Fibonacci number implemented with recursive function async timer recursive function

asyncTimerRecursiveFibo.js

// Utility function to make fake async processing
const delay = fn => new Promise(r => setTimeout(() => r(fn()), 100))

const isConstant = n => delay(() => n < 2)

const fibo = async n => {
  if (await isConstant(n)) return n === 0 ? 0 : 1
  const [prev1, prev2] = await Promise.all([fibo(n - 1), fibo(n - 2)])
  return delay(() => prev1 + prev2)
}

fibo(10).then(n => console.log(n)) // 55