Reading analog values of Quest 2 controller trigger and grip buttons

How do I read the analog values from the controller trigger and grip buttons?

The values (0.0 → 1.0) are returned in

let TriggerAnalog = this.gamepadLeft.buttons[0].value;
let GripAnalog = this.gamepadLeft.buttons[1].value;

where, before this call, the gamepadLeft is set
this.gamepadLeft = inputSource.gamepad;

usually set within the xrstart function
WL.onXRSessionStart.push(this.xrstart.bind(this));