navigator.xr
  .requestSession({ optionalFeatures: ["hand-tracking"] })
  .then(/** … */);

function renderFrame(session, frame) {
  // …

  for (const inputSource of session.inputSources) {
    if (inputSource.hand) {
      const indexFingerTipJoint = inputSource.hand.get("index-finger-tip"); // XRJointSpace
      indexFingerTipJoint.jointName; // "index-finger-tip"
      frame.getJointPose(indexFingerTipJoint, referenceSpace); // XRJointPose
    }
  }
}