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Exception Handling In Javascript

In this article, we will discuss how to handle exceptions in JavaScript.

What Are Exceptions?

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

Benefits Of Handling Exceptions

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

Syntax

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

Try Catch In Javascript

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 the 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: -

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

Try It

Here aa is not a valid element, so the following exception will appear when you execute the code above.

Picture showing the alert box showing the exception caught in JavaScript

Throw In Javascript

The throw keyword is used to throw an exception explicitly.

Syntax

throw [Exception_Message];

Here Exception_Message is a string message. Whenever a throw statement is encountered the code post the statement does not execute. You can see the workflow in the picture below.

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

Consider the following code: -

<script>
    try {
        alert('Before Throw');
    throw 'Error';
    alert('After Throw');
}
    catch (e) {
        alert(e);
}
</script>

Try It

If we check the output of the above script in the browser then you will see that 2 alerts would be triggered i.e., Before Throw and Error. The code that is written after the throw i.e., After throw alert will not be displayed.

Posted By -	Karan Gupta

Posted On -	Thursday, July 19, 2012

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