I’ve never been a big fan of using the Internet Explorer CSS expressions, under the belief that you can use regular JavaScript to do the same thing whenever you want. However, recently I fell victim to the belief expressions may be useful and decided to give it a try. Then proceeded to spend three days tracking down a JavaScript bug that was related directly to the expression I forgot I used. Here’s what happened.

I had two elements on the page, and the expression used these two elements to create a measurement of width for a third. I removed one of the two elements without remembering I had used it in an expression. When I loaded up the page in IE, I got a JavaScript error, one of the very useful, “‘variableName’ is undefined.” It gave line 1 and character 1 as the offending code. Naturally, I went into all the JavaScript files and looked…no mention of that variable. Stepped into the debugger at the time the error occurred, the debugger said there was no source available for this location. Lot of help that was. I systematically went through adding and removing elements on the page before I finally called over another set of eyes. Yep, it was the expression that I forgot I had used making a reference to the element I had removed.

So, if you ever run into a problem where you get a JavaScript error that you can’t step into a debugger to see and the offending code is said to be on line 1 and character 1 in IE, check your CSS for expressions. It could save you a lot of time and effort.