diff --git a/src/domain/session/room/timeline/TilesCollection.js b/src/domain/session/room/timeline/TilesCollection.js
index b3696fad..8dce4125 100644
--- a/src/domain/session/room/timeline/TilesCollection.js
+++ b/src/domain/session/room/timeline/TilesCollection.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {BaseObservableList} from "../../../../observable/list/BaseObservableList";
-import {sortedIndex} from "../../../../utils/sortedIndex.js";
+import {sortedIndex} from "../../../../utils/sortedIndex";
 
 // maps 1..n entries to 0..1 tile. Entries are what is stored in the timeline, either an event or fragmentboundary
 // for now, tileCreator should be stable in whether it returns a tile or not.
diff --git a/src/observable/list/SortedArray.js b/src/observable/list/SortedArray.js
index 874d1b04..1201845c 100644
--- a/src/observable/list/SortedArray.js
+++ b/src/observable/list/SortedArray.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {BaseObservableList} from "./BaseObservableList";
-import {sortedIndex} from "../../utils/sortedIndex.js";
+import {sortedIndex} from "../../utils/sortedIndex";
 import {findAndUpdateInArray} from "./common";
 
 export class SortedArray extends BaseObservableList {
diff --git a/src/observable/list/SortedMapList.js b/src/observable/list/SortedMapList.js
index 2421419e..38900380 100644
--- a/src/observable/list/SortedMapList.js
+++ b/src/observable/list/SortedMapList.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {BaseObservableList} from "./BaseObservableList";
-import {sortedIndex} from "../../utils/sortedIndex.js";
+import {sortedIndex} from "../../utils/sortedIndex";
 
 /*
 
diff --git a/src/utils/sortedIndex.js b/src/utils/sortedIndex.ts
similarity index 93%
rename from src/utils/sortedIndex.js
rename to src/utils/sortedIndex.ts
index e0acb5ff..0022cb87 100644
--- a/src/utils/sortedIndex.js
+++ b/src/utils/sortedIndex.ts
@@ -22,7 +22,7 @@ limitations under the License.
  * Based on Underscore.js 1.8.3 <http://underscorejs.org/LICENSE>
  * Copyright Jeremy Ashkenas, DocumentCloud and Investigative Reporters & Editors
  */
-export function sortedIndex(array, value, comparator) {
+export function sortedIndex<T>(array: T[], value: T, comparator: (x:T, y:T) => number): number {
     let low = 0;
     let high = array.length;