Function.prototype.memoize.js

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


(function(slice){

Function.prototype.memoize = function(hashFn, bind){
	var cache = {}, self = bind ? this.bind(bind) : this, hashFn = hashFn || JSON.stringify;
	return function(){
		return ((key = hashFn(slice.call(arguments))) in cache) ? cache[key] : (cache[key] = self(key));
	};
};

})(Array.prototype.slice);

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);