diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 20469f24ac..385d3c98ba 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -219,7 +219,7 @@ EXEMPT_FILES = make_set(
         "web/src/url-template.d.ts",
         "web/src/user_group_create.js",
         "web/src/user_group_create_members.js",
-        "web/src/user_group_create_members_data.js",
+        "web/src/user_group_create_members_data.ts",
         "web/src/user_group_edit.js",
         "web/src/user_group_edit_members.js",
         "web/src/user_group_ui_updates.js",
diff --git a/web/src/user_group_create_members_data.js b/web/src/user_group_create_members_data.ts
similarity index 65%
rename from web/src/user_group_create_members_data.js
rename to web/src/user_group_create_members_data.ts
index c24a871548..d60354d0db 100644
--- a/web/src/user_group_create_members_data.js
+++ b/web/src/user_group_create_members_data.ts
@@ -1,21 +1,25 @@
+import assert from "minimalistic-assert";
+
 import {page_params} from "./page_params";
 import * as people from "./people";
+import type {User} from "./people";
 
-let user_id_set;
+let user_id_set: Set<number>;
 
-export function initialize_with_current_user() {
+export function initialize_with_current_user(): void {
     const current_user_id = page_params.user_id;
-    user_id_set = new Set();
+    user_id_set = new Set<number>();
+    assert(current_user_id !== undefined, "Current user's id is undefined");
     user_id_set.add(current_user_id);
 }
 
-export function sorted_user_ids() {
+export function sorted_user_ids(): number[] {
     const users = people.get_users_from_ids([...user_id_set]);
     people.sort_but_pin_current_user_on_top(users);
     return users.map((user) => user.user_id);
 }
 
-export function get_all_user_ids() {
+export function get_all_user_ids(): number[] {
     const potential_members = people.get_realm_users();
     const user_ids = potential_members.map((user) => user.user_id);
     // sort for determinism
@@ -23,17 +27,17 @@ export function get_all_user_ids() {
     return user_ids;
 }
 
-export function get_principals() {
+export function get_principals(): number[] {
     // Return list of user ids which were selected by user.
     return [...user_id_set];
 }
 
-export function get_potential_members() {
+export function get_potential_members(): User[] {
     const potential_members = people.get_realm_users();
     return potential_members.filter((user) => !user_id_set.has(user.user_id));
 }
 
-export function add_user_ids(user_ids) {
+export function add_user_ids(user_ids: number[]): void {
     for (const user_id of user_ids) {
         if (!user_id_set.has(user_id)) {
             const user = people.maybe_get_user_by_id(user_id);
@@ -44,7 +48,7 @@ export function add_user_ids(user_ids) {
     }
 }
 
-export function remove_user_ids(user_ids) {
+export function remove_user_ids(user_ids: number[]): void {
     for (const user_id of user_ids) {
         user_id_set.delete(user_id);
     }