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.

readonly    attribute long                index;

window.addEventListener("gamepadconnected", function() {
  var gp = navigator.getGamepads()[0];
  gamepadInfo.innerHTML = "Gamepad connected at index " + gp.index + ": " + gp.id + ".";
});

A number.

Using the Gamepad API