Fast Fibonacci.js

/*
Description:
Fast Fibonacci
The generic implementation of the fibonacci algorithm is usually something like the following

function fib(num) {
  if (num < 2) return num;
  return fib(num - 1) + fib(num - 2);
}
Now thats all and well and good but that function isn't too efficient. If I wanted to get the 1000th number in the series, I'd have to wait... days? maybe years?

Your task
Write a more efficient fibonacci function that can calculate the 1000th+ number series without breaking a sweat. Read up on tail call optimization for some help.

Starting values
fib(0) = 0;
fib(1) = 1;
*/
function fib(num) {
  const arr=[0,1];
  for (let i=0;i<num;i++){
  arr.push(arr[i]+arr[i+1])
  }
  return arr[arr.length-2]
}