fibonacci-2-ways.js

function fib(n) {
  if (n < 1) {
    return 1;
  }
  let a = 1, b = 0;
  while (n >= 0) {
    [ a, b ] = [ b, a + b ];
    n--;
  }
  return b;
};

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

function fibCheck(num){
  var a = 1, b = 0, temp;

  while (num >= 0){
    temp = a;
    a = a + b;
    b = temp;
    num--;
  }

  return b;
}

console.log(fib(8), fibCheck(8));
console.log(fib(18), fibCheck(18));
console.log(fib(2), fibCheck(2));
console.log(fib(1), fibCheck(1));

console.log(fibRecursive(8), fibCheck(8));
console.log(fibRecursive(18), fibCheck(18));
console.log(fibRecursive(2), fibCheck(2));
console.log(fibRecursive(1), fibCheck(1));