Production built code fails - how to debug?

I have a problem where the application production build fails but works fine otherwise.

I get an unhelpful console error in "TypeError: q is undefined" in firefox, and "Uncaught TypeError: Cannot read property 'indexOf' of undefined" in Chrome.

I am commenting out blocks of code and trying to revert back any recent changes to see where the problem started occurring but still no luck.

My question is if there are any other good way to debug this issue. I feel it will be very difficult to put together a test case until I have narrowed it down to a few lines of code, at which point I will have probably resolved the issue..

I can't really stop and trace in the production code so am a bit at loss given the lack of clues in the error message....