message_list_view: Refactor me message calculation for typescript.

We're going to need to be able to build the message list view
in pieces and put it all together in the end, instead of assigning
values directly to a half-formed object (which is hard to type).

This is part of the work towards that.

docs/subsystems/widgets.md

@@ -23,7 +23,7 @@ These are the least complex. We use our markdown processors to
 detect if a message is a `/me` message, plumb the flag through
 the message object (as `is_me_message`) and have the clients
 format it correctly. Related code (for the web app) lies in
-`message_list_view.js` in `_maybe_format_me_message`.
+`message_list_view.js` in `_maybe_get_me_message`.
 
 ## Polls, todo lists, and games
 

web/src/message_list_view.js

@@ -527,7 +527,10 @@ export class MessageListView {
             );
         }
 
-        this._maybe_format_me_message(message_container);
+        Object.assign(
+            message_container,
+            this._maybe_get_me_message(message_container.is_hidden, message_container.msg),
+        );
         // Once all other variables are updated
         this._add_msg_edited_vars(message_container);
     }
@@ -1656,22 +1659,25 @@ export class MessageListView {
         }
     }
 
-    _maybe_format_me_message(message_container) {
+    _maybe_get_me_message(is_hidden, message) {
         // If the message is to be hidden anyway, no need to render
         // it differently.
-        if (!message_container.is_hidden && message_container.msg.is_me_message) {
+        if (!is_hidden && message.is_me_message) {
             // Slice the '<p>/me ' off the front, and '</p>' off the first line
             // 'p' tag is sliced off to get sender in the same line as the
             // first line of the message
-            const msg_content = message_container.msg.content;
+            const msