A Javascript debugging tip for Firebug (or “Stop using alert()!”)

Did you know that if you’ve got Firebug for Firefox installed, you can use it for debugging your own code? By calling

console.log("Here's a message!");

Firebug will print the message to it’s internal message log. Neat!

Firebug message console

Now, that’s all good. But let’s say you’re on a project that’s not using a Javascript preprocessor to minify and strip debugging code — which would be the best option. You want the benefits of debugging, but not having to constantly remove debugging code for deployment. If a user doesn’t have Firebug installed, they won’t have a console object. So obviously leaving your debug code in is going to cause an error.

Or will it?

Try adding this code before any of your other Javascript. It will set up a fake console so that if Firebug is not installed, there won’t be any errors.

if(!console || !console.log) {
var console = new Array();
console.log = function () {}

Bonus idea! If you combine the idea shown here, with my previous post about debugging Firefox extensions, you can see how to prevent extension debugging from getting in the way of your users.

if(!Firebug || !Firebug.Console || !Firebug.Console.log) {
var Firebug = new Array();
Firebug.Console = new Array();
Firebug.Console.log = function () {}

Firefox extension debugging

One hugely important thing in coding is debugging. Unfortunately, a lot of Javascript debugging gets done via alert() calls. This gets awkward quickly, with the alerts affecting timing, and just being annoying if you have to dump large amounts of data out.

Firebug is a great development tool, and has a really handy logging interface that you can dump debugging info to. Just calling console.log(whatever) will dump it to the main Firebug interface as text that you can copy/paste, scroll through, etc.

If you’re developing a Firefox extension, this debugging capability is really useful. Except, calling console.log() doesn’t work, console isn’t defined for the browser, only for each window.

The trick? Call it directly from the Firebug extension object.


Be sure to capitalize both Firebug and Console, and you’ll be good to go. In addition to having great capabilities for logging, the console will prevent your debugging messages from popping up to your users, in case you leave some code where it shouldn’t be.

By the way — if you found this helpful, check back here in a few days. I’ve submitted a presentation proposal to SXSW for Firefox extension development, where I’ve got tons of info for creating extensions for web applications. They collect votes from the community, and I’d like your support. Plus, if the presentation goes through, I’ll be collecting lots of my best tips and putting them online as a resource for the attendees. That means you’ll get all of them too, and you don’t have to go anywhere! 😀

Edit (2008-08-21: Added link for SXSW voting panel)