Does this array alternate?

main.js

function alternates(arr) {
  if (arr.length <= 2) return true;

  let a = arr[0];
  let b = arr[1];

  for (let i = 2; i < arr.length; i++) {
    if (arr[i] != a) return false;
    [a, b] = [b, arr[i]];
  }

  return true;
}