Tweaking console.assert in IE9

Sunday, Jan 30, 2011 3 minute read Tags: ie9 javascript web
Hey, thanks for the interest in this post, but just letting you know that it is over 3 years old, so the content in here may not be accurate.

Today while writing some JavaScript I was using the console.assert method to work out the state of things at different points in time.

If you’re not familiar with console.assert here’s the method signature:

console.assert(expression, message[, object])

What this allows you to do is pass in an expression to be evaluated, a message to display when the expression is false and an optional object to dump.

This is really useful if you’re writing large chunks of JavaScript and you can’t/ don’t want to attach the debugger (common if you’re working with timeouts and intervals), you can have the application assertion results to the console to be observed.

I was using this in Chrome and FireFox (since the machine I have at work only has XP so no IE9 :() and found it really useful to be able to log out the optional object.

When doing so you end up with something like this:

Chrome console.assert

As you can see you can inspect into the object that you dumped out. Sweet!

Something you may already be aware of is that IE9 also includes a console object (yay, no more alert debugging :P), and it also contains an implementation of console.assert. So I decided to test and see how it goes in IE9, and here’s what it looks like:

IE9 sad face

Oh dear, [object Object], where’s my object to inspect? This isn’t good now is it. The problem is that the IE9 console.assert method calls toString() on your object, resulting in the [object Object] output. It’s also right up against message.

Well let’s fix it, the best thing about JavaScript is that you can just change stuff if you don’t like it. So here’s a method you can run in JavaScript to replace the out-of-the-box console.assert method:

(function(assert) {

	console.assert = function(expression, message, object) {
		if(object) {
			//we only want to do this if they did provide an object
			assert(expression, message, ' >>> ' + JSON.stringify(object));
		} else {
			assert(expression, message);


Here we’re creating an anonymous function that we’ll immediately execute, pass in the standard console.assert and then augment it with using JSON.stringify. The beauty of this is that the native method is still being called, but if you’re passing in an object we’re converting it to a JSON string first.

Now when you do a console.assert and provide an object you get this:

IE9 happy

It’s not perfect, you can’t inspect into the object since it’s just a string, but it does suite for a lot of purposes.

Just don’t be silly and pass in jQuery as the object, you’ll end up with something quite large :P.

Disclaimer: This was done against the IE9 Beta (build 9.0.7930.16406) so it may change by the time of official release.

Disclaimer 2: Tested against the RC and it still doesn’t produce an object inspection so this work around is still handy.