In any programming language, your code will throw a run-time exception when trying to execute an illegal command. One of the most trivial illegal operations you may attempt is accessing a null object. Your operating system will definitely complain when trying to access a null object. Accessing an array element that is out of the array's legal range is another example.

More often than not, you want to avoid system errors. System messages are usually cryptic and do not make sense for the average user. In fact, system messages are bad for your reputation as a programmer, as they are clear and tangible evidence of your bugs. Naturally, you'll want to avoid these messages by checking for them in your code, before they hit the operating system. Checking for exceptions in your code will surely make it more cumbersome. Mingling error-checking with the normal flow of your code is inevitable when the language does not support exception handling. The following skeleton code checks for two errors: accessing a null object and out of range array element (the code is just for making a point, do not try compiling with jsc):


var newObject = createObject(dataPiece1, dataPiece2, dataPiece3);
if (newObject == null) {
// do something here when the object was not created
...
}
else {
// getElementIndex return an index or -1 for an error
var index = newObject.getElementIndex();
if (index == -1) {
// handle the error case
...
}
else {
// finally, you can do something with index
...
}
}

ripped: http://www.webreference.com/js/tips/020912.html
(please leave link when posting articles)

The example code is just error handling - nothing to do with exceptions, which are handled with try...catch blocks in JavaScript.