JavaScript Exception Handling - Anatomy Of An Exception (
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Normally, when a JavaScript program encounters an error, be it syntactical
or logical, it exits the program at that stage itself with a message indicating
the cause of the error. Now, while this behaviour is acceptable during the development
phase, it cannot continue once a program has been released to actual users.
In these "live" situations, it is unprofessional to display cryptic
error messages (which are usually incomprehensible to non-technical users);
rather, it is more professional to intercept these errors and either resolve
them (if resolution is possible), or notify the user with a clear error message
(if not).
The term "exceptions" refers to those errors which can be tracked
and controlled. For example, if a function attempts an unsupported operation
on a built-in JavaScript object (say, trying to assign values to a non-existent
array index), JavaScript will generate a "TypeError" exception, together
with a message explaining the problem. Exceptions like these can be caught by
the application, and appropriately diverted to an exception-handling routine.
In earlier versions of JavaScript, exception handling was almost non-existent
- developers were stuck with the standard error construct provided by the browser,
and had little or no control over what happened with the JavaScript interpreter
encountered an error. With JavaScript 1.5, all of that changed - the language
now supports standard "try-catch" exception-handling constructs, and
provides a far greater degree of control over how errors are processed.
An example might make this clearer. Consider the following simple line of code,
<script language="JavaScript">
colours[2] = "red";
</script>
In the example above, JavaScript tries to change the value of the second element
of the array variable "colours". Now, this variable does not exist
in the variable namespace. What does JavaScript do? Rather than quietly packing
up and moving to a new neighbourhood, JavaScript decides to generate an exception.
An error has occurred in the script on this page.
Line: 8
Char: 1
Error: 'colours' is undefined
Code: 0
URL: http://localhost/errors.html
You can see this from the first few lines of the output, which describe the
exception, together with the variable that caused it.
From the above output, it's pretty clear what happened - an error occurred
because the "colours" variable doesn't exist. And JavaScript also
provides primitive debugging information on the error, such as the line on which
the errant code resides. A little analysis of this debug data, and you'll quickly
be able to identify the lines of code involved in the error.
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