A Fibonacci implementation that uses memoization.

fibonacci_memoized.js

this.fibonacci = (function (n) {
  var fibo = {};
  var rec = function (n) {
    var tmp;
    
    if (n in fibo) {
      tmp = fibo[n];
    } else if (n === 0 || n === 1) {
      tmp = n;
    } else {
      tmp = rec(n-1) + rec(n-2);
    }
    
    fibo[n] = tmp;
    return tmp;
  };
  
  return rec;
})();