// A memoize function to store function results of heavy functions

Function.prototype.memoize = function(hashFn, bind){
    var memo = {},
        self = this;

    if (!hashFn) hashFn = function(arg){
        return arg;
    };

    return function(){
        var key = hashFn.apply(self, arguments);
        return (key in memo) ? memo[key] : (memo[key] = self.apply(bind, arguments));
    };
};

var isPrime = function(num){
    var i = num - 2;
    while (i--) if (num % (i + 2) == 0) return false;
    console.log('Prime calculated of ' + num);
    return true;
}

var isPrimeFast = isPrime.memoize();

// Calculate the first primes twice
// since they will not change over time, we can use the memoized function
for (var i = 0; i < 1000; i++) if (isPrimeFast(i)) console.log('prime found: ' + i);
for (var i = 0; i < 1000; i++) if (isPrimeFast(i)) console.log('prime found again: ' + i);