When browsing Webpages on the internet, we all have seen a Java Script alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message may be useful for developers but not for visiter. When users see errors, they often leave the Webpage.

This Academic Tutorial chapter will teach you how to trap and handle Java Script error messages, so you don't lose your audience.

There are two ways of catching errors in a Webpage:

By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6)

By using the onerror event. This is the old standard solution to catch errors

The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.

try...catch is written in lowercase letters in the Java Script. Using uppercase letters will generate a Java Script error!

The example below contains a Java Script that is supposed to display the message "Welcome Vyom!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:

To take more appropriate action when an error occurs, you can add a try...catch statement.

The example below contains the "Welcome Vyom!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, the catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:

The next example given bellow uses a confirm box to display a custom message telling visiters they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:

The onerror event can be explained soon, but first you will learn how to use the throw statements to create an exception. The throw statements can be used together with the try...catch statement.