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Today's bug hunt

Today I worked on finding and fixing a bug that is difficult to reproduce. In fact, I couldn't reproduce it. So I had to look at the code and reason where the bug could possibly be located. From the error message in the bug report, I narrowed it down to a particular JavaScript file. And it sort of looked impossible that the bug could ever occur. Basically, the error message said that a variable was undefined, but we were clearly defining the variable before we were using it.

Or were we? It turns out that one of the places that used the variable was in a public method. So after thinking about it for a bit, it dawned on me that this method could be called before the object finished getting fully initialized. To prevent this, we always make sure to put all JavaScript that must run on page load in an "addOnRequire" callback. Anyway, the team that wrote this code doesn't work on this product much so they didn't do that.

So I thought about emailing the team to make them aware that they should use addOnRequire(). But then, they are not currently working on the product, and might not be for weeks or months, so how would they remember? So then I thought, I'll write a test that scans the JavaScript files to catch this pattern. Since our tests run frequently, we'll catch this problem any time it arises in the future. Cool!

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