Greatest common divisor in Javascript @see: https://en.wikipedia.org/wiki/Binary_GCD_algorithm

gcd.js

var gcd = function(a, b) {
    var coef = 1;

    while (
        a !== b && 
        a !== 0 && 
        b !== 0 && 
        a !== 1 && 
        b !== 1
    ) {
        if ( ! (a % 2) && ! (b % 2)) {
            coef *= 2;
            a /= 2;
            b /= 2;
        } else if ( ! (a % 2)) {
            a /= 2;
        } else if ( ! (b % 2)) {
            b /= 2;
        } else if (a > b) {
            a = (a - b) / 2;
        } else if (b > a) {
            b = (b - a) / 2;
        }
    }

    return coef * ((a === 1 || b === 1) ? 1 : (a || b));
};