Ynote

// This is just a short reminder of this great explanation:
// http://www.2ality.com/2015/06/tail-call-optimization.html

// not TCO
function factorial(n) {
  if (n <= 0) return 1;
  
  return n * factorial(n-1); // here, the main recursive call not in a tail position because of the `n` context.
}