gistfile1.js

// Solution to Part 1.

Function.prototype.cached = function(){
  var self = this, cache = {};
  return function(arg){
    if(arg in cache) return cache[arg];
    return cache[arg] = self(arg);
  }
}

// A tracing version:

Function.prototype.cached = function(){
  var self = this, cache = {};
  return function(arg){
    if(arg in cache) {
      console.log('Cache hit for '+arg);
      return cache[arg];
    } else {
      console.log('Cache miss for '+arg);
      return cache[arg] = self(arg);
    }
  }
}

// Solution to Part 2.

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

isPrime = isPrime.cached();

memoize sugar +1

(from our JavaScript Master Class!)