javascript

Chapter 14: Error Handling and Debugging
This type of error typically occurs when JavaScript is being dynamically generated by a server - side
component. Many server - side languages automatically insert HTML into the output if a runtime error
occurs, and such output clearly breaks JavaScript syntax. If you ’ re having trouble tracking down a
syntax error, double - check each external JavaScript file to be sure that it doesn ’ t contain HTML inserted
by the server because of an error.
The System Cannot Locate the Resource Specified
Perhaps one of the least useful error messages is “ The system cannot locate the resource
specified. ” This error occurs when JavaScript is used to request a resource by URL and the URL is
longer than IE ’ s maximum URL length of 2083 characters. This URL length limit applies not just to
JavaScript but to IE in general (other browsers do not limit URL length so tightly). There is also a URL
path limit of 2048 characters. The following example causes this error:
function createLongUrl(url){
var s = “?”;
for (var i=0, len=2500; i < len; i++){
s += “a”;
}
}
return url + s;
var x = new XMLHttpRequest();
x.open(“get”, createLongUrl(“http://www.somedomain.com/”), true);
x.send(null);
In this code, the XMLHttpRequest object attempts to make a request to a URL that exceeds the maximum
URL limit. The error occurs when open() is called. One workaround for this type of error is to shorten
the query string necessary for the request to succeed, either by decreasing the size of the named query
string arguments or by eliminating unnecessary data. Another workaround is to change the request to a
POST and send the data as the request body instead of in the query string. Ajax, the XMLHttpRequest
object, and issues such as this are discussed fully in Chapter 17 .
Debugging Tools
JavaScript debugging has come a long way from its roots using alert dialogs. IE, Firefox, and Safari all
support full - featured JavaScript debuggers that include stack information, variable inspection, and
stepping through scripts. As of version 9.5, Opera does not have a JavaScript debugger.
Internet Explorer Debugger
As of version 8, IE ships with a JavaScript debugger as part of its development tools. By default,
JavaScript debugging is turned off, so you must first enable it. To do so, click the Tools menu and go into
Internet Options. Click on the Advanced tab, and make sure that the check box next to “ Disable script
debugging (Internet Explorer) ” is unchecked. Click OK to exit the dialog box.
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