The below formula can be used to get the LCM of two numbers. ^{See Reduction by the greatest common divisor section in Wikipedia}

               a * b
lcm(a, b) = ―――――――――――
             gcd(a, b)

GCD can be calculated by using the Euclidean Algorithm^{See Using Euclid's algorithm section in Wikipedia}.

gcd(a, b) = gcd(b, a % b)

Using these two formulas, the LCM can be calculated as follow

console.time('new');

function gcd(a, b) {
    while (b !== 0) {
        var temp = a;
        a = b;
        b = temp % b;

        // Can be written in ES6 as
        // [a, b] = [b, a % b];
    }

    return a;
}

function lcm(a, b) {
    return a * b / gcd(a, b);
}
var range = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20];
console.log(range.reduce(lcm, 1));
console.timeEnd('new');

This is very efficient than using while and for, see this JsFiddle demo.

Note: This answer is inspired by this and this posts.

The below formula can be used to get the LCM of two numbers. ^{See Reduction by the greatest common divisor section in Wikipedia}

               a * b
lcm(a, b) = ―――――――――――
             gcd(a, b)

GCD can be calculated by using the Euclidean Algorithm^{See Using Euclid's algorithm section in Wikipedia}.

gcd(a, b) = gcd(b, a % b)

Using these two formulas, the LCM can be calculated as follow

console.time('new');

function gcd(a, b) {
    while (b !== 0) {
        var temp = a;
        a = b;
        b = temp % b;

        // Can be written in ES6 as
        // [a, b] = [b, a % b];
    }

    return a;
}

function lcm(a, b) {
    return a * b / gcd(a, b);
}
var range = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20];
console.log(range.reduce(lcm, 1));
console.timeEnd('new');

This is very efficient than using while and for, see this JsFiddle demo.

Note: This answer is inspired by this and this posts.