make_fast implementation

make_fast.js

function make_fast(fn) {
  var memo = {};
  return function (val) {
    if(memo[val] === undefined) {
      memo[val] = fn(val);
    }
   
    return memo[val];
  };
}

function expensive(val) {
  var result = 0;
  for (var i = 0; i < 2000000000; ++i) {
    result += val;
  }
  return result;
}

// Wrap function "expensive" to make subsequent calls with the same parameter fast
var cheap = make_fast(expensive);

// This is still slow
cheap(100);

// But, this is fast
cheap(100);