Javascript memoizer

memoizer.js

var memoizer = function(memo, fundamental){
  var shell = function (n) {
    var result = memo[n];
    if (typeof result !== 'number'){
      result = fundamental(shell, n);
      memo[n] = result;
    }
    return result;
  };
  return shell;
};

var fibonacci =
  memoizer([0,1], function(recur, n){
    return recur(n - 1) + recur(n - 2);
  });

fibonacci(40);