The GamepadEvent interface of the Gamepad API contains references to gamepads connected to the system, which is what the gamepad events gamepadconnected and gamepaddisconnected are fired in response to.

GamepadEvent()

Returns a new GamepadEvent object.

GamepadEvent.gamepad Read only

Returns a Gamepad object, providing access to the associated gamepad data for the event fired.

The gamepad property being called on a fired gamepadconnected event.

window.addEventListener("gamepadconnected", (e) => {
  console.log(
    "Gamepad connected at index %d: %s. %d buttons, %d axes.",
    e.gamepad.index,
    e.gamepad.id,
    e.gamepad.buttons.length,
    e.gamepad.axes.length,
  );
});

And on a gamepaddisconnected event.

window.addEventListener("gamepaddisconnected", (e) => {
  console.log(
    "Gamepad disconnected from index %d: %s",
    e.gamepad.index,
    e.gamepad.id,
  );
});

Using the Gamepad API