javascript fibonacci memoization

Question

To calculate the nth term of the fibonacci sequence, I have the familiar recursive function:

var fibonacci = function(index){
    if(index<=0){ return 0; }
    if(index===1){ return 1; }
    if(index===2){ return 2; }

    return fibonacci(index-2) + fibonacci(index-1);
}

This works as expected. Now, I am trying to store calculated indices in an object:

var results = {
  0: 0,
  1: 1,
  2: 2
};

var fibonacci = function(index){
    if(index<=0){ return 0; }
    if(index===1){ return 1; }
    if(index===2){ return 2; }

    if(!results[index]){
        results[index] = fibonacci(index-2) + fibonacci(index-1);
    }
}

I know this isn't actually increasing performance since I'm not accessing the results object, but I wanted to check first if my results object was being populated correctly before memoizing. Unfortunately, it isn't. For fibonacci(9), I get:

Object {0: 0, 1: 1, 2: 2, 3: 3, 4: NaN, 5: NaN, 6: NaN, 7: NaN, 8: NaN, 9: NaN}

Why am I getting NaN for indices past 3?

Answer 1

Here's a solution using "Helper Method Recursion":

function fib(n) {
  const memorize = {};

  function helper(n) {
    if (n in memorize) return memorize[n];
    if (n < 3) return 1;
    return memorize[n] = helper(n - 1) + helper(n - 2);
  }

  return helper(n);
}