Introduce constant TRACKER_TYPE

src/trackers/image-tracker/image-tracker.ts

     resolution: 'sm'
 };
 
+/** Tracker type */
+const TRACKER_TYPE: keyof TrackerType = 'image-tracker';
+
 
 
 
      */
     get type(): keyof TrackerType
     {
-        return 'image-tracker';
+        return TRACKER_TYPE;
     }
 
     /**
      */
     is<T extends keyof TrackerType>(type: T): this is TrackerType[T]
     {
-        return type === this.type;
+        return type === TRACKER_TYPE;
     }
 
     /**

src/trackers/pointer-tracker/pointer-tracker.ts

     space: 'normalized'
 };
 
+/** Tracker type */
+const TRACKER_TYPE: keyof TrackerType = 'pointer-tracker';
+
 
 
 
      */
     get type(): keyof TrackerType
     {
-        return 'pointer-tracker';
+        return TRACKER_TYPE;
     }
 
     /**
      */
     is<T extends keyof TrackerType>(type: T): this is TrackerType[T]
     {
-        return type === this.type;
+        return type === TRACKER_TYPE;
     }
 
     /**