How to Throw an Exception in JavaScript

If you've ever want to "throw" a tantrum while writing a script with the JavaScript programming language, it's perfectly understandable. Unfortunately, tantrums don't work well in JavaScript. You can certainly throw an exception, however, as long as you know where and why it's appropriate to do so.

Things You'll Need

• JavaScript editor
• Web browser

Instructions

Throw an Exception in JavaScript

1. Understand what throwing an exception in JavaScript really is: another way of controlling how a JavaScript responds to data. An exceptions tells the web browser, "Hey--if a certain thing happens on this web page, then follow my command." You can "throw" all kinds of "exceptions," including errors, text, number, Boolean values and even objects.

2. Decide what kind of exception you want to throw. In the examples below, the exception will be an error named "whoops."

3. A response to something is needed so that the example script will have something to call an error. So go on and create a variable that stores a response. Example: X = prompt("Enter a number less than 5:",""). This example asks a visitor to enter a number less than 5 in a prompt box. If the answer is greater than 5, then the example script will have something to throw an exception (or error) at.

4. Decide when you want to throw the exception. Example: try
   }
   if(x>5)
   throw "whoops"
   

5. Catch the error if it occurs and store it in a new variable. Example: catch(wronganswer)
   

6. Tell the script how to respond to a caught error. Example: if(wronganswer == "whoops")
   alert("BZZZZzzzzZZZZTT!")
   

7. Study the following example that demonstrates throwing an exception in JavaScript. It will display "BZZZZzzzzZZZZTT" in an alert box if the prompt box response is greater than 5.
   x = prompt("Enter a number less than 5:","")
   try
   {
   if(x>5)
   throw "whoops"
   }
   catch(wronganswer)
   {
   if(wronganswer=="whoops")alert("BZZZZzzzzZZZZTT!")
   }
   
   

8. Copy the code above and paste it onto your web pages. You can simply replace the variables in the sample code with your own values.

Tips & Warnings

• A "throw" must be written in lowercase only. Uppercase throws will generate errors.