Beginning Ajax With ASP.NET (2006).pdf

The XMLHttpRequest Object
}
// Execute the request
xmlHttpObj.send(null);
}
}
Here, you have opted to define the XMLHttpRequest object as a “global” variable xmlHttpObj, rather
than redeclaring and creating this object in each function. The function to assign a valid XMLHttpRequest
instance to the variable is separated into its own discrete function. This might typically be included as
part of your common script library rather than having to rewrite this in every page.
Using a “global” object to hold a reference to your XMLHttpRequest object can expose your clientside
application to a potential bug or flaw in its operation if a subsequent request is issued using the
same XMLHttpRequest object before the first request has completed.
In addition, since the LoadCustomers() function is executed as part of the load event of the page, and the
page is not usable until this function has executed, you make the server call in a synchronous manner using
the false parameter, as shown in the line below:
// We want this request synchronous
xmlHttpObj.open(“GET”,”http://” + location.host + “/XmlHttp_Chap4/DataFile.xml”,
false);
You execute the call, and then you check the status of the call by examining the status property of the
XMLHttpRequest object. This property contains the standard HTTP status code returned by the server as a
result of the request being made. In this example, you check to ensure that a status code of 200 was returned
as part of the call:
// If the request was ok (ie. equal to a Http Status code of 200)
if (xmlHttpObj.status == 200)
{
// rest of function . . .
The status code of 200 returned from the server indicates a successful server request has been made.
Because you have defined this literal value in the script include file, the code would read:
// If the request was ok (ie. equal to a Http Status code of 200)
if (xmlHttpObj.status == HTTPSTATUS_OK)
{
// rest of function . . .
For a list of all the valid HTTP status codes defined by the W3C, see www.w3.org/Protocols/
rfc2616/rfc2616-sec10.html.
You then use the XML DOM method selectNodes to execute an X Path expression over the XML data
to find each Lastname node for each Customer node and return a list of matching nodes:
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