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Welcome to Quality Research

Try...Catch Statement

Test a block of code for errors.
Try block contains the code to be run,
Catch block contains the code to be executed if an error occurs.
try...catch is in lowercase letters.

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:

<html>

<head>

<script type="text/JavaScript">

function message(){adddlert("Welcome guest!");}</script></head>

<body>

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

</html>

Catch block catches the error and displays a custom error message informing user what happened

<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>

Example 2:

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:

<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>

Throw Statement

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.
Throw(exception). Exception can be a string, integer, Boolean or an object.

Example 1

If value of x is higher than 10 or lower than 0 we are going to throw an error.
Error is 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>