diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 76c442cf37..31099659b0 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -243,7 +243,7 @@ EXEMPT_FILES = make_set(
         "web/src/stream_settings_containers.ts",
         "web/src/stream_settings_ui.js",
         "web/src/stream_ui_updates.js",
-        "web/src/submessage.js",
+        "web/src/submessage.ts",
         "web/src/subscriber_api.ts",
         "web/src/timerender.ts",
         "web/src/tippyjs.ts",
diff --git a/web/src/submessage.js b/web/src/submessage.ts
similarity index 84%
rename from web/src/submessage.js
rename to web/src/submessage.ts
index 93014a535e..ddc8f788e1 100644
--- a/web/src/submessage.js
+++ b/web/src/submessage.ts
@@ -3,6 +3,8 @@ import z from "zod";
 import * as blueslip from "./blueslip";
 import * as channel from "./channel";
 import * as message_store from "./message_store";
+import type {Message} from "./message_store";
+import type {Submessage} from "./types";
 import * as widgetize from "./widgetize";
 
 export const zform_widget_extra_data_schema = z.object({
@@ -47,7 +49,9 @@ const submessages_event_schema = z
     .tuple([widget_data_event_schema])
     .rest(inbound_data_event_schema);
 
-export function get_message_events(message) {
+type SubmessageEvents = z.infer<typeof submessages_event_schema>;
+
+export function get_message_events(message: Message): SubmessageEvents | undefined {
     if (message.locally_echoed) {
         return undefined;
     }
@@ -70,18 +74,19 @@ export function get_message_events(message) {
     return clean_events;
 }
 
-export function process_submessages(in_opts) {
+export function process_submessages(in_opts: {$row: JQuery; message_id: number}): void {
     // This happens in our rendering path, so we try to limit any
     // damage that may be triggered by one rogue message.
     try {
-        return do_process_submessages(in_opts);
+        do_process_submessages(in_opts);
+        return;
     } catch (error) {
         blueslip.error("Failed to do_process_submessages", undefined, error);
-        return undefined;
+        return;
     }
 }
 
-export function do_process_submessages(in_opts) {
+export function do_process_submessages(in_opts: {$row: JQuery; message_id: number}): void {
     const message_id = in_opts.message_id;
     const message = message_store.get(message_id);
 
@@ -129,7 +134,7 @@ export function do_process_submessages(in_opts) {
     });
 }
 
-export function update_message(submsg) {
+export function update_message(submsg: Submessage): void {
     const message = message_store.get(submsg.message_id);
 
     if (message === undefined) {
@@ -154,7 +159,7 @@ export function update_message(submsg) {
     message.submessages.push(submsg);
 }
 
-export function handle_event(submsg) {
+export function handle_event(submsg: Submessage): void {
     // Update message.submessages in case we haven't actually
     // activated the widget yet, so that when the message does
     // come in view, the data will be complete.
@@ -184,11 +189,13 @@ export function handle_event(submsg) {
     });
 }
 
-export function make_server_callback(message_id) {
-    return function (opts) {
+export function make_server_callback(
+    message_id: number,
+): (opts: {msg_type: string; data: string}) => void {
+    return function (opts: {msg_type: string; data: string}) {
         const url = "/json/submessage";
 
-        channel.post({
+        void channel.post({
             url,
             data: {
                 message_id,