Try…. Catch Statement, Throw Statement

The try statement lets you test a block of code for errors.

The catch statement lets you handle the error.

The throw statement lets you create custom errors.

The finally statement lets you execute code, after try and catch, regardless of the result.

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Errors Will Happen!

When executing JavaScript code, different errors can occur.

Errors can be coding errors made by the programmer, errors due to wrong input, and other unforeseeable things.

Example

In this example we have written alert as adddlert to deliberately produce an error:

<p id="demo"></p>

<script>
try {
    adddlert("Welcome guest!");
}
catch(err) {
    document.getElementById("demo").innerHTML = err.message;
}</script>

JavaScript catches adddlert as an error, and executes the catch code to handle it.

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JavaScript try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs:

try {
    Block of code to try}
catch(err) {
    Block of code to handle errors}

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JavaScript Throws Errors

When an error occurs, JavaScript will normally stop and generate an error message.

The technical term for this is: JavaScript will  throw an exception (throw an error).

JavaScript will actually create an Error object with two properties: name and message.

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The throw Statement

The throw statement allows you to create a custom error.

Technically you can throw an exception (throw an error).

The exception can be a JavaScript String, a Number, a Boolean or an Object:

throw "Too big";    // throw a textthrow 500;          // throw a number

If you use throw together with try and catch, you can control program flow and generate custom error messages.