diff --git a/tools/test-js-with-node b/tools/test-js-with-node
index 2be722b86e..be881d3b05 100755
--- a/tools/test-js-with-node
+++ b/tools/test-js-with-node
@@ -73,7 +73,7 @@ EXEMPT_FILES = make_set(
         "web/src/compose_fade.ts",
         "web/src/compose_notifications.ts",
         "web/src/compose_popovers.js",
-        "web/src/compose_recipient.js",
+        "web/src/compose_recipient.ts",
         "web/src/compose_reply.js",
         "web/src/compose_setup.js",
         "web/src/compose_state.ts",
diff --git a/web/src/compose_recipient.js b/web/src/compose_recipient.ts
similarity index 78%
rename from web/src/compose_recipient.js
rename to web/src/compose_recipient.ts
index ce37e1c452..9d176b0b80 100644
--- a/web/src/compose_recipient.js
+++ b/web/src/compose_recipient.ts
@@ -1,7 +1,9 @@
 /* Compose box module responsible for the message's recipient */
 
 import $ from "jquery";
-import _ from "lodash";
+import _, {isNumber} from "lodash";
+import assert from "minimalistic-assert";
+import type {Instance, Placement} from "tippy.js";
 
 import render_inline_decorated_stream_name from "../templates/inline_decorated_stream_name.hbs";
 
@@ -10,9 +12,11 @@ import * as compose_fade from "./compose_fade";
 import * as compose_pm_pill from "./compose_pm_pill";
 import * as compose_state from "./compose_state";
 import * as compose_ui from "./compose_ui";
+import type {ComposePlaceholderOptions, ComposeTriggeredOptions} from "./compose_ui";
 import * as compose_validate from "./compose_validate";
 import * as drafts from "./drafts";
 import * as dropdown_widget from "./dropdown_widget";
+import type {Option} from "./dropdown_widget";
 import {$t} from "./i18n";
 import * as narrow_state from "./narrow_state";
 import * as people from "./people";
@@ -23,19 +27,29 @@ import * as sub_store from "./sub_store";
 import * as ui_util from "./ui_util";
 import * as util from "./util";
 
-function composing_to_current_topic_narrow() {
+type MessageType = "stream" | "private";
+type DirectMessagesOption = {
+    is_direct_message: boolean;
+    unique_id: string | number;
+    name: string;
+};
+
+function composing_to_current_topic_narrow(): boolean {
     return (
-        util.lower_same(compose_state.stream_name(), narrow_state.stream_name() || "") &&
-        util.lower_same(compose_state.topic(), narrow_state.topic() || "")
+        util.lower_same(compose_state.stream_name(), narrow_state.stream_name() ?? "") &&
+        util.lower_same(compose_state.topic(), narrow_state.topic() ?? "")
     );
 }
 
-function composing_to_current_private_message_narrow() {
+function composing_to_current_private_message_narrow(): boolean {
     const compose_state_recipient = compose_state.private_message_recipient();
     const narrow_state_recipient = narrow_state.pm_emails_string();
+    if (narrow_state_recipient === undefined) {
+        return false;
+    }
     return (
-        compose_state_recipient &&
-        narrow_state_recipient &&
+        Boolean(compose_state_recipient) &&
+        Boolean(narrow_state_recipient) &&
         _.isEqual(
             compose_state_recipient
                 .split(",")
@@ -49,7 +63,7 @@ function composing_to_current_private_message_narrow() {
     );
 }
 
-export function update_narrow_to_recipient_visibility() {
+export function update_narrow_to_recipient_visibility(): void {
     const message_type = compose_state.get_message_type();
     if (message_type === "stream") {
         const stream_exists = Boolean(compose_state.stream_id());
@@ -76,7 +90,7 @@ export function update_narrow_to_recipient_visibility() {
     $(".narrow_to_compose_recipients").toggleClass("invisible", true);
 }
 
-function update_fade() {
+function update_fade(): void {
     if (!compose_state.composing()) {
         return;
     }
@@ -92,14 +106,14 @@ function update_fade() {
     compose_fade.update_all();
 }
 
-export function update_on_recipient_change() {
+export function update_on_recipient_change(): void {
     update_fade();
     update_narrow_to_recipient_visibility();
     drafts.update_compose_draft_count();
     check_posting_policy_for_compose_box();
 }
 
-export function get_posting_policy_error_message() {
+export function get_posting_policy_error_message(): string {
     if (compose_state.selected_recipient_id === "direct") {
         const recipients = compose_pm_pill.get_user_ids_string();
         if (!people.user_can_direct_message(recipients)) {
@@ -110,6 +124,10 @@ export function get_posting_policy_error_message() {
         return "";
     }
 
