Power of closures (from Secrets of the JavaScript Ninja)

MemoizeWClosure.js

			Function.prototype.memoized = function(key) { 
				this._values = this._values || {};
				return this._values[key] !== undefined ? 
					this._values[key] :
					this._values[key] = this.apply(this, arguments);
			}

			Function.prototype.memoize = function() { 
				var fn = this;
				return function() { 
					return fn.memoized.apply(fn, arguments);
				}
			}

			var isPrime = (function(num) { 
				var prime = num != 1; 
				for (var i = 2; i < num; i++) { 
					if(num % i == 0) {
						prime = false;
						break;
					}
				}
				return prime;
			}).memoize();