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Exception Handling In Javascript

In this article I will explain how to do exception handling in JavaScript.

Software Requirements

Before you start reading this tutorial you should make sure that

Windows any version is installed on your machine.
Notepad or any text editor for writing code.

Prerequisite Knowledge

Before you go ahead, it is required that you should have

Basic knowledge of JavaScript.
Basic knowledge of HTML.
Knowledge of any programming language is an added advantage.

What Are Exceptions?

Exceptions are runtime errors which occur during the execution of the program.

Benefits Of Handling Exceptions

Exception handling block gives information about the exception as well as the point (or line of code) where the exception occurs.

Try Catch In Javascript

Syntax

try {
    //  Write your code here
} catch (e) {
    //  Write error handling code here
}

You can write your code inside the try block. If an exception occurs then code inside the catch block gets executed (skipping the code execution from the line of code where exception occurs till the last line of code inside the try block). The code inside the catch block should be the error handling code ideally like an alert message which informs the user about the error in some user friendly way.

Consider the following code segment

try {
    document.getElementById('aa').value = 'test value';
} catch (e) {
    alert(e);
}

Here aa is not a valid element so when you run the code above, following error will come

Picture showing the alert box showing the exception caught in JavaScript

Throw In Javascript

throw keyword is used to explicitly throw an exception.

Syntax

throw '<Exception_Message>';

Here Exception_Message is a string message. Whenever throw statement is encountered the code post the statement doesn’t execute see the figure below.

Picture showing the cycle of throw keyword in exception handling in JavaScript

Consider the following code

try {
    alert('Before Throw');
    throw 'Error';
    alert('After Throw');
} catch (e) {
    alert(e);
}

Here 2 alerts will prompt ‘Before Throw’ and ‘Error’. The line of code which comes after throw won’t execute.

Posted By -	Karan Gupta

Posted On -	Thursday, July 19, 2012

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