diff --git a/docs/contributing/code-style.md b/docs/contributing/code-style.md
index 2e512b6ffa..22bff0c157 100644
--- a/docs/contributing/code-style.md
+++ b/docs/contributing/code-style.md
@@ -70,7 +70,7 @@ To speed things up, you can [pass specific files or directories
 to the linter](../testing/linters.md):
 
 ```
-$ ./tools/lint web/src/compose.js
+$ ./tools/lint web/src/compose.ts
 ```
 
 If you'd like, you can also set up a local Git commit hook that
diff --git a/docs/testing/linters.md b/docs/testing/linters.md
index 6c033f8bf0..326e40c594 100644
--- a/docs/testing/linters.md
+++ b/docs/testing/linters.md
@@ -43,7 +43,7 @@ You can also run them individually or pass specific files:
 
 ```bash
 ./tools/lint
-./tools/lint web/src/compose.js
+./tools/lint web/src/compose.ts
 ./tools/lint web/src/
 ```
 
diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 4a71e4001c..405a5fefd1 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -69,7 +69,7 @@ EXEMPT_FILES = make_set(
         "web/src/channel_folders_ui.ts",
         "web/src/click_handlers.ts",
         "web/src/color_picker_popover.ts",
-        "web/src/compose.js",
+        "web/src/compose.ts",
         "web/src/compose_actions.ts",
         "web/src/compose_banner.ts",
         "web/src/compose_call_ui.ts",
diff --git a/web/src/compose.js b/web/src/compose.ts
similarity index 78%
rename from web/src/compose.js
rename to web/src/compose.ts
index 47ffebb7f2..bf882df528 100644
--- a/web/src/compose.js
+++ b/web/src/compose.ts
@@ -2,6 +2,8 @@
 
 import autosize from "autosize";
 import $ from "jquery";
+import assert from "minimalistic-assert";
+import * as z from "zod/mini";
 
 import render_success_message_scheduled_banner from "../templates/compose_banner/success_message_scheduled_banner.hbs";
 import render_wildcard_mention_not_allowed_error from "../templates/compose_banner/wildcard_mention_not_allowed_error.hbs";
@@ -14,7 +16,9 @@ import * as compose_ui from "./compose_ui.ts";
 import * as compose_validate from "./compose_validate.ts";
 import * as drafts from "./drafts.ts";
 import * as echo from "./echo.ts";
+import type {PostMessageAPIData} from "./echo.ts";
 import * as message_events from "./message_events.ts";
+import type {LocalMessage} from "./message_helper.ts";
 import * as onboarding_steps from "./onboarding_steps.ts";
 import * as scheduled_messages from "./scheduled_messages.ts";
 import * as sent_messages from "./sent_messages.ts";
@@ -27,17 +31,45 @@ import * as zcommand from "./zcommand.ts";
 
 // Docs: https://zulip.readthedocs.io/en/latest/subsystems/sending-messages.html
 
-export function clear_invites() {
+// This is similar to transmit.SendMessageData but not quite the same.
+// We might want to try to consolidate them at some point.
+export type SendMessageData = {
+    sender_id: number;
+    queue_id: string | null;
+    to: string;
+    content: string;
+    resend?: boolean;
+    locally_echoed?: boolean;
+    draft_id: string;
+} & (
+    | {
+          type: "stream";
+          stream_id: number;
+          topic: string;
+      }
+    | {
+          type: "private";
+          reply_to: string;
+          private_message_recipient: string;
+          to_user_ids: string | undefined;
+          // TODO: It would be nice to only require stream_id and topic
+          // on "stream" objects, but that requires some refactoring.
+          stream_id: undefined;
+          topic: "";
+      }
+);
+
+export function clear_invites(): void {
     $(
         `#compose_banners .${CSS.escape(compose_banner.CLASSNAMES.recipient_not_subscribed)}`,
     ).remove();
 }
 
-export function clear_private_stream_alert() {
+export function clear_private_stream_alert(): void {
     $(`#compose_banners .${CSS.escape(compose_banner.CLASSNAMES.private_stream_warning)}`).remove();
 }
 
