Error Handling in JavaScript - Try-Catch and Throw Statements

Error Handling in JavaScript - Try-Catch and Throw Statements
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Debugging in JavaScript

Now that we have covered almost all the basics of JavaScript including advanced operators, conditional statements, looping constructs, alerts and functions, we move on to exception or error handling in JavaScript. All of us must be familiar with JavaScript errors that either stop the page from loading completely or prompt us to enter the JavaScript Debug console to rectify the error.

These errors occur when the JavaScript code hasn’t been written properly. As JavaScript is an interpreted and not a compiled language, we cannot know where the error lies until we test run it. Even then, in cases concerning user input, errors can creep in if the user enters a null value and the required cases are not taken into consideration by the programmer.

Normally, if there is an error in the JavaScript code, the code executes until that point and the page stops loading where the error occurs. The error is generally not directed at the user level but at the developer level. Here’s how you can handle such errors without letting the user know that anything went wrong.

Error Handling in Javascript

The basic error handling mechanism in Javascript is the try .. catch statement.

Try Catch Statement

The try catch statement allows you to test your code for errors. All the code which needs to be checked for errors is placed in the try block. The code which handles the error condition in case there is an error goes into the catch block.

Here’s the basic syntax for the try catch statement

try

{code to be tried; if any error occurs, it is passed on to the catch statement. }

catch(errorvariable)

{error handling code}

Here’s an example:

docutypoment.write(“This is an error as document is misspelled as docutypoment.”);

This leads to an error in the page which prevents it from loading.

Now we use a try catch statement to handle the error.

In this case, the page loads fully and prints the error description using the .description attribute. We haven’t actually handled the error, just printed what it was. Depending on what the error was, the developer can write proper error handling code.

Throw Statement

Javascript also allows you to define your own exceptions. You can create your own exceptions for unexpected events like a user trying to enter an invalid value, trying to divide by zero, etc. The exceptions can be thrown in a try block and can be handled in the catch block.

Basic Syntax:

throw(exceptionname);

Example:

In this code sample, if the data is invalid (not in the range 10, 20) we throw an error depending on whether it is high or low. The error is handled in the catch block and the user is alerted.

This post is part of the series: JavaScript - Tutorials for Beginners

This collection contains several basic JavaScript tutorials for beginners. It explains fundamental concepts, such as operators, variables, loops, events, errors and conditional statements in JavaScript. It assumes no prior programming knowledge.

  1. Conditional Statements in JavaScript
  2. Looping Constructs in JavaScript
  3. Advanced Operators in JavaScript
  4. Alerts and Functions in JavaScript
  5. Error Handling in JavaScript