How to really learn from debugging


2006-1-27

Everybody who has ever done any kind of programming knows the wild hunt for the cause of an error. You change a thing here and try. Same effect. Chage something there. Try. Same problem, but now also something is wrong there. Change back, try this route. Repeat until the problem is gone. Properly done with a good staretegy this is called "debugging", but very often we get so caught up in trying this and that until it somehow magically works. Sometimes this process is so long and the path so twisted with so many false starts and deadends that in the end we no longer know what exactly the resolution was. What actually caused the problem, and how can we avoid it in the future? Here's some ideas about how to learn, really learn from debugging:

I'm sure that list can go on for some time. But my main point really is: make sure you are aware of the actual fix and remember only that. Make sure you leave only the changes that actually fixed the problem in your code.

Here's what inspired this post: I was just debugging an AJAX app that suddently no longer worked in IE but everywhere else. I commented things out to find the actual point things broke, re-wrote portions of functions I thought might have broken etc. In the end I discovered what had caused all the trouble: a HTML closing tag was missing it's '>' bracket. In Safari and all Mozilla based browsers this was no problem. But in IE this lead to a situation where the following HTML element never appeared in the DOM. And that element was crucial for my code. So there really was nothing wrong with the JS I was debugging. I could have left it the way it was! But in the meantime I had developed so many "possible solutions" I really needed to make the mental leap to flush them all out. I needed top realize and focus on the fact that unclean HTML had cause this, not bad JS code.

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