fibonacciBetter.js

function fibonacci(n) {
  //0,1,1,2,3,5,8,13,21,34,55
  var start = [0, 1];
  var i, j, k, x;
  for (i = 0, j = 1, k = 0; k < n; i = j, j = x, k++) {
    x = i + j;
    start.push(x);
  }  
  return start[n];
}
fibonacci(10);

// ---------------------------------------

function fibMemo(index, cache) {
  cache = cache || [];
  if (cache[index]) return cache[index];
  else {
    if (index < 3) return 1;
    else {
      cache[index] = fibMemo(index - 1, cache) + fibMemo(index - 2, cache);
    }
  }

  return cache[index];
}
fibMemo(7);

fibonacciInteresting.md

1. there are 4 examples written about fibonacci series
2. 2 worse, 2 better

the references are from UdemyCourse: LearningAlgorithmsInJavascriptFromScratch stackoverflow.com/questions/7944239/

Prove Upper Bound (Big O) for Fibonacci's Sequence?

The result is O(n) = 1.6^n

A beginner's guide to Big O notation

Big-O of Fibonacci with memoization? tell when having cache (aka memorization) that the result would be O(n) at ex4 instead of the O(2^n) that template at ex1, ex2

fibonacciWorse.js

// it's adorable but it may be shutdown as n becoming bigger.
function fibonacci(n) {
   return n < 1 ? 0
        : n <= 2 ? 1
        : fibonacci(n - 1) + fibonacci(n - 2);
}

// ---------------------------------------

// shutdown, too
function fibonacci2(index) {
  if (index === 0) return 0;
  if (index < 3 &&) return 1;
  else return fibonacci2(index - 1) + fibonacci2(index - 2);
}
fibonacci2(3); //2

if using Generator

function* fibonacci() {
  var fn1 = 0;
  var fn2 = 1;
  while (true) {  
    var current = fn1;
    fn1 = fn2;
    fn2 = current + fn1;
    var reset = yield current;
    if (reset) {
        fn1 = 0;
        fn2 = 1;
    }
  }
}

var sequence = fibonacci();
console.log(sequence.next().value);     // 0
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 2
console.log(sequence.next().value);     // 3
console.log(sequence.next().value);     // 5
console.log(sequence.next().value);     // 8
console.log(sequence.next(true).value); // 0
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 1
console.log(sequence.next().value);     // 2