Fib Straight forward

fibsimple.js

// 0 1 1 2 3 5 8 13 21 34
function fibValue(index) {
  let [prev, next] = [0, 1]

  if (index < 2) {
    return index
  }

  for (var x = 2; x < index; x++) {
    const result = prev + next
    prev = next
    next = result
  }

  return prev + next
}