JavaScript Exception Handling - Raising The Bar (
Page 7 of 8 )
Thus far, you've been working with JavaScript's built-in exceptions, which
can handle most logical or syntactical errors. However, JavaScript also allows
you to get creative with exceptions, by generating your own custom exceptions
if the need arises.
This is accomplished via JavaScript's "throw" statement, which is
used to raise errors which can be detected and resolved by the "try"
family of exception handlers. The "throw" statement needs to be passed
an error type. When the exception is raised, this exception name and description
will be made available to the defined exception handler.
Let's go to a quick example. In the following piece of code, if the value entered
into the form field is greater than 99, the code will manually generate a RangeError
exception, which will be caught and displayed appropriately.
<html>
<head>
<script language="JavaScript">
function checkAge()
{
try {
// if incorrect value in form
// generate an error
if (document.forms[0].age.value > 99)
{
throw RangeError;
}
} catch (e) {
// catch all thrown errors and print error type
alert(e.name);
}
}
</script>
</head>
<body>
<form>
<input type="text" name="age" onBlur="checkAge()">
</form>
</body>
</html>
You can also create your own exceptions, via the JavaScript Error constructor.
The following example demonstrates, by creating a custom error type called idiotUserError.
In this case, when you run the script, a new Error object will be created named
idiotUserError, with the message specified in the Error object constructor.
This error can now be thrown using the regular "throw" statement,
as in the example above.
Let's try a more useful example:
<html>
<head>
<script language="JavaScript">
// create new Error objects
badNameError = new Error ("System user name does not match actual user
name"); noNameError = new Error ("System user name cannot be blank");
// create generic exception handler
// to display error message
// you may want to filter down error types
// further inside this and handle each type
// differently
function mainExceptionHandler(e)
{
alert (e.message);
}
// general form validation functions
function validateForm()
{
checkName();
}
// check form values
// if errors, throw appropriate exception
function checkName()
{
try {
if (document.forms[0].username.value == "")
{
throw noNameError;
}
if (document.forms[0].username.value != "john")
{
throw badNameError;
}
} catch (e) {
// any and all errors will go to this handler
mainExceptionHandler(e);
}
}
</script>
</head>
<body>
<form onSubmit="validateForm()">
Enter your system username: <input type="text" name="username">
<br> <input type="submit" name="submit" value="Go">
</form>
</body>
</html>
In this case, if the value entered into the form is blank or not "john",
a custom exception will be thrown by the code. This custom exception can be
caught by a generic exception handler such as the one used above, and the exception
can be routed and resolved appropriately
/ 206 
