diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index a21cb4bb48..78bc31d540 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -244,7 +244,7 @@ EXEMPT_FILES = make_set(
         "web/src/timerender.ts",
         "web/src/tippyjs.ts",
         "web/src/todo_widget.js",
-        "web/src/topic_list.js",
+        "web/src/topic_list.ts",
         "web/src/topic_popover.js",
         "web/src/tutorial.js",
         "web/src/types.ts",
diff --git a/web/src/topic_list.js b/web/src/topic_list.ts
similarity index 66%
rename from web/src/topic_list.js
rename to web/src/topic_list.ts
index 8f08326b3a..684e4caf01 100644
--- a/web/src/topic_list.js
+++ b/web/src/topic_list.ts
@@ -1,5 +1,6 @@
 import $ from "jquery";
 import _ from "lodash";
+import assert from "minimalistic-assert";
 
 import render_more_topics from "../templates/more_topics.hbs";
 import render_more_topics_spinner from "../templates/more_topics_spinner.hbs";
@@ -11,28 +12,29 @@ import * as scroll_util from "./scroll_util";
 import * as stream_topic_history from "./stream_topic_history";
 import * as stream_topic_history_util from "./stream_topic_history_util";
 import * as topic_list_data from "./topic_list_data";
+import type {TopicInfo} from "./topic_list_data";
 import * as vdom from "./vdom";
 
 /*
-    Track all active widgets with a Map.
+    Track all active widgets with a Map by stream_id.
 
     (We have at max one for now, but we may
     eventually allow multiple streams to be
     expanded.)
 */
 
-const active_widgets = new Map();
+const active_widgets = new Map<number, TopicListWidget>();
 
 // We know whether we're zoomed or not.
 let zoomed = false;
 
-export function update() {
+export function update(): void {
     for (const widget of active_widgets.values()) {
         widget.build();
     }
 }
 
-export function clear() {
+export function clear(): void {
     popover_menus.get_topic_menu_popover()?.hide();
 
     for (const widget of active_widgets.values()) {
@@ -42,12 +44,12 @@ export function clear() {
     active_widgets.clear();
 }
 
-export function close() {
+export function close(): void {
     zoomed = false;
     clear();
 }
 
-export function zoom_out() {
+export function zoom_out(): void {
     zoomed = false;
 
     const stream_ids = [...active_widgets.keys()];
@@ -59,75 +61,96 @@ export function zoom_out() {
 
     const stream_id = stream_ids[0];
     const widget = active_widgets.get(stream_id);
+    assert(widget !== undefined);
     const parent_widget = widget.get_parent();
 
     rebuild(parent_widget, stream_id);
 }
 
-export function keyed_topic_li(conversation) {
-    const render = () => render_topic_list_item(conversation);
+type ListInfoNodeOptions =
+    | {
+          type: "topic";
+          conversation: TopicInfo;
+      }
+    | {
+          type: "more_items";
+          more_topics_unreads: number;
+      }
+    | {
+          type: "spinner";
+      };
 
-    const eq = (other) => _.isEqual(conversation, other.conversation);
+type ListInfoNode = vdom.Node<ListInfoNodeOptions>;
+
+export function keyed_topic_li(conversation: TopicInfo): ListInfoNode {
+    const render = (): string => render_topic_list_item(conversation);
+
+    const eq = (other: ListInfoNode): boolean =>
+        other.type === "topic" && _.isEqual(conversation, other.conversation);
 
     const key = "t:" + conversation.topic_name;
 
     return {
         key,
         render,
+        type: "topic",
         conversation,
         eq,
     };
 }
 
 export function more_li(
-    more_topics_unreads,
-    more_topics_have_unread_mention_messages,
-    more_topics_unread_count_muted,
-) {
-    const render = () =>
+    more_topics_unreads: number,
+    more_topics_have_unread_mention_messages: boolean,
+    more_topics_unread_count_muted: boolean,
+): ListInfoNode {
+    const render = (): string =>
         render_more_topics({
             more_topics_unreads,
             more_topics_have_unread_mention_messages,
             more_topics_unread_count_muted,
         });
 
