memoried function

Function.prototype.memorized = function(key) { this._values = this._values || {}; if (this._values[key] !== undefined) { return this._values[key] } else { //"this" is parent function object this._values[key] = this.apply(this, arguments); /* the "this" passed as context object is optional? */ return this._values[key]; } }; function isPrime(num) { alert(this); var prime = num != 1; for (var i = 2; i < num; i++) { if (num % i == 0) { prime = false; break; } } return prime; } var a = isPrime.memorized(5); alert(a); var b = isPrime.memorized(5); alert(b);