function memoize(fn) {
var cache = {};
  return function(arg) {
  var query = JSON.stringify(arg);
  if (!cache[query]) {
  cache[query] = fn(arg);
  }
  return cache[query];
  }
}


// example. factorial
function factorial(n) {
if (n < 2) {
  return n;
  }

  return n * factorial(n - 1);
}

factorial(20);
console.time('not memoized');
factorial(20);
console.timeEnd('not memoized');

var factorialMemoized = memoize(factorial);

factorialMemoized(20);
console.time('memoized');
factorialMemoized(20);
console.timeEnd('memoized');