fibo.js

const fibonacciRecursive = n => n < 2 
  ? 1
  : fibonacciRecursive(n-1) + fibonacciRecursive(n-2)

const fibonacciArray = n => {
  let arr = [1, 1]
  for (let i = 0; i < n-1; i++) {
    arr[i + 2] = arr[i + 1] + arr[i];
  }
  return arr[n]
}

const fibonacciCycle = n => {
  let prevNumber = 1;
  let currentNumber = 1;
  let nextNumber = 1;
  for (let i = 0; i < n-1; i++) {
    nextNumber = prevNumber + currentNumber;
    prevNumber = currentNumber;
    currentNumber = nextNumber;
  }
  return nextNumber;
}

const testCases = [1, 1, 2, 3, 5, 8, 13, 21, 34]

testCases.forEach((testCase, index) => {
  console.assert(testCase === fibonacciRecursive(index))
  console.assert(testCase === fibonacciArray(index))
  console.assert(testCase === fibonacciCycle(index))
})

Один вариант забыл =)
const fibonacciFunc = (n, tmp=Math.sqrt(5)) => (Math.pow((1+tmp)/2, n + 1) - Math.pow((1-tmp)/2, n + 1)) / tmp