-export function clear_preview_area() {
+export function clear_preview_area(): void {
     $("textarea#compose-textarea").trigger("focus");
     $("#compose .undo_markdown_preview").hide();
     $("#compose .preview_message_area").hide();
@@ -51,7 +83,7 @@ export function clear_preview_area() {
     $("#compose .preview_mode_disabled .compose_control_button").attr("tabindex", 0);
 }
 
-export function show_preview_area() {
+export function show_preview_area(): void {
     // Disable unneeded compose_control_buttons as we don't
     // need them in preview mode.
     $("#compose").addClass("preview_mode");
@@ -64,9 +96,10 @@ export function show_preview_area() {
     render_preview_area();
 }
 
-export function render_preview_area() {
-    const $compose_textarea = $("textarea#compose-textarea");
+export function render_preview_area(): void {
+    const $compose_textarea = $<HTMLTextAreaElement>("textarea#compose-textarea");
     const content = $compose_textarea.val();
+    assert(content !== undefined);
     const $preview_message_area = $("#compose .preview_message_area");
     compose_ui.render_and_show_preview(
         $("#compose"),
@@ -79,7 +112,7 @@ export function render_preview_area() {
     $preview_message_area.show();
 }
 
-export function clear_compose_box() {
+export function clear_compose_box(): void {
     /* Before clearing the compose box, we reset it to the
      * default/normal size. Note that for locally echoed messages, we
      * will have already done this action before echoing the message
@@ -102,7 +135,16 @@ export function clear_compose_box() {
     $(".needs-empty-compose").removeClass("disabled-on-hover");
 }
 
-export function send_message_success(sent_message, data) {
+export type SentMessageData = SendMessageData & {
+    local_id: string;
+    locally_echoed: boolean;
+    resend: boolean;
+};
+
+export function send_message_success(
+    sent_message: SentMessageData | LocalMessage,
+    data: PostMessageAPIData,
+): void {
     if (!sent_message.locally_echoed) {
         clear_compose_box();
     }
@@ -117,7 +159,10 @@ export function send_message_success(sent_message, data) {
             }
             // topic has been automatically unmuted or followed. No need to
             // suggest the user to unmute. Show the banner and return.
-            compose_notifications.notify_automatic_new_visibility_policy(sent_message, data);
+            compose_notifications.notify_automatic_new_visibility_policy(sent_message, {
+                ...data,
+                automatic_new_visibility_policy: data.automatic_new_visibility_policy,
+            });
             return;
         }
 
@@ -132,7 +177,7 @@ export function send_message_success(sent_message, data) {
     }
 }
 
-export let send_message = () => {
+export let send_message = (): void => {
     // Changes here must also be kept in sync with echo.try_deliver_locally
     compose_state.set_recipient_edited_manually(false);
     compose_state.set_is_content_unedited_restored_draft(false);
@@ -148,9 +193,10 @@ export let send_message = () => {
         // message can just disappear into the void.
         force_save: true,
     });
-    const message_type = compose_state.get_message_type();
+    assert(draft_id !== undefined);
 
-    let message_data;
+    const message_type = compose_state.get_message_type();
+    let message_data: SendMessageData;
     if (message_type === "private") {
         const recipient_emails = compose_state.private_message_recipient_emails();
         const recipient_ids = compose_state.private_message_recipient_ids();
@@ -168,7 +214,9 @@ export let send_message = () => {
             stream_id: undefined,
         };
     } else {
+        assert(message_type === "stream");
         const stream_id = compose_state.stream_id();
+        assert(stream_id !== undefined);
         const topic = compose_state.topic();
         message_data = {
             type: message_type,
@@ -182,37 +230,43 @@ export let send_message = () => {
         };
     }
 
