memo_es6_fib.js

const fib = (num, cache = {}) => {
  if (num < 2) return num;
  // if the cached result exists, return the cached result
  if (cache[num]) return cache[num];
  // otherwise, create a new entry in the cache with the new result
  return Object.assign(
    cache,
    { [num]: (fib(num - 1, cache) + fib(num - 2, cache)) },
  )[num];
  // ^^ and return the result so recursion can continue
};