Gamepad.index

The Gamepad.index property of the Gamepad interface returns an integer that is auto-incremented to be unique for each device currently connected to the system.

This can be used to distinguish multiple controllers; a gamepad that is disconnected and reconnected will retain the same index.

window.addEventListener("gamepadconnected", () => {
  const gp = navigator.getGamepads()[0];
  gamepadInfo.textContent = `Gamepad connected at index ${gp.index}: ${gp.id}.`;
});