-    const eq = (other) => other.more_items && more_topics_unreads === other.more_topics_unreads;
+    const eq = (other: ListInfoNode): boolean =>
+        other.type === "more_items" && more_topics_unreads === other.more_topics_unreads;
 
     const key = "more";
 
     return {
         key,
-        more_items: true,
+        type: "more_items",
         more_topics_unreads,
         render,
         eq,
     };
 }
 
-export function spinner_li() {
-    const render = () => render_more_topics_spinner();
+export function spinner_li(): ListInfoNode {
+    const render = (): string => render_more_topics_spinner();
 
-    const eq = (other) => other.spinner;
+    const eq = (other: ListInfoNode): boolean => other.type === "spinner";
 
     const key = "more";
 
     return {
         key,
-        spinner: true,
+        type: "spinner",
         render,
         eq,
     };
 }
 
 export class TopicListWidget {
-    prior_dom = undefined;
+    prior_dom?: vdom.Tag<ListInfoNodeOptions> = undefined;
+    $parent_elem: JQuery;
+    my_stream_id: number;
 
-    constructor($parent_elem, my_stream_id) {
+    constructor($parent_elem: JQuery, my_stream_id: number) {
         this.$parent_elem = $parent_elem;
         this.my_stream_id = my_stream_id;
     }
 
-    build_list(spinner) {
+    build_list(spinner: boolean): vdom.Tag<ListInfoNodeOptions> {
         const list_info = topic_list_data.get_list_info(
             this.my_stream_id,
             zoomed,
@@ -143,7 +166,7 @@ export class TopicListWidget {
             list_info.items.length === num_possible_topics &&
             stream_topic_history.is_complete_for_stream_id(this.my_stream_id);
 
-        const attrs = [["class", "topic-list"]];
+        const attrs: [string, string][] = [["class", "topic-list"]];
 
         const nodes = list_info.items.map((conversation) => keyed_topic_li(conversation));
 
@@ -167,28 +190,29 @@ export class TopicListWidget {
         return dom;
     }
 
-    get_parent() {
+    get_parent(): JQuery {
         return this.$parent_elem;
     }
 
-    get_stream_id() {
+    get_stream_id(): number {
         return this.my_stream_id;
     }
 
-    remove() {
+    remove(): void {
         this.$parent_elem.find(".topic-list").remove();
         this.prior_dom = undefined;
     }
 
-    build(spinner) {
+    build(spinner = false): void {
         const new_dom = this.build_list(spinner);
 
-        const replace_content = (html) => {
+        const replace_content = (html?: string): void => {
+            assert(html !== undefined);
             this.remove();
             this.$parent_elem.append(html);
         };
 
-        const find = () => this.$parent_elem.find(".topic-list");
+        const find = (): JQuery => this.$parent_elem.find(".topic-list");
 
         vdom.update(replace_content, find, new_dom, this.prior_dom);
 
@@ -202,7 +226,7 @@ export class TopicListWidget {
     }
 }
 
-export function clear_topic_search(e) {
+export function clear_topic_search(e: JQuery.Event): void {
     e.stopPropagation();
     const $input = $("#filter-topic-input");
     if ($input.length) {
@@ -215,13 +239,14 @@ export function clear_topic_search(e) {
 
         const stream_id = stream_ids[0];
         const widget = active_widgets.get(stream_id);
+        assert(widget !== undefined);
         const parent_widget = widget.get_parent();
 
         rebuild(parent_widget, stream_id);
     }
 }
 
-export function active_stream_id() {
+export function active_stream_id(): number | undefined {
     const stream_ids = [...active_widgets.keys()];
 
     if (stream_ids.length !== 1) {
@@ -231,7 +256,7 @@ export function active_stream_id() {
     return stream_ids[0];
 }
 
