[rusk]

If you have the time, make sure that you have adequate coverage from a suite of automated tests. This goes doubly so for a dynamic language like javascript. This will free you up to make changes and will make the whole process of introducing changes less nerve wracking.

[speedplane]

You beat me to the punch. Yes if I'm hired to work on a large, buggy, and antiquated code-base, the first thing I'd do is build tests. Many of them, both unit tests and big functional selenium-style tests. I would definitely try to setup some coverage analysis, but I wouldn't be religious about it. The focus should be to cover popular features, not lines of code.

With those in place, you can start chopping away left and right. Even if you make big changes, when you see all of those tests pass green (or blue if you're on Jenkins), it gives you a level of comfort that no careful reading of the code can supply.