.
   
2. The object will be instantiated with thenew keyword.
   
3. Itsonreadystatechangeevent listener (see the section “XMLHttpRequest” earlier in this chapter) will be defined as a function namedhandleResponse(). 
   
4. The code calls the request object’sopen()andsend()methods.

What about Internet Explorer users?

Microsoft Internet Explorer–related blogs mentioned, at the time this book went to publication, that IE 7 would support a native XMLHttpRequest object.