Getting additional details on an error

While you may very well be content with simply using the onerror event to suppress JavaScript errors, this event provides additional details on the error that just occurred should you want them, by way of 3 parameters that get indirectly passed into the function on the right. By default when an JavaScript error occurs on the page, the browser informs the user the url of the page containing the error, the line where the error occurred, and a (less than helpful, I might add) message attempting to explain why the error occurred:

When you utilize the onerror event to intercept JavaScript errors on the page, you have access to these 3 pieces of information as well. The trick is to make room for 3 parameters (can be named anything) in the function assigned to window.onerror. Using our trusted example from the previous pages again, here's how to modify it to gain access to details about the error:

<script type="text/javascript">

window.onerror=function(msg, url, linenumber){
 alert('Error message: '+msg+'\nURL: '+url+'\nLine Number: '+linenumber)
 return true

<script type="text/javascript">
document.write('hi there'

Click here to see a live demo. The 3 parameters provide the following nuggets of information:

Parameter (in order) Description
Error Message Contains a message explaining why the error occurred.
Error URL Contains the url of the page with the error script
Error Line Number Contains the line number where the error occurred