function fib (n) {
  if (n < 1) return 0
  if (n <= 2) return 1

  let fib_1 = 0
  let fib_2 = 1
  let fib_N = n

  for (let i=2; i < n; i++>) {
    fib_N = fib_1 + fib_2
    fib_1 = fib_2
    fib_2 = fib_N
  }
  return fib_N
}

function fib (n) {
  if (n < 1) return 0
  if (n <= 2) return 1
  return fib(n-1) + fib(n-2)
}

function fib (n) {
  const memo = [0, 1]
  const fibon = n => {
    if (memo[n] != null) return memo[n]
    return momo[n] = fib(n-1, memo) + fib(n-2, memo)
  }
  return fibon
}