To memo-ize previous function calls, we can use a nifty memoize function.

What we're doing in memoize is utilizing a hash map (in this case, a plain old JS object) to store previous calculations. The key will be the arguments called-- for example, 5 in fib(5), and the value attached will be the result of the callback. In this example, we utilize the closure pattern so that ...args are the parameters of the callback function, and not of memoize.

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const memoize = (callback) => {

  let memo = {};

  return (...args) => {

    if (memo[args]) {

      return memo[args];

    } else {

      memo[args] = callback(args);

      return memo[args];

    }

  };

};