Number of digits of factorial - https://codefights.com/challenge/KW8NZKvNFZ4HdTdSZ

digits_factorial.md

function length_of_factorial(N) {
    if (N < 2) return 1;
    sum = 0;
    for (i = 2; i <= N; i++) {
        sum += Math.log(i) / Math.log(10)
    }

    return Math.floor(sum) + 1
}

reference: https://www.quora.com/What-is-the-formula-for-finding-the-total-number-of-digits-in-a-natural-number-N

57 chars

N = _fArgs[0]
M = Math
s = 1

for (;N;)
  s += M.log(N--) / M.LN10
  
return s | 0

52 chars

return ~~((Math.log(n = _fArgs[0]) * (.5 + n) - n)/2.30258) + 1