diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 61fb25a7e3..62e0bea6a4 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -292,7 +292,7 @@ EXEMPT_FILES = make_set(
         "web/src/user_sort.ts",
         "web/src/user_status.ts",
         "web/src/user_status_ui.ts",
-        "web/src/user_topic_popover.js",
+        "web/src/user_topic_popover.ts",
         "web/src/user_topics.ts",
         "web/src/user_topics_ui.ts",
         "web/src/views_util.ts",
diff --git a/web/src/user_topic_popover.js b/web/src/user_topic_popover.ts
similarity index 83%
rename from web/src/user_topic_popover.js
rename to web/src/user_topic_popover.ts
index 2317157691..80a1155fe3 100644
--- a/web/src/user_topic_popover.js
+++ b/web/src/user_topic_popover.ts
@@ -1,4 +1,5 @@
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import render_change_visibility_policy_popover from "../templates/popovers/change_visibility_policy_popover.hbs";
 
@@ -8,7 +9,7 @@ import {parse_html} from "./ui_util";
 import * as user_topics from "./user_topics";
 import * as util from "./util";
 
-export function initialize() {
+export function initialize(): void {
     popover_menus.register_popover_menu(".change_visibility_policy", {
         theme: "popover-menu",
         placement: "bottom",
@@ -28,15 +29,18 @@ export function initialize() {
             popover_menus.popover_instances.change_visibility_policy = instance;
             popover_menus.on_show_prep(instance);
             const $elt = $(instance.reference).closest(".change_visibility_policy").expectOne();
-            const stream_id = $elt.attr("data-stream-id");
-            const topic_name = $elt.attr("data-topic-name");
+            const stream_id_str = $elt.attr("data-stream-id");
             $elt.addClass("visibility-policy-popover-visible");
+            assert(stream_id_str !== undefined);
+
+            const stream_id = Number.parseInt(stream_id_str, 10);
+            const topic_name = $elt.attr("data-topic-name")!;
 
             instance.setContent(
                 parse_html(
                     render_change_visibility_policy_popover(
                         popover_menus_data.get_change_visibility_policy_popover_content_context(
-                            Number.parseInt(stream_id, 10),
+                            stream_id,
                             topic_name,
                         ),
                     ),
@@ -46,8 +50,11 @@ export function initialize() {
         onMount(instance) {
             const $popper = $(instance.popper);
             const $elt = $(instance.reference).closest(".change_visibility_policy").expectOne();
-            const stream_id = Number.parseInt($elt.attr("data-stream-id"), 10);
-            const topic_name = $elt.attr("data-topic-name");
+            const stream_id_str = $elt.attr("data-stream-id");
+            assert(stream_id_str !== undefined);
+
+            const stream_id = Number.parseInt(stream_id_str, 10);
+            const topic_name = $elt.attr("data-topic-name")!;
 
             if (!stream_id) {
                 popover_menus.hide_current_popover_if_visible(instance);
@@ -58,11 +65,11 @@ export function initialize() {
                 const start_time = Date.now();
 
                 const visibility_policy = Number.parseInt(
-                    $(e.currentTarget).attr("data-visibility-policy"),
+                    $(e.currentTarget).attr("data-visibility-policy")!,
                     10,
                 );
 
-                const success_cb = () => {
+                const success_cb = (): void => {
                     setTimeout(
                         () => {
                             popover_menus.hide_current_popover_if_visible(instance);
@@ -71,7 +78,8 @@ export function initialize() {
                     );
                 };
 
-                const error_cb = () => {
+                const error_cb = (): void => {
+                    assert(stream_id !== undefined);
                     const prev_visibility_policy = user_topics.get_topic_visibility_policy(
                         stream_id,
                         topic_name,
@@ -86,7 +94,7 @@ export function initialize() {
                         util.get_remaining_time(start_time, 500),
                     );
                 };
-
+                assert(stream_id !== undefined);
                 user_topics.set_user_topic_visibility_policy(
                     stream_id,
                     topic_name,
@@ -105,7 +113,7 @@ export function initialize() {
                 .expectOne()
                 .removeClass("visibility-policy-popover-visible");
             instance.destroy();
-            popover_menus.popover_instances.change_visibility_policy = undefined;
+            popover_menus.popover_instances.change_visibility_policy = null;
 
             // If the reference is in recent view / inbox, we would ideally restore focus
             // to the reference icon here but we don't do that because there are a lot of