JavaScript：ATM machines allow 4 or 6 digit PIN codes and PIN codes cannot contain anything but exactly 4 digits or exactly 6 digits. If the function is passed a valid PIN string, return true, else return false

Certainly! Here's a JavaScript function that checks if a PIN string is valid according to the given conditions:

function validatePIN(pin) {
  if (pin.length === 4 || pin.length === 6) {
    if (/^\d+$/.test(pin)) {
      return true;
    }
  }
  return false;
}

You can use this function by passing a PIN string to it. It will return `true` if the PIN is valid (contains exactly 4 or 6 digits only), and `false` otherwise.