-    let local_id;
-    let locally_echoed;
+    let local_id: string;
 
     const message = echo.try_deliver_locally(message_data, message_events.insert_new_messages);
+    const locally_echoed = Boolean(message);
     if (message) {
         // We are rendering this message locally with an id
         // like 92l99.01 that corresponds to a reasonable
         // approximation of the id we'll get from the server
         // in terms of sorting messages.
+        assert(message.local_id !== undefined);
         local_id = message.local_id;
-        locally_echoed = true;
     } else {
         // We are not rendering this message locally, but we
         // track the message's life cycle with an id like
         // loc-1, loc-2, loc-3,etc.
-        locally_echoed = false;
         local_id = sent_messages.get_new_local_id();
     }
 
-    const request = {
-        ...message_data,
-        local_id,
-        locally_echoed,
-        resend: false,
-    };
-
-    function success(data) {
-        send_message_success(request, data);
+    function success(data: unknown): void {
+        const parsed_data = z
+            .object({
+                id: z.number(),
+                automatic_new_visibility_policy: z.optional(z.number()),
+            })
+            .parse(data);
+        send_message_success(
+            {
+                ...message_data,
+                local_id,
+                locally_echoed,
+                resend: false,
+            },
+            parsed_data,
+        );
     }
 
-    function error(response, server_error_code) {
+    function error(response: string, server_error_code: string): void {
         // Error callback for failed message send attempts.
         if (!locally_echoed) {
             if (server_error_code === "TOPIC_WILDCARD_MENTION_NOT_ALLOWED") {
@@ -245,35 +299,45 @@ export let send_message = () => {
             return;
         }
 
+        assert(message !== undefined);
         echo.message_send_error(message.id, response);
 
         // We might not have updated the draft count because we assumed the
         // message would send. Ensure that the displayed count is correct.
         drafts.sync_count();
 
-        const draft = drafts.draft_model.getDraft(message_data.draft_id);
+        assert(draft_id !== undefined);
+        const draft = drafts.draft_model.getDraft(draft_id);
+        assert(draft !== false);
         draft.is_sending_saving = false;
-        drafts.draft_model.editDraft(message_data.draft_id, draft);
+        drafts.draft_model.editDraft(draft_id, draft);
     }
 
-    transmit.send_message(request, success, error);
+    transmit.send_message(
+        {...message_data, local_id, locally_echoed, resend: false},
+        success,
+        error,
+    );
     server_events_state.assert_get_events_running(
         "Restarting get_events because it was not running during send",
     );
 
     if (locally_echoed) {
         clear_compose_box();
+        assert(message !== undefined);
         // Schedule a timer to display a spinner when the message is
         // taking a longtime to send.
-        setTimeout(() => echo.display_slow_send_loading_spinner(message), 5000);
+        setTimeout(() => {
+            echo.display_slow_send_loading_spinner(message);
+        }, 5000);
     }
 };
 
-export function rewire_send_message(value) {
+export function rewire_send_message(value: typeof send_message): void {
     send_message = value;
 }
 
-export function handle_enter_key_with_preview_open(cmd_or_ctrl_pressed = false) {
+export function handle_enter_key_with_preview_open(cmd_or_ctrl_pressed = false): void {
     if (
         (user_settings.enter_sends && !cmd_or_ctrl_pressed) ||
         (!user_settings.enter_sends && cmd_or_ctrl_pressed)
@@ -289,7 +353,7 @@ export function handle_enter_key_with_preview_open(cmd_or_ctrl_pressed = false)
 // Common entrypoint for asking the server to send the message
 // currently drafted in the compose box, including for scheduled
 // messages.
-export let finish = (scheduling_message = false) => {
+export let finish = (scheduling_message = false): boolean | undefined => {
     if (compose_ui.compose_spinner_visible) {
         // Avoid sending a message twice in parallel in races where
         // the user clicks the `Send` button very quickly twice or
@@ -329,18 +393,18 @@ export let finish = (scheduling_message = false) => {
     return true;
 };
 
-export function rewire_finish(value) {
+export function rewire_finish(value: typeof finish): void {
     finish = value;
 }
 
-export function do_post_send_tasks() {
+export function do_post_send_tasks(): void {
     clear_preview_area();
     // TODO: Do we want to fire the event even if the send failed due
     // to a server-side error?
     $(document).trigger("compose_finished.zulip");
 }
 
