diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index f7eee13b8a..c6a007c921 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -185,7 +185,7 @@ EXEMPT_FILES = make_set(
         "web/src/resize.js",
         "web/src/resize_handler.js",
         "web/src/rows.ts",
-        "web/src/scheduled_messages.js",
+        "web/src/scheduled_messages.ts",
         "web/src/scheduled_messages_feed_ui.js",
         "web/src/scheduled_messages_overlay_ui.js",
         "web/src/scheduled_messages_popover.js",
diff --git a/web/src/scheduled_messages.js b/web/src/scheduled_messages.ts
similarity index 73%
rename from web/src/scheduled_messages.js
rename to web/src/scheduled_messages.ts
index d2d64ef456..31ae225db2 100644
--- a/web/src/scheduled_messages.js
+++ b/web/src/scheduled_messages.ts
@@ -2,15 +2,35 @@ import * as channel from "./channel";
 import {$t} from "./i18n";
 import * as timerender from "./timerender";
 
+type ScheduledMessage = {
+    scheduled_message_id: number;
+    type: string;
+    to: number[] | number;
+    topic: string;
+    content: string;
+    rendered_content: string;
+    scheduled_delivery_timestamp: number;
+    failed: boolean;
+};
+
+type TimeKey =
+    | "today_nine_am"
+    | "today_four_pm"
+    | "tomorrow_nine_am"
+    | "tomorrow_four_pm"
+    | "monday_nine_am";
+
+type SendOption = {[key in TimeKey]?: {text: string; stamp: number}};
+
 export const MINIMUM_SCHEDULED_MESSAGE_DELAY_SECONDS = 5 * 60;
 
-// scheduled_messages_data is a dictionary where key=scheduled_message_id and value=scheduled_messages
-export const scheduled_messages_data = new Map();
+// scheduled_messages_data is a map where key=scheduled_message_id and value=scheduled_messages
+export const scheduled_messages_data = new Map<number, ScheduledMessage>();
 
-let selected_send_later_timestamp;
+let selected_send_later_timestamp: number | undefined;
 
-function compute_send_times(now = new Date()) {
-    const send_times = {};
+function compute_send_times(now = new Date()): Record<TimeKey, number> {
+    const send_times: Record<string, number> = {};
 
     const today = new Date(now);
     const tomorrow = new Date(new Date(now).setDate(now.getDate() + 1));
@@ -33,19 +53,19 @@ function compute_send_times(now = new Date()) {
     return send_times;
 }
 
-export function add_scheduled_messages(scheduled_messages) {
+export function add_scheduled_messages(scheduled_messages: ScheduledMessage[]): void {
     for (const scheduled_message of scheduled_messages) {
         scheduled_messages_data.set(scheduled_message.scheduled_message_id, scheduled_message);
     }
 }
 
-export function remove_scheduled_message(scheduled_message_id) {
+export function remove_scheduled_message(scheduled_message_id: number): void {
     if (scheduled_messages_data.has(scheduled_message_id)) {
         scheduled_messages_data.delete(scheduled_message_id);
     }
 }
 
-export function update_scheduled_message(scheduled_message) {
+export function update_scheduled_message(scheduled_message: ScheduledMessage): void {
     if (!scheduled_messages_data.has(scheduled_message.scheduled_message_id)) {
         return;
     }
@@ -53,18 +73,23 @@ export function update_scheduled_message(scheduled_message) {
     scheduled_messages_data.set(scheduled_message.scheduled_message_id, scheduled_message);
 }
 
-export function delete_scheduled_message(scheduled_msg_id, success = () => {}) {
-    channel.del({
+export function delete_scheduled_message(scheduled_msg_id: number, success?: () => void): void {
+    void channel.del({
         url: "/json/scheduled_messages/" + scheduled_msg_id,
         success,
     });
 }
 
-export function get_count() {
+export function get_count(): number {
     return scheduled_messages_data.size;
 }
 
-export function get_filtered_send_opts(date) {
+export function get_filtered_send_opts(date: Date): {
+    possible_send_later_today: SendOption | false;
+    send_later_tomorrow: SendOption;
+    possible_send_later_monday: SendOption | false;
+    send_later_custom: {text: string};
+} {
     const send_times = compute_send_times(date);
 
     const day = date.getDay(); // Starts with 0 for Sunday.
@@ -142,8 +167,8 @@ export function get_filtered_send_opts(date) {
         text: $t({defaultMessage: "Custom"}),
     };
 
-    let possible_send_later_today = {};
-    let possible_send_later_monday = {};
+    let possible_send_later_today: SendOption | false = {};
+    let possible_send_later_monday: SendOption | false = {};
 
     const minutes_into_day = date.getHours() * 60 + date.getMinutes();
     // Show Today send options based on time of day
@@ -171,28 +196,30 @@ export function get_filtered_send_opts(date) {
     };
 }
 
-export function get_selected_send_later_timestamp() {
+export function get_selected_send_later_timestamp(): number | undefined {
     if (!selected_send_later_timestamp) {
         return undefined;
     }
     return selected_send_later_timestamp;
 }
 
-export function get_formatted_selected_send_later_time() {
+export function get_formatted_selected_send_later_time(): string | undefined {
     if (!selected_send_later_timestamp) {
         return undefined;
     }
     return timerender.get_full_datetime(new Date(selected_send_later_timestamp * 1000), "time");
 }
 
-export function set_selected_schedule_timestamp(timestamp) {
+export function set_selected_schedule_timestamp(timestamp: number): void {
     selected_send_later_timestamp = timestamp;
 }
 
-export function reset_selected_schedule_timestamp() {
+export function reset_selected_schedule_timestamp(): void {
     selected_send_later_timestamp = undefined;
 }
 
-export function initialize(scheduled_messages_params) {
+export function initialize(scheduled_messages_params: {
+    scheduled_messages: ScheduledMessage[];
+}): void {
     add_scheduled_messages(scheduled_messages_params.scheduled_messages);
 }