JS functional find prime factors

functional_find_prime_factors.js

// Calling find_prime_factors on numbers that functional languages can easily handle 
// will cause a stack size exception. JS can't handle functional programming. :)
// http://jsbin.com/qakodivepe/edit?js,console
// try: 
//  primeFactors(10)
//  primeFactors(7775460)
//  primeFactors(7919)
//  primeFactors(1056884)  <- Boom, stack size exception

// Entry point
function find_prime_factors(num){ facs(num, 2) }

// n is current number
// f is factor to check
function facs(n, f) {
  // if f is bigger than half n, n is prime... return it
  if (f > n / 2) return [n];
  // if f is a factor of n
  if (n % f === 0) {
    // return it and the prime factors of n/f
    return [f].concat(facs(n / f, 2));
  } else {
    // otherwise try the next factor, skipping all evens except 2
    return facs(n, f + (f === 2 ? 1 : 2));
  }
};