+    if (!isNumber(compose_state.selected_recipient_id)) {
+        return "";
+    }
+
     const stream = sub_store.get(compose_state.selected_recipient_id);
     if (stream && !stream_data.can_post_messages_in_stream(stream)) {
         return $t({
@@ -119,7 +137,7 @@ export function get_posting_policy_error_message() {
     return "";
 }
 
-export function check_posting_policy_for_compose_box() {
+export function check_posting_policy_for_compose_box(): void {
     const banner_text = get_posting_policy_error_message();
     if (banner_text === "") {
         compose_validate.set_recipient_disallowed(false);
@@ -135,14 +153,15 @@ export function check_posting_policy_for_compose_box() {
     compose_banner.show_error_message(banner_text, banner_classname, $("#compose_banners"));
 }
 
-function switch_message_type(message_type) {
+function switch_message_type(message_type: MessageType): void {
     $("#compose-content .alert").hide();
 
     compose_state.set_message_type(message_type);
 
     const opts = {
         message_type,
-        stream_id: compose_state.stream_id(),
+        trigger: "switch_message_type",
+        stream_id: compose_state.stream_id()!,
         topic: compose_state.topic(),
         private_message_recipient: compose_state.private_message_recipient(),
     };
@@ -151,7 +170,7 @@ function switch_message_type(message_type) {
     compose_ui.set_focus(opts);
 }
 
-function update_recipient_label(stream_id) {
+function update_recipient_label(stream_id: number): void {
     const stream = stream_data.get_sub_by_id(stream_id);
     if (stream === undefined) {
         $("#compose_select_recipient_widget .dropdown_widget_value").text(
@@ -164,7 +183,7 @@ function update_recipient_label(stream_id) {
     }
 }
 
-export function update_compose_for_message_type(opts) {
+export function update_compose_for_message_type(opts: ComposeTriggeredOptions): void {
     if (opts.message_type === "stream") {
         $("#compose-direct-recipient").hide();
         $("#compose_recipient_box").show();
@@ -192,10 +211,10 @@ export function update_compose_for_message_type(opts) {
     compose_banner.clear_uploads();
 }
 
-export function on_compose_select_recipient_update() {
+export function on_compose_select_recipient_update(): void {
     const prev_message_type = compose_state.get_message_type();
 
-    let curr_message_type = "stream";
+    let curr_message_type: MessageType = "stream";
     if (compose_state.selected_recipient_id === compose_state.DIRECT_MESSAGE_ID) {
         curr_message_type = "private";
     }
@@ -207,20 +226,23 @@ export function on_compose_select_recipient_update() {
     if (curr_message_type === "stream") {
         // Update stream name in the recipient box.
         const stream_id = compose_state.stream_id();
+        assert(stream_id !== undefined);
         update_recipient_label(stream_id);
     }
 
     update_on_recipient_change();
 }
 
-export function possibly_update_stream_name_in_compose(stream_id) {
+export function possibly_update_stream_name_in_compose(stream_id: number): void {
     if (compose_state.selected_recipient_id === stream_id) {
         on_compose_select_recipient_update();
     }
 }
 
-function item_click_callback(event, dropdown) {
-    let recipient_id = $(event.currentTarget).attr("data-unique-id");
+function item_click_callback(event: JQuery.ClickEvent, dropdown: Instance): void {
+    const recipient_id_str = $(event.currentTarget).attr("data-unique-id");
+    assert(recipient_id_str !== undefined);
+    let recipient_id: string | number = recipient_id_str;
     if (recipient_id !== compose_state.DIRECT_MESSAGE_ID) {
         recipient_id = Number.parseInt(recipient_id, 10);
     }
@@ -231,8 +253,9 @@ function item_click_callback(event, dropdown) {
     event.stopPropagation();
 }
 
-function get_options_for_recipient_widget() {
-    const options = stream_data.get_options_for_dropdown_widget();
+function get_options_for_recipient_widget(): Option[] {
+    const options: (Option | DirectMessagesOption)[] =
+        stream_data.get_options_for_dropdown_widget();
 
     const direct_messages_option = {
         is_direct_message: true,
@@ -251,7 +274,7 @@ function get_options_for_recipient_widget() {
     return options;
 }
 
