fib.js

function fib_rec(n) {
  if (n <= 1) {
    return 1;
  }

  return fib_rec(n - 1) + fib_rec(n - 2);
}

function fib_loop(n) {
  let [pre, cur] = [0, 1];

  for (let i = 0; i < n; i++) {
    [pre, cur] = [cur, pre + cur];
  }
  return cur;
}