diff --git a/src/randomstring.js b/src/randomstring.ts
similarity index 88%
rename from src/randomstring.js
rename to src/randomstring.ts
index 7ebe4ed78..3362c0b02 100644
--- a/src/randomstring.js
+++ b/src/randomstring.ts
@@ -1,5 +1,6 @@
 /*
 Copyright 2018 New Vector Ltd
+Copyright 2019 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,7 +15,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-export function randomString(len) {
+export function randomString(len: number): string {
     let ret = "";
     const chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";