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JavaScript Try-Catch Statement

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The try...catch statement marks a block of statements to try, and specifies one or more responses should an exception be thrown. If an exception is thrown, the try...catch statement catches it.

A D V E R T I S E M E N T

The try...catch statement allows you to test a block of code for there is error or not.



JavaScript - Catching Errors

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

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

There are two ways of catching errors in a Webpage:

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



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
}


try...catch is written in lowercase letters in the Java Script. Using uppercase letters will generate a Java Script error!


Example:1

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

<html>
<head>
<script type="text/javascript">
function message()
{
adddlert("Welcome Vyom!")
}
</script>
</head>

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

</html>


To take more appropriate action when an error occurs, you can add a try...catch statement.

The example below contains the "Welcome Vyom!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, 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:

<html>
<head>
<script type="text/javascript">
var txt=""
function message()
{
try
{
adddlert("Welcome Vyom!")
}
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>


Example 2

The next example given bellow uses a confirm box to display a custom message telling visiters 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:

<html>
<head>
<script type="text/javascript">
var txt=""
function message()
{
try
{
adddlert("Welcome Vyom!")
}
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.bestebooksworld. com/"
}
}
}
</script>
</head>

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

</html>


The onerror Event

The onerror event can be explained soon, but first you will learn how to use the throw statements to create an exception. The throw statements can be used together with the try...catch statement.


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Keywords:javascript try catch error message, javascript try catch languages, javascript try catch ajax, javascript try catch user

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