fix: undefined difference function

lib/client/ground.db.js

@@ -343,12 +343,13 @@ Ground.Collection = class GroundCollection {
     // Map each cursor id's into one flat array
     const keepIds = arrayOfCursors.map((cursor) => cursor.map((doc) => strId(doc._id))).flat();
     // Remove all other documents from the collection
-    difference(currentIds, keepIds).forEach((id) => {
+    const arrays = [currentIds, keepIds];
+    for (const id of arrays.reduce((a, b) => a.filter((c) => !b.includes(c)))) {
       // Remove it from in memory
       delete this._collection._docs._map[id];
       // Remove it from storage
       this.saveDocument({ _id: id }, true);
-    });
+    }
 
     this.invalidate();
   }