code: function fibonacci(num, memo = {}) {
  	if (memo[num]) return memo[num];
  	if (num <= 1) return 1;
  
  	return (memo[num] = fibonacci(num - 1, memo) + fibonacci(num - 2, memo));
  }
docs: High performance fibonacci from https://medium.com/developers-writing/fibonacci-sequence-algorithm-in-javascript-b253dc7e320e