I started work at Mozilla in May, and haven't had a chance to update my blog since then. I've been working on a project to implement the DOM in JavaScript. One of the interesting side-projects that has come out of this is a code coverage tool for SpiderMonkey (Mozilla's js interpreter). I'm calling it CoverMonkey, and have just created a repo for it on github. Take a look at some example output.
Debug builds of SpiderMonkey have a -D option that causes them to output opcode-by-opcode disassembly of every file they've run, along with execution counts for every opcode. CoverMonkey analyzes this output and displays coverage statistics and generates color-coded versions of your source code to highlight uncovered and partially covered lines.
The first big caveat about CoverMonkey is that this is not a tool you can use in the browser. It works with standalone versions of SpiderMonkey only. So it is only useful if you're using pure JavaScript. The second caveat is that you'll need a debug build of SpiderMonkey, which means that you'll probably have to build it yourself. Another, smaller caveat: CoverMonkey is a JavaScript program, but unfortunately SpiderMonkey cannot create files, so CoverMonkey requires Node.
@__DavidFlanagan
