Give that recursive function everything it needs!

fibonacci_tail.js

// feel free to paste this in your JS console!

const slowfib = x => {
  if (x === 0) return null
  if (x < 5) return [0,1,1,2][x -1]
  return slowfib(x - 2) + slowfib(x - 1)
}

const F = () => false
const add = (x, y) => !y ? z => x + z : x + y
const dec = add(-1)

const fib = x => {
  const fibLookup = {
    0: () => null,
    1: (a, _) => a,
    2: (_, b) => b,
  }
  const _fib = (y, a, b) =>
    fibLookup[y] === undefined
      ? _fib(dex(y), b, add(a, b))
      : fibLookup[y](a, b)
  return _fib(x, 0, 1)
}

console.time('slowfib')
slowfib(45)
console.timeEnd('slowfib')

console.time('fib')
fib(45)
console.timeEnd('fib')