recursion2.js

/*Using a Memo to Avoid Repetitious Calculation*/
(function() {

  function calcFibonacci(n, memo) {
    // If we've got the answer in our memo, no need to recalculate
    if (memo[n] !== -1) {
      return memo[n];
    }

    // Otherwise, calculate the answer and store it in memo
    memo[n] = calcFibonacci(n - 2, memo) + calcFibonacci(n - 1, memo);

    // We still need to return the answer we calculated
    return memo[n];
  }

  function fib(n) {
    var memo = new Array(n + 1).fill(-1);

    memo[0] = 0;
    memo[1] = 1;

    return calcFibonacci(n, memo);
  }

  console.log(fib(5));
  console.log(fib(11));
  console.log(fib(17));
  console.log(fib(31));

})();

recursion2a.js

(function() {

  var util = {
    fib: function(n) {
      var memo = new Array(n + 1).fill(-1);

      memo[0] = 0
      memo[1] = 1

      return util.calcFibonacci(n, memo)
    },

    calcFibonacci: function(n, memo) {
      // If we've got the answer in our memo, no need to recalculate
      if (memo[n] != -1) {
        return memo[n]
      }

      // Otherwise, calculate the answer and store it in memo    
      memo[n] = this.calcFibonacci(n - 2, memo) + this.calcFibonacci(n - 1, memo)

      // We still need to return the answer we calculated
      return memo[n]
    }
  };

  console.log(util.fib(5));
  console.log(util.fib(11));
  console.log(util.fib(17));
  console.log(util.fib(31));
})();