JavaScript Exception Handling - Being Verbose (
Page 4 of 8 )
Now, what you just saw was a very primitive exception handler, albeit one which
isn't very useful on a production site. Ideally, you'd want the message generated
by the exception handler to be a little more descriptive, and to contain some
information on the source of the error.
JavaScript satisfies this requirement via two special, pre-defined properties
of the Error object generated by an exception - the "name" and "message"
properties. Take a look:
<script language="JavaScript">
try {
colours[2] = "red";
} catch (e) {
alert("An exception occurred in the script. Error name: " + e.name
+ ". Error message: " + e.message); }
</script>
Here's the output:
An exception occurred in the script. Error name: TypeError. Error message: 'colours' is undefined
Don't want to use the alert() box because it's disruptive and annoying? Write
the output directly to the page itself with the writeln() method instead, as
below:
It's also possible to write an exception handler to ignore all errors generated
in your script - the following example demonstrates:
<script language="JavaScript">
try {
// generate an error
eval("some gibberish")
} catch (e) {
// ignore it
}
</script>
In this case, it doesn't matter what type of exception JavaScript generates
- the generic handler will catch it, ignore it and continue to process the rest
of the script.
It should be noted, however, that this approach, although extremely simple,
is *not* recommended for general use. It is poor programming practice to trap
all errors, regardless of type, and ignore them; it is far better - and more
professional - to anticipate the likely errors ahead of time, and use the "try-catch"
construct to isolate and resolve them.
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