JavaScript Errors - Throwing the Ball (and Exceptions)
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The Throw statement allows you to create an exception. This is useful when trying to generate informative error messages.
Let's say you wanted to create a program that asked a user a question when the page first loaded. If they answer it right, they get to view the page; if not something else happens to them. Here it is in code:
<html>
<body>
<script type="text/javascript">
var name=prompt("Enter your name:","")
try
{
if(name == "James")
throw "FirstError"
else if(name == "Houston")
throw "SecondError"
else if(isNaN(name))
throw "ThirdError"
}
catch(er)
{
if(er=="FirstError")
alert("Alert! Alert! James is too heavy for the page to load!")
if(er == "SecondError")
alert("Alert! Alert! You may not haxor our bases!")
if(er == "ThirdError")
alert("Greetings...we've been expecting you.")
}
</script>
</body>
</html>
This code displays a prompt asking for the user's name. Depending on the name entered, a different message will occur prior to the page loading. If the user types in James, it throws the FirstError exception, and the following alert box will appear:
If the user types in Houston, it triggers the SecondError exception, and this alert is displayed:
And lastly, if the user enter anything else (even if they type in james in lowercase), the ThirdError exception is triggered and this alert will appear:
Note that the word throw must be written in lowercase or you will receive Javascript errors, which of course is what we are trying to avoid.
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