How to do factorial

factorial.js

function factorial(n) {
    return n < 2 ? 1 : n * factorial(n - 1);
};

var fastFac = (function () {
    var memo = {};
    
    return function (n) {
        if (memo[n]) {
            return memo[n];
        }
        
        memo[n] = factorial(n);
        
        return memo[n];
    };
})();