GamepadEvent.gamepad

The GamepadEvent.gamepad property of the GamepadEvent interface returns a Gamepad object, providing access to the associated gamepad data for fired gamepadconnected and gamepaddisconnected events.

readonly    attribute Gamepad gamepad;

The gamepad property being called on a fired Window.gamepadconnected event.

window.addEventListener("gamepadconnected", function(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);
});

A Gamepad object.

Using the Gamepad API