Ajax - Beginning JavaScript - page 497

{
var oHttp = new XMLHttpRequest();
return oHttp;
}
else if (window.ActiveXObject)
{
var versions =
[
“MSXML2.XmlHttp.6.0”,
“MSXML2.XmlHttp.3.0”
];
for (var i = 0; i < versions.length; i++)
{
try
{
var oHttp = new ActiveXObject(versions[i]);
return oHttp;
}
catch (error)
{
//do nothing here
}
}
}
return null;
}
This new code fi rst checks to see if window.XMLHttpRequest exists. If it does, then the function creates
an XMLHttpRequest object with the XMLHttpRequest constructor. If not, the code checks for window
.ActiveXObject for IE 5 and 6 and tries to create an object with the latest XMLHttp version. If no
XMLHttpRequest object can be created any browser, then the function returns null .
The order in which browsers are tested is important; test for window.XMLHttpRequest fi rst because
IE 7+ supports both window.XMLHttpRequest and window.ActiveXObject .
Regardless of the user's browser, if it supports XMLHttpRequest , this revised function creates an
XMLHttpRequest object.
Using the XMLHttpRequest Object
Once you create the XMLHttpRequest object, you are ready to start requesting data with it. The fi rst step
in this process is to call the open() method to initialize the object.
oHttp.open(requestType, url, async);
This method accepts three arguments. The fi rst, requestType , is a string value consisting of the type
of request to make. The values can be either GET or POST . The second argument is the URL to send the
request to, and the third is a true or false value indicating whether the request should be made in
asynchronous or synchronous mode. For more on synchronous and asynchronous modes, see