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JavaScript try/catch/finally Statement

❮ JavaScript Statements Reference

Example

In this example, we have made a typo in the code (in the try block).

The example is supposed to alert "Welcome guest!", but alert is misspelled.

The catch block catches the error, and executes a code to handle it:

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

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

</body>
</html>
Try it Yourself »

More "Try it Yourself" examples below.


Definition and Usage

The try/catch/finally statement handles some or all of the errors that may occur in a block of code, while still running code.

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

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

Note: The catch and finally statements are both optional, but you need to use one of them (if not both) while using the try statement.

Tip: When an error occurs, JavaScript will normally stop, and generate an error message. Use the throw statement to create a custom error (throw an exception). If you use throw together with try and catch, you can control program flow and generate custom error messages.

For more information about JavaScript errors, read out JavaScript Errors Tutorial.


Browser Support

Statement
try/catch/finally Yes Yes Yes Yes Yes

Syntax

try {
  tryCode - Block of code to try
}
catch(err) {
  catchCode - Block of code to handle errors
}
finally {
  finallyCode - Block of code to be executed regardless of the try / catch result
}

Parameter Values

Parameter Description
tryCode Required. Block of code to be tested for errors while it is being executed
err Required if used with catch. Specifies a local variable that refers to the error. The variable can refer to the Error object (contains information about the occurred error, like the message "'addlert' is not defined"). If the exception was created by the throw statement, the variable refers to the object specified in the throw statement (see "More Examples")
catchCode Optional. Block of code to be executed, if an error occurs in the try block. If no error occurs, this block of code is never executed
finallyCode Optional. Block of code to be executed regardless of the try / catch result


Technical Details

JavaScript Version: ECMAScript 3

More Examples

Example

This example examines input. If the value is wrong, an exception (err) is thrown.

The exception (err) is caught by the catch statement and a custom error message is displayed:

<!DOCTYPE html>
<html>
<body>

<p>Please input a number between 5 and 10:</p>

<input id="demo" type="text">
<button type="button" onclick="myFunction()">Test Input</button>
<p id="message"></p>

<script>
function myFunction() {
  var message, x;
  message = document.getElementById("message");
  message.innerHTML = "";
  x = document.getElementById("demo").value;
  try {
    if(x == "") throw "is Empty";
    if(isNaN(x)) throw "not a number";
    if(x > 10) throw "too high";
    if(x < 5) throw "too low";
  }
  catch(err) {
    message.innerHTML = "Input " + err;
  }
}
</script>

</body>
</html>
Try it Yourself »

Example

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

function myFunction()
  var message, x;
  message = document.getElementById("message");
  message.innerHTML = "";
  x = document.getElementById("demo").value;
  try {
    if(x == "") throw "Empty";
    if(isNaN(x)) throw "Not a number";
    if(x > 10) throw "Too high";
    if(x < 5) throw "Too low";
  }
  catch(err) {
    message.innerHTML = "Error: " + err + ".";
  }
  finally {
    document.getElementById("demo").value = "";
  }
}
Try it Yourself »

Related Pages

JavaScript Tutorial: JavaScript Errors

JavaScript Reference: JavaScript throw Statement


❮ JavaScript Statements Reference