-function schedule_message_to_custom_date() {
+function schedule_message_to_custom_date(): void {
     const deliver_at = scheduled_messages.get_formatted_selected_send_later_time();
     const scheduled_delivery_timestamp = scheduled_messages.get_selected_send_later_timestamp();
 
@@ -373,13 +437,15 @@ function schedule_message_to_custom_date() {
         update_count: false,
         is_sending_saving: true,
     });
+    assert(draft_id !== undefined);
 
     const $banner_container = $("#compose_banners");
-    const success = function (data) {
+    const success = function (data: unknown): void {
         drafts.draft_model.deleteDrafts([draft_id]);
         clear_compose_box();
+        const {scheduled_message_id} = z.object({scheduled_message_id: z.number()}).parse(data);
         const new_row_html = render_success_message_scheduled_banner({
-            scheduled_message_id: data.scheduled_message_id,
+            scheduled_message_id,
             minimum_scheduled_message_delay_minutes:
                 scheduled_messages.MINIMUM_SCHEDULED_MESSAGE_DELAY_SECONDS / 60,
             deliver_at,
@@ -390,7 +456,7 @@ function schedule_message_to_custom_date() {
         compose_banner.append_compose_banner_to_banner_list($(new_row_html), $banner_container);
     };
 
-    const error = function (xhr) {
+    const error = function (xhr: JQuery.jqXHR<unknown>): void {
         const response = channel.xhr_error_message("Error sending message", xhr);
         compose_ui.hide_compose_spinner();
         compose_banner.show_error_message(
@@ -400,6 +466,7 @@ function schedule_message_to_custom_date() {
             $("textarea#compose-textarea"),
         );
         const draft = drafts.draft_model.getDraft(draft_id);
+        assert(draft !== false);
         draft.is_sending_saving = false;
         drafts.draft_model.editDraft(draft_id, draft);
     };
@@ -412,6 +479,6 @@ function schedule_message_to_custom_date() {
     });
 }
 
-export function is_topic_input_focused() {
+export function is_topic_input_focused(): boolean {
     return $("#stream_message_recipient_topic").is(":focus");
 }
diff --git a/web/src/compose_notifications.ts b/web/src/compose_notifications.ts
index 9a1c3249a0..e23aefbd56 100644
--- a/web/src/compose_notifications.ts
+++ b/web/src/compose_notifications.ts
@@ -8,6 +8,7 @@ import render_message_sent_banner from "../templates/compose_banner/message_sent
 import render_unmute_topic_banner from "../templates/compose_banner/unmute_topic_banner.hbs";
 
 import * as blueslip from "./blueslip.ts";
+import type {SentMessageData} from "./compose.ts";
 import * as compose_banner from "./compose_banner.ts";
 import * as hash_util from "./hash_util.ts";
 import {$t} from "./i18n.ts";
@@ -78,7 +79,7 @@ export function notify_above_composebox(
 }
 
 export function notify_automatic_new_visibility_policy(
-    message: Message,
+    message: Message | (SentMessageData & {type: "stream"}),
     data: {automatic_new_visibility_policy: number; id: number},
 ): void {
     const followed =
@@ -108,7 +109,9 @@ export function notify_automatic_new_visibility_policy(
 
 // Note that this returns values that are not HTML-escaped, for use in
 // Handlebars templates that will do further escaping.
-function get_message_recipient(message: Message): MessageRecipient {
+function get_message_recipient(
+    message: Message | (SentMessageData & {type: "stream"}),
+): MessageRecipient {
     if (message.type === "stream") {
         const channel_message_recipient: MessageRecipient = {
             message_type: "channel",
@@ -149,7 +152,7 @@ function get_message_recipient(message: Message): MessageRecipient {
     return direct_message_recipient;
 }
 
