fibonacci.js

function fibonacci(n) {
  return n < 2 ? n : fibonacci(n - 2) + fibonacci(n - 1);
}

function fibonacci(n) {
  var prev, curr = 0, next = 1;
  while (n-- > 0) {
    prev = curr;
    curr = next;
    next = prev + curr;
  }
  return curr;
}