@aral Just because you run through every line of code while testing, doesn't mean at all these are bug free. Most tests in TDD are positive tests ("Putting A into function B results in behaviour C"), confining undefined behaviour especially in dynamicly typed languages, is a very different beast :D
The simple line:
result = input + 1
in JS has some many potential results, that you have to cover this line multiple times :D
@sheogorath That was my point also :)