-export function get_muted_narrow(message: Message): string | undefined {
+export function get_muted_narrow(message: Message | SentMessageData): string | undefined {
     if (
         message.type === "stream" &&
         stream_data.is_muted(message.stream_id) &&
@@ -313,7 +316,9 @@ export function notify_local_mixes(
     }
 }
 
-function get_above_composebox_narrow_url(message: Message): string {
+function get_above_composebox_narrow_url(
+    message: Message | (SentMessageData & {type: "stream"}),
+): string {
     let above_composebox_narrow_url;
     if (message.type === "stream") {
         above_composebox_narrow_url = hash_util.by_stream_topic_url(
diff --git a/web/src/compose_send_menu_popover.js b/web/src/compose_send_menu_popover.js
index 1ed1a6c212..079454fd09 100644
--- a/web/src/compose_send_menu_popover.js
+++ b/web/src/compose_send_menu_popover.js
@@ -6,7 +6,7 @@ import render_send_later_popover from "../templates/popovers/send_later_popover.
 
 import * as blueslip from "./blueslip.ts";
 import * as channel from "./channel.ts";
-import * as compose from "./compose.js";
+import * as compose from "./compose.ts";
 import * as compose_state from "./compose_state.ts";
 import * as compose_validate from "./compose_validate.ts";
 import * as drafts from "./drafts.ts";
diff --git a/web/src/compose_setup.js b/web/src/compose_setup.js
index 710e1225c2..cb68723f54 100644
--- a/web/src/compose_setup.js
+++ b/web/src/compose_setup.js
@@ -5,7 +5,7 @@ import {unresolve_name} from "../shared/src/resolved_topic.ts";
 import render_add_poll_modal from "../templates/add_poll_modal.hbs";
 import render_add_todo_list_modal from "../templates/add_todo_list_modal.hbs";
 
-import * as compose from "./compose.js";
+import * as compose from "./compose.ts";
 import * as compose_actions from "./compose_actions.ts";
 import * as compose_banner from "./compose_banner.ts";
 import * as compose_call from "./compose_call.ts";
diff --git a/web/src/echo.ts b/web/src/echo.ts
index ab30afe634..158dda5752 100644
--- a/web/src/echo.ts
+++ b/web/src/echo.ts
@@ -8,6 +8,7 @@ import render_message_controls_failed_msg from "../templates/message_controls_fa
 import * as alert_words from "./alert_words.ts";
 import * as blueslip from "./blueslip.ts";
 import * as browser_history from "./browser_history.ts";
+import type * as compose from "./compose.ts";
 import * as compose_notifications from "./compose_notifications.ts";
 import * as compose_ui from "./compose_ui.ts";
 import * as echo_state from "./echo_state.ts";
@@ -59,15 +60,15 @@ type PrivateMessageObject = {
     reply_to: string;
     private_message_recipient: string;
     to_user_ids: string | undefined;
+    display_recipient?: DisplayRecipientUser[];
 };
 
 type StreamMessageObject = {
     type: "stream";
     stream_id: number;
+    display_recipient?: string;
 };
 
-type MessageRequest = MessageRequestObject & (PrivateMessageObject | StreamMessageObject);
-
 type LocalEditRequest = Partial<{
     raw_content: string | undefined;
     content: string;
@@ -98,12 +99,10 @@ export type RawLocalMessage = MessageRequestObject & {
     id: number;
     topic_links: TopicLink[];
     reactions: MessageReaction[];
-} & (
-        | (StreamMessageObject & {display_recipient?: string})
-        | (PrivateMessageObject & {display_recipient?: DisplayRecipientUser[]})
-    );
+    draft_id: string | undefined;
+} & (StreamMessageObject | PrivateMessageObject);
 
-type PostMessageAPIData = z.output<typeof send_message_api_response_schema>;
+export type PostMessageAPIData = z.output<typeof send_message_api_response_schema>;
 
