The engineering culture at Twitter requires tests. Lots of tests. I haven’t had formal experience with JavaScript testing before Twitter, so I’ve been learning a lot as I go. In particular, a number of patterns I used to use, write about, and encourage have turned out to be bad for writing testable code. So I thought it would be worthwhile to share a few of the most important principles I’ve developed for writing testable JavaScript. The examples I provide are based on QUnit, but should be just as applicable to any JavaScript testing framework.