Back to Basics – JavaScript onerror Event

Back to Basics – JavaScript onerror Event

The JavaScript window.onerror event is a very useful event that I find very strange that sometimes client-side developers don’t know. The event is fired most of the times (yep, not every time) an error occurs in the JavaScript code. You can wire a handler to the event that will help you to prevent errors from bubbling to the browser or to send the errors to some logger for further investigation.

The onerror Event

The window.onerror is an event handler for error events that are sent to the window. It isn’t supported in all the browsers (mostly in old browsers) so you can’t take it for granted that it will work, but most of the time it will. It takes three optional parameters:

• An error message 
• A URL where the error was raised 
• The line number of the line that raised the error

Here is a simple code sample which will suppress most of the raised JavaScript errors:

window.onerror = function(msg, url, lineNumber) {   
    return true; // prevents browser error messages  
};

While the previous code will stop the bubbling of the errors to the browser, I prefer to do more than just preventing the annoying browser error dialogs. You can add an AJAX call in order to send the error to a logger that is sitting on your site. This will help you to monitor and diagnose JavaScript errors and will help you to fix JavaScript bugs.

Summary

The onerror event handler can be very useful and might help prevent browser JavaScript error dialogs. Even so, you might be careful when using it since not all of the browsers support it. Combining it with a logging framework such as log4js, log4javascript or just plain AJAX functionality for logging might help you monitor the JavaScript errors in your web site/application.