Summary

This article looks at the code that creates XMLHttpRequest object on the Yahoo Maps and Yahoo Mail pages. This code has a clever way of iterating through the ActiveX objects on older Microsoft browsers.

Getting the XMLHttpRequest Object from Yahoo Maps

The following code is used on Yahoo Maps to obtain the XMLHttpRequest object on a given web browser.

Code Listing 1. Getting the XMLHttpRequest Object from Yahoo Maps

createXhrObject:function(E)
{
    var D,A;
    try
    {
        A=new XMLHttpRequest();
        D={conn:A,tId:E};
    }
    catch(C)
    {
        for(var B=0;B<this._msxml_progid.length;++B)
        {
            try
            {
                A=new ActiveXObject(this._msxml_progid[B]);
                D={conn:A,tId:E};
                break;
            }
            catch(C)
            {
            }
        }
    }
    finally
    {
        return D;
    }
}

_msxml_progid:["Microsoft.XMLHTTP","MSXML2.XMLHTTP.3.0","MSXML2.XMLHTTP"]

Analyzing the XMLHttpRequest Code used on Yahoo Maps

The following figure shows various sections of the function that creates the XMLHttpRequest object for a given web browser on the Yahoo Maps page.

Figure 1. Analysis of Creating XMLHttpRequest Object on Yahoo Maps

Get the XMLHttpRequest on newer browsers

This code tries first to get the XMLHttpRequest object that is available on all the newer browsers (Internet Explorer 7, FireFox, Safari, and Opera).

var D,A;
try
{
A=new XMLHttpRequest();
D={conn:A,tId:E};
}

Get the XMLHttpRequest on older Microsoft browsers

If the current browser is older versions of Internet Explorer, the following code gets executed. This code has a clever way of using an array (_msxml_progid) to store various names of the XMLHTTP object used on the older IE versions. In newer browsers you can instantiate XMLHttpRequest() object; however in the older IE browsers, you would need to instantiate an ActiveXObject and pass it an XMLHTTP string.

for(var B=0;B<this._msxml_progid.length;++B)
{
    try
    {
        A=new ActiveXObject(this._msxml_progid[B]);
        D={conn:A,tId:E};
        break;
    }
    catch(C)
    {
    }
}

Array of XMLHTTP strings

Various XMLHTTP strings from MSXML are stored in array (outside the above function). This facilitates the creation of ActiveXObject that is appropriate for the version of IE.

_msxml_progid:["Microsoft.XMLHTTP","MSXML2.XMLHTTP.3.0","MSXML2.XMLHTTP"]

Getting the XMLHttpRequest Object from Yahoo Mail

Following is yet another way of getting the XMLHttpRequest object. This time, it’s on Yahoo Mail.

Code Listing 2. Getting the XMLHttpRequest Object from Yahoo Mail

_newHTTP: function(bt)
{
    var ret = null, e;

    try
    {
ret = new XMLHttpRequest();
    }
    catch (e)
    {
        e = ret = null;
    }

if (!ret)
        try
        {
ret = new ActiveXObject(activeX.xmlhttp);
        }
        catch (e)
        {
            ret = null;
        }

return (bt) ? {err:e,msg:"xmlhttp:failure\n"} : ret;
}

xmlhttp:’MSXML2.XMLHTTP.3.0’