-export function get_stream_li() {
+export function get_stream_li(): JQuery | undefined {
     const widgets = [...active_widgets.values()];
 
     if (widgets.length !== 1) {
@@ -242,7 +267,7 @@ export function get_stream_li() {
     return $stream_li;
 }
 
-export function rebuild($stream_li, stream_id) {
+export function rebuild($stream_li: JQuery, stream_id: number): void {
     const active_widget = active_widgets.get(stream_id);
 
     if (active_widget) {
@@ -259,19 +284,20 @@ export function rebuild($stream_li, stream_id) {
 
 // For zooming, we only do topic-list stuff here...let stream_list
 // handle hiding/showing the non-narrowed streams
-export function zoom_in() {
+export function zoom_in(): void {
     zoomed = true;
 
     const stream_id = active_stream_id();
-    if (!stream_id) {
+    if (stream_id === undefined) {
         blueslip.error("Cannot find widget for topic history zooming.");
         return;
     }
 
     const active_widget = active_widgets.get(stream_id);
+    assert(active_widget !== undefined);
 
-    function on_success() {
-        if (!active_widgets.has(stream_id)) {
+    function on_success(): void {
+        if (!active_widgets.has(stream_id!)) {
             blueslip.warn("User re-narrowed before topic history was returned.");
             return;
         }
@@ -285,7 +311,7 @@ export function zoom_in() {
             return;
         }
 
-        active_widget.build();
+        active_widget!.build();
     }
 
     scroll_util.get_scroll_element($("#left_sidebar_scroll_container")).scrollTop(0);
@@ -296,15 +322,20 @@ export function zoom_in() {
     stream_topic_history_util.get_server_history(stream_id, on_success);
 }
 
-export function get_topic_search_term() {
-    const $filter = $("#filter-topic-input");
-    if ($filter.val() === undefined) {
+export function get_topic_search_term(): string {
+    const $filter = $<HTMLInputElement>("input#filter-topic-input");
+    const filter_val = $filter.val();
+    if (filter_val === undefined) {
         return "";
     }
-    return $filter.val().trim();
+    return filter_val.trim();
 }
 
-export function initialize({on_topic_click}) {
+export function initialize({
+    on_topic_click,
+}: {
+    on_topic_click: (stream_id: number, topic?: string) => void;
+}): void {
     $("#stream_filters").on(
         "click",
         ".sidebar-topic-check, .topic-name, .topic-markers-and-controls",
@@ -317,7 +348,9 @@ export function initialize({on_topic_click}) {
             }
 
             const $stream_row = $(e.target).parents(".narrow-filter");
-            const stream_id = Number.parseInt($stream_row.attr("data-stream-id"), 10);
+            const stream_id_string = $stream_row.attr("data-stream-id");
+            assert(stream_id_string !== undefined);
+            const stream_id = Number.parseInt(stream_id_string, 10);
             const topic = $(e.target).parents("li").attr("data-topic-name");
             on_topic_click(stream_id, topic);
 
@@ -325,7 +358,9 @@ export function initialize({on_topic_click}) {
         },
     );
 
-    $("body").on("input", "#filter-topic-input", () => {
-        active_widgets.get(active_stream_id()).build();
+    $("body").on("input", "#filter-topic-input", (): void => {
+        const stream_id = active_stream_id();
+        assert(stream_id !== undefined);
+        active_widgets.get(stream_id)?.build();
     });
 }
diff --git a/web/src/topic_list_data.ts b/web/src/topic_list_data.ts
index 2378d77439..1ccd24aecf 100644
--- a/web/src/topic_list_data.ts
+++ b/web/src/topic_list_data.ts
@@ -13,7 +13,7 @@ import * as util from "./util";
 const max_topics = 8;
 const max_topics_with_unread = 12;
 
-type TopicInfo = {
+export type TopicInfo = {
     topic_name: string;
     topic_resolved_prefix: string;
     topic_display_name: string;