diff --git a/src/matrix/room/members/MemberList.js b/src/matrix/room/members/MemberList.js
index de07adf5..9923fb87 100644
--- a/src/matrix/room/members/MemberList.js
+++ b/src/matrix/room/members/MemberList.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {ObservableMap} from "../../../observable/map/ObservableMap.js";
-import {RetainedValue} from "../../../utils/RetainedValue.js";
+import {RetainedValue} from "../../../utils/RetainedValue";
 
 export class MemberList extends RetainedValue {
     constructor({members, closeCallback}) {
diff --git a/src/utils/RetainedValue.js b/src/utils/RetainedValue.ts
similarity index 82%
rename from src/utils/RetainedValue.js
rename to src/utils/RetainedValue.ts
index b3ed7a91..a67b848b 100644
--- a/src/utils/RetainedValue.js
+++ b/src/utils/RetainedValue.ts
@@ -15,16 +15,18 @@ limitations under the License.
 */
 
 export class RetainedValue {
-    constructor(freeCallback) {
+    private readonly _freeCallback: () => void;
+    private _retentionCount: number = 1;
+
+    constructor(freeCallback: () => void) {
         this._freeCallback = freeCallback;
-        this._retentionCount = 1;
     }
 
-    retain() {
+    retain(): void {
         this._retentionCount += 1;
     }
 
-    release() {
+    release(): void {
         this._retentionCount -= 1;
         if (this._retentionCount === 0) {
             this._freeCallback();