-function compose_recipient_dropdown_on_show(dropdown) {
+function compose_recipient_dropdown_on_show(dropdown: Instance): void {
     // Offset to display dropdown above compose.
     let top_offset = 5;
     const window_height = window.innerHeight;
@@ -263,12 +286,12 @@ function compose_recipient_dropdown_on_show(dropdown) {
     // pixels below compose starting from top of compose box.
     const bottom_space = window_height - recipient_input_top - search_box_and_padding_height;
     // Show dropdown on top / bottom based on available space.
-    let placement = "top-start";
+    let placement: Placement = "top-start";
     if (bottom_space > top_space) {
         placement = "bottom-start";
         top_offset = -30;
     }
-    const offset = [-10, top_offset];
+    const offset: [number, number] = [-10, top_offset];
     dropdown.setProps({placement, offset});
     const height = Math.min(
         dropdown_widget.DEFAULT_DROPDOWN_HEIGHT,
@@ -278,16 +301,16 @@ function compose_recipient_dropdown_on_show(dropdown) {
     $popper.find(".dropdown-list-wrapper").css("max-height", height + "px");
 }
 
-export function open_compose_recipient_dropdown() {
+export function open_compose_recipient_dropdown(): void {
     $("#compose_select_recipient_widget").trigger("click");
 }
 
-function focus_compose_recipient() {
+function focus_compose_recipient(): void {
     $("#compose_select_recipient_widget_wrapper").trigger("focus");
 }
 
 // NOTE: Since tippy triggers this on `mousedown` it is always triggered before say a `click` on `textarea`.
-function on_hidden_callback() {
+function on_hidden_callback(): void {
     if (compose_state.get_message_type() === "stream") {
         // Always move focus to the topic input even if it's not empty,
         // since it's likely the user will want to update the topic
@@ -302,13 +325,13 @@ function on_hidden_callback() {
     }
 }
 
-export function handle_middle_pane_transition() {
-    if (compose_state.composing) {
+export function handle_middle_pane_transition(): void {
+    if (compose_state.composing()) {
         update_narrow_to_recipient_visibility();
     }
 }
 
-export function initialize() {
+export function initialize(): void {
     new dropdown_widget.DropdownWidget({
         widget_name: "compose_select_recipient",
         get_options: get_options_for_recipient_widget,
@@ -334,18 +357,28 @@ export function initialize() {
     });
 }
 
-export function update_placeholder_text() {
+export function update_placeholder_text(): void {
     // Change compose placeholder text only if compose box is open.
     if (!$("textarea#compose-textarea").is(":visible")) {
         return;
     }
+    const message_type = compose_state.get_message_type();
+    assert(message_type !== undefined);
 
-    const opts = {
-        message_type: compose_state.get_message_type(),
-        stream_id: compose_state.stream_id(),
-        topic: compose_state.topic(),
-        direct_message_user_ids: compose_pm_pill.get_user_ids(),
-    };
+    let opts: ComposePlaceholderOptions;
+    if (message_type === "stream") {
+        const stream_id = compose_state.stream_id();
+        opts = {
+            message_type,
+            stream_id,
+            topic: compose_state.topic(),
+        };
+    } else {
+        opts = {
+            message_type,
+            direct_message_user_ids: compose_pm_pill.get_user_ids(),
+        };
+    }
 
     $("textarea#compose-textarea").attr("placeholder", compose_ui.compute_placeholder_text(opts));
 }
diff --git a/web/src/compose_ui.ts b/web/src/compose_ui.ts
index c6f80ef0a7..124ab8b7df 100644
--- a/web/src/compose_ui.ts
+++ b/web/src/compose_ui.ts
@@ -22,7 +22,7 @@ import * as stream_data from "./stream_data";
 import * as user_status from "./user_status";
 import * as util from "./util";
 
-type ComposeTriggeredOptions = {
+export type ComposeTriggeredOptions = {
     trigger: string;
 } & (
     | {
@@ -35,7 +35,7 @@ type ComposeTriggeredOptions = {
           private_message_recipient: string;
       }
 );
-type ComposePlaceholderOptions =
+export type ComposePlaceholderOptions =
     | {
           message_type: "stream";
           stream_id: number | undefined;
diff --git a/web/src/dropdown_widget.ts b/web/src/dropdown_widget.ts
index 83b8bb1f4b..b6b74b7d97 100644
--- a/web/src/dropdown_widget.ts
+++ b/web/src/dropdown_widget.ts
@@ -60,7 +60,7 @@ type DropdownWidgetOptions = {
     tippy_props?: Partial<tippy.Props>;
     // NOTE: Any value other than `undefined` will be rendered when class is initialized.
     default_id?: string | number;
-    unique_id_type: DataTypes;
+    unique_id_type?: DataTypes;
     // Text to show if the current value is not in `get_options()`.
     text_if_current_value_not_in_options?: string;
     hide_search_box?: boolean;
@@ -90,7 +90,7 @@ export class DropdownWidget {
     instance: tippy.Instance | undefined;
     default_id: string | number | undefined;
     current_value: string | number | undefined;
-    unique_id_type: DataTypes;
+    unique_id_type: DataTypes | undefined;
     $events_container: JQuery;
     text_if_current_value_not_in_options: string;
     hide_search_box: boolean;