diff --git a/back/src/Model/Group.ts b/back/src/Model/Group.ts
index 16cd5ffe..4c597557 100644
--- a/back/src/Model/Group.ts
+++ b/back/src/Model/Group.ts
@@ -68,7 +68,7 @@ export class Group {
 
     leave(user: UserInterface): void
     {
-        let success = this.users.delete(user);
+        const success = this.users.delete(user);
         if (success === false) {
             throw new Error("Could not find user "+user.id+" in the group "+this.id);
         }
@@ -84,7 +84,7 @@ export class Group {
      */
     destroy(): void
     {
-        for (let user of this.users) {
+        for (const user of this.users) {
             this.leave(user);
         }
     }
diff --git a/back/src/Model/World.ts b/back/src/Model/World.ts
index e4ce5c32..6d4fc205 100644
--- a/back/src/Model/World.ts
+++ b/back/src/Model/World.ts
@@ -20,7 +20,7 @@ export class World {
 
     // Users, sorted by ID
     private readonly users: Map<string, UserInterface>;
-    private readonly groups: Map<string, Group>;
+    private readonly groups: Set<Group>;
 
     private readonly connectCallback: ConnectCallback;
     private readonly disconnectCallback: DisconnectCallback;
@@ -35,7 +35,7 @@ export class World {
                 groupDeletedCallback: GroupDeletedCallback)
     {
         this.users = new Map<string, UserInterface>();
-        this.groups = new Map<string, Group>();
+        this.groups = new Set<Group>();
         this.connectCallback = connectCallback;
         this.disconnectCallback = disconnectCallback;
         this.minDistance = minDistance;
@@ -99,7 +99,7 @@ export class World {
                         user,
                         closestUser
                     ], this.connectCallback, this.disconnectCallback);
-                    this.groups.set(group.getId(), group);
+                    this.groups.add(group);
                 }
             }
 
@@ -132,10 +132,10 @@ export class World {
         if (group.isEmpty()) {
             this.groupDeletedCallback(group.getId(), user);
             group.destroy();
-            if (!this.groups.has(group.getId())) {
+            if (!this.groups.has(group)) {
                 throw new Error("Could not find group "+group.getId()+" referenced by user "+user.id+" in World.");
             }
-            this.groups.delete(group.getId());
+            this.groups.delete(group);
         } else {
             this.groupUpdatedCallback(group);
         }