 // These retry spinner functions return true if and only if the
 // spinner already is in the requested state, which can be used to
@@ -159,7 +158,7 @@ function resend_message(
         on_send_message_success,
         send_message,
     }: {
-        on_send_message_success: (request: Message, data: PostMessageAPIData) => void;
+        on_send_message_success: typeof compose.send_message_success;
         send_message: typeof transmit.send_message;
     },
 ): void {
@@ -250,7 +249,7 @@ export function track_local_message(message: LocalMessage): void {
 }
 
 export function insert_local_message(
-    message_request: MessageRequest,
+    message_request: compose.SendMessageData,
     local_id_float: number,
     insert_new_messages: (opts: InsertNewMessagesOpts) => Message[],
 ): Message {
@@ -295,7 +294,7 @@ export function is_slash_command(content: string): boolean {
 }
 
 export let try_deliver_locally = (
-    message_request: MessageRequest,
+    message_request: compose.SendMessageData,
     insert_new_messages: (opts: InsertNewMessagesOpts) => Message[],
 ): Message | undefined => {
     // Checks if the message request can be locally echoed, and if so,
@@ -656,7 +655,7 @@ export function initialize({
     on_send_message_success,
     send_message,
 }: {
-    on_send_message_success: (request: Message, data: PostMessageAPIData) => void;
+    on_send_message_success: typeof compose.send_message_success;
     send_message: typeof transmit.send_message;
 }): void {
     function on_failed_action(
@@ -668,7 +667,7 @@ export function initialize({
                 on_send_message_success,
                 send_message,
             }: {
-                on_send_message_success: (request: Message, data: PostMessageAPIData) => void;
+                on_send_message_success: typeof compose.send_message_success;
                 send_message: typeof transmit.send_message;
             },
         ) => void,
diff --git a/web/src/hotkey.js b/web/src/hotkey.js
index 20fe20e354..c4d643affb 100644
--- a/web/src/hotkey.js
+++ b/web/src/hotkey.js
@@ -6,7 +6,7 @@ import * as activity_ui from "./activity_ui.ts";
 import * as browser_history from "./browser_history.ts";
 import * as color_picker_popover from "./color_picker_popover.ts";
 import * as common from "./common.ts";
-import * as compose from "./compose.js";
+import * as compose from "./compose.ts";
 import * as compose_actions from "./compose_actions.ts";
 import * as compose_banner from "./compose_banner.ts";
 import * as compose_recipient from "./compose_recipient.ts";
diff --git a/web/src/reload_setup.js b/web/src/reload_setup.js
index 22ff992eb4..352a5049a9 100644
--- a/web/src/reload_setup.js
+++ b/web/src/reload_setup.js
@@ -1,6 +1,6 @@
 import * as activity from "./activity.ts";
 import * as blueslip from "./blueslip.ts";
-import * as compose from "./compose.js";
+import * as compose from "./compose.ts";
 import * as compose_actions from "./compose_actions.ts";
 import {localstorage} from "./localstorage.ts";
 import * as message_fetch from "./message_fetch.ts";
diff --git a/web/src/ui_init.js b/web/src/ui_init.js
index 2ba087c24d..cc158bcbcc 100644
--- a/web/src/ui_init.js
+++ b/web/src/ui_init.js
@@ -26,7 +26,7 @@ import * as channel_folders_popover from "./channel_folders_popover.ts";
 import * as click_handlers from "./click_handlers.ts";
 import * as color_picker_popover from "./color_picker_popover.ts";
 import * as common from "./common.ts";
-import * as compose from "./compose.js";
+import * as compose from "./compose.ts";
 import * as compose_closed_ui from "./compose_closed_ui.ts";
 import * as compose_notifications from "./compose_notifications.ts";
 import * as compose_paste from "./compose_paste.ts";
diff --git a/web/src/zcommand.ts b/web/src/zcommand.ts
index 6be2cb4419..43a45e1286 100644
--- a/web/src/zcommand.ts
+++ b/web/src/zcommand.ts
@@ -144,7 +144,7 @@ export function process(message_content: string): boolean {
     }
 
     // It is incredibly important here to return false
-    // if we don't see an actual zcommand, so that compose.js
+    // if we don't see an actual zcommand, so that compose.ts
     // knows this is a normal message.
     return false;
 }