code: function updatePointer(eventType, pointerEvent) {
      if (eventType == EVENT_END || (eventType != EVENT_END && pointerEvent.buttons !== 1)) {
        delete this.pointers[pointerEvent.pointerId];
      } else {
        pointerEvent.identifier = pointerEvent.pointerId;
        this.pointers[pointerEvent.pointerId] = pointerEvent;
      }
    }
docs: update the position of a pointer
  @param {String} eventType matches `EVENT_START|MOVE|END`
  @param {Object} pointerEvent