tail.hs

var fib = function(n) {
  if (n <= 1) {
    return n;
  } else {
    return fib_tail_recursive(n, 0, 1);
  }
}

var fib_tail_recursive = function(n, a, b) {
  if (n === 0) {
    return a;
  } else {
    return fib_tail_recursive(n-1, b, a+b);
  }
}