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");
      const radius = frame.getJointPose(
        indexFingerTipJoint,
        referenceSpace,
      ).radius;
    }
  }
}