Javascript - Try Catch, Throw

Note: many of the contents of this page are taken from w3school website.

JavaScript Try...Catch Statement

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.

This chapter will teach you how to trap and handle JavaScript error messages, so you don't lose your audience.

There are two ways of catching errors in a Web page:

By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6)
By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3)

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

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!

Example 1

The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:

Example script

<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>

The HTML:

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

JavaScript Throw Statement

The throw statement allows you to create an exception.

The Throw Statement

The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.

Syntax

throw(exception)

The exception can be a string, integer, Boolean or an object.

Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!

Example 1

The example below determines the value of a variable called x. If the value of x is higher than 10 or lower than 0 we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:

<html>
<body>
<script type="text/javascript">
var x=prompt("Enter a number between 0 and 10:","")
try
{
  if(x>10)
    throw "Err1"
  else if(x<0)
    throw "Err2"
}
catch(er)
{
  if(er=="Err1")
    alert("Error! The value is too high")
  if(er == "Err2")
    alert("Error! The value is too low")
}
</script>
</body>
</html>

JS - Try Catch, Throw