Project Euler problem 3: Largest prime factor in JavaScript
Your code looks pretty, and it is a very literal naïve implementation of the requirements. Unfortunately, the algorithm you are using is extremely inefficient.

Factoring large numbers is hard work! The difficulty of factoring large numbers is, in fact, what keeps cryptographic algorithms like RSA safe.  In your findFactors() function, just counting up to 600 billion will take a while; having to check for divisibility within the loop is even slower!

For a quick algorithm that will give the answer in about one second, see this answer.