message_list_data: Potentially return undefined in first_unread_message_id.

web/src/message_list.js

@@ -1,5 +1,6 @@
 import autosize from "autosize";
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import {all_messages_data} from "./all_messages_data";
 import * as blueslip from "./blueslip";
@@ -131,7 +132,9 @@ export class MessageList {
             // message. Regardless of whether the messages are new or
             // old, we want to select a message as though we just
             // entered this view.
-            this.select_id(this.first_unread_message_id(), {then_scroll: true, use_closest: true});
+            const first_unread_message_id = this.first_unread_message_id();
+            assert(first_unread_message_id !== undefined);
+            this.select_id(first_unread_message_id, {then_scroll: true, use_closest: true});
         }
 
         return render_info;

web/src/message_list_data.ts

@@ -1,5 +1,3 @@
-import assert from "minimalistic-assert";
-
 import * as blueslip from "./blueslip";
 import {FetchStatus} from "./fetch_status";
 import type {Filter} from "./filter";
@@ -285,7 +283,7 @@ export class MessageListData {
         this._items = this.unmuted_messages(this._all_items);
     }
 
-    first_unread_message_id(): number {
+    first_unread_message_id(): number | undefined {
         const first_unread = this._items.find((message) => unread.message_unread(message));
 
         if (first_unread) {
@@ -293,9 +291,7 @@ export class MessageListData {
         }
 
         // if no unread, return the bottom message
-        const last = this.last();
-        assert(last !== undefined);
-        return last.id;
+        return this.last()?.id;
     }
 
     has_unread_messages(): boolean {

web/src/narrow.js

@@ -1,5 +1,6 @@
 import * as Sentry from "@sentry/browser";
 import $ from "jquery";
+import assert from "minimalistic-assert";
 
 import {all_messages_data} from "./all_messages_data";
 import * as blueslip from "./blueslip";
@@ -730,6 +731,8 @@ export function update_selection(opts) {
     if (msg_id === undefined) {
         msg_id = message_lists.current.first_unread_message_id();
     }
+    // There should be something since it's not visibly empty.
+    assert(msg_id !== undefined);
 
     const preserve_pre_narrowing_screen_position =
         message_lists.current.get(msg_id) !== undefined && select_offset !== undefined;
@@ -1122,6 +1125,7 @@ export function deactivate() {
                 }
                 message_id_to_select = message_lists.current.selected_id();
             }
+            assert(message_id_to_select !== undefined);
             message_lists.current.select_id(message_id_to_select, select_opts);
         }