Answers to Exercises - Beginning JavaScript - page 645

if (tempRequest.status == 200)
{
fpCallback(tempRequest.responseText);
}
else
{
alert(“An error occurred while attempting to “ +
“contact the server.”);
}
}
}
this.request.onreadystatechange = request_readystatechange;
}
this.request.send(null);
if (!this.async)
{
this.callBack(this.request.responseText);
}
}
It's possible to add synchronous communication to your HttpRequest module in a variety of ways.
The approach in this solution refactors the code to accommodate a new property called async , which
contains either true or false . If it contains true , then the underlying XMLHttpRequest object uses
asynchronous communication to retrieve the fi le. If false , the module uses synchronous communica-
tion. In short, this property resembles an XML DOM's async property for determining how an XML
document is loaded.
The fi rst change made to the module is in the constructor itself. The original constructor initializes
and readies the XMLHttpRequest object to send data. This will not do for this new version, however.
Instead, the constructor merely initializes all the properties.
function HttpRequest(sUrl, fpCallback)
{
this.url = sUrl;
this.callBack = fpCallback;
this.async = true;
this.request = this.createXmlHttpRequest();
}
There are three new properties here. The fi rst, url , contains the URL that the XMLHttpRequest object
should attempt to request from the server. The callBack property contains a reference to the callback
function, and the async property determines the type of communication the XMLHttpRequest object
uses. Setting async to true in the constructor gives the property a default value. Therefore, you can
send the request in asynchronous mode without setting the property externally.
The new constructor and properties are actually desirable, as they enable you to reuse the same
HttpRequest object for multiple requests. If you wanted to make a request to a different URL, all