lowest common multiple of numbers less than another number

lcm.js

function gcd(a, b) {
    var a_ = a;
    var b_ = b;
    while (a_ != 0 && b_ != 0) {
        if (a_ > b_) {
            a_ = a_ % b_;
        } else {
            b_ = b_ % a_;
        }
    }
    if (a_ == 0) {
        return b_;
    } else {
        return a_;
    }
}

function lcm(a, b) {
    return a / gcd(a, b) * b;
}

function lcm_rec(k) {
    if (k == 1) {
        return 1;
    } else {
        return lcm(k, lcm_rec(k-1));
    }
}

var then = performance.now();
var n = 20;
console.log("lowest common multiple of numbers <= ", n, lcm_rec(n))
console.log("that took: ", performance.now() - then, " milliseconds");