Factorial

A factorial is the product of the integers 1 through $n$.

For example: $5! = 1 * 2 * 3 * 4 * 5 = 120$

It is useful to calculate the amount of different combinations of a set. Specially useful when doing permutations and combinations.

Iterative factorial

var factorial = function(n) {
    var result = 1;

    for(var i = 1; i <= n; i++) {
        result *= i;
    }

    return result;
};

Recursive factorial

We don't need to run a loop.

var factorial = function(n) {
    // base case: 
    if (n === 0) {
        return 1;
    }

    // recursive case:
    if (n > 1) {
        return n * factorial(n - 1);    
    }

    return 1;
};