The try...catch statement allows you to test a block of code for errors.




JavaScript - Catching Errors

When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page.


The try...catch Statement

The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.

Syntax
Code:
try
  {
  //Run some code here
  }
catch(err)
  {
  //Handle errors here
  }
Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error!

Examples
The example below is supposed to alert "Welcome guest!" when the button is clicked. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs. The catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:
Example
Code:
<html>
<head>
<script type="text/javascript">
var txt="";
function message()
{
try
      {
      adddlert("Welcome guest!");
      }
catch(err)
      {
      txt="There was an error on this page.\n\n";
      txt+="Error description: " + err.description + "\n\n";
      txt+="Click OK to continue.\n\n";
      alert(txt);
      }
}
</script>
</head>

<body>
<input type="button" value="View message" onclick="message()" />
</body>

</html>
Try it yourself

The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:
Example
Code:
<html>
<head>
<script type="text/javascript">
var txt="";
function message()
{
try
      {
      adddlert("Welcome guest!");
      }
catch(err)
      {
      txt="There was an error on this page.\n\n";
      txt+="Click OK to continue viewing this page,\n";
      txt+="or Cancel to return to the home page.\n\n";
      if(!confirm(txt))
            {
            document.location.href="http://www.w3schools.com/";
            }
      }
}
</script>
</head>

<body>
<input type="button" value="View message" onclick="message()" />
</body>

</html>
Try it yourself


The throw Statement

The throw statement can be used together with the try...catch statement, to create an exception for the error.