Can I define custom types for user-defined exceptions in JavaScript? If so, how would I do it?
Use the throw statement.
JavaScript doesn't care what the exception type is (as Java does). JavaScript just notices, there's an exception and when you catch it, you can "look" what the exception "says".
If you have different exception types you have to throw, I'd suggest to use variables which contain the string/object of the exception i.e. message. Where you need it use "throw myException" and in the catch, compare the caught exception to myException.