GamepadButton.value

The GamepadButton.value property of the GamepadButton interface returns a double value used to represent the current state of analog buttons on many modern gamepads, such as the triggers.

The values are normalized to the range 0.0 — 1.0, with 0.0 representing a button that is not pressed, and 1.0 representing a button that is fully pressed.

    readonly    attribute double  value;

var gp = navigator.getGamepads()[0];

if(gp.buttons[0].value > 0) {
  // respond to analog button being pressed in
} 

A double.

Using the Gamepad API