Web Workers

Chapter 6. Debug Your Workers

Because Web Workers JavaScript files aren’t part of the files you will find in Firefox’s firebug or in Safari’s web inspector, it will be difficult to debug them. These script files aren’t part of the current page scope, and the browser won’t show them to us. Luckily, in Chrome (15+) we have a great option to debug Web Workers in the Chrome Dev Tools. We will see how to use this tool later in this chapter.

If you aren’t using Chrome, there is an option to gain information when an error occurs. The Web Workers specification^[9] shows us that an error event (onerror handler) should be fired when a runtime script error occurs in a Worker. The main properties in the onerror handler are the following:

message

The error message itself.

lineno

The number of the line inside our Web Worker that caused the error.

filename

The name of the file inside the Worker in which the error occurred.

That should be all the information you need to be able to fix your “errors,” right?

You can override the onerror function with a version that will throw an error with enough information to help us see what’s happening inside the Worker. You may also add some other parameters that are specific to your case. For example, if your Worker is handling connections, you can add their data (e.g., number of open connections, number of ideal connections, etc.) to this error message. This is a simple way to understand where your code is broken.

var worker = new Worker("worker.js"); worker.onerror ...