JavaScript Errors – Throw and Try to Catch

The try statement lets you test a block of code  for errors.

The catch statement lets you handle the error.

The throw statement lets you create custom  errors.

 Errors Will Happen!

When the JavaScript engine is executing JavaScript code, different errors can  occur:

It can be syntax errors, typically coding errors or typos made by the  programmer.

It can be misspelled or missing features in the language (maybe due to  browser differences).

It can be errors due to wrong input, from a user, or from an Internet server.

And, of course, it can be many other unforeseeable things.

JavaScript Throws Errors

When an error occurs, when something goes wrong, the JavaScript engine will  normally stop, and generate an error message.

The technical term for this is: JavaScript will throw an  error.

JavaScript try and catch

The try statement allows you to define a block of code to be  tested for errors while it is being executed.

The catch statement allows you to define a block of code to  be executed, if an error occurs in the try block.

The JavaScript statements try and catch come in pairs.

Syntax

try   {   //Run some code here   } catch(err)   {   //Handle errors here   }

The Throw Statement

The throw statement allows you to create a custom error.

The correct technical term is to create or throw an exception.

If you use the throw statement together with try and catch, you can control program  flow and generate custom error messages.

Syntaxthrow exception

The exception can be a JavaScript String, a Number, a Boolean or an Object.

Example

This example examines the value of an input variable. If the value is wrong,  an exception (error) is thrown. The error is caught by the catch statement and a custom error message is displayed:

Example

<script>function myFunction(){try  {   var x=document.getElementById("demo").value;  if(x=="")    throw "empty";  if(isNaN(x)) throw "not a number";  if(x>10)     throw "too high";  if(x<5)      throw "too low";  }catch(err)  {  var y=document.getElementById("mess");  y.innerHTML="Error: " + err + ".";  }}</script><h1>My First JavaScript</h1><p>Please input a number between 5 and 10:</p><input id="demo" type="text"><button type="button" onclick="myFunction()">Test Input</button><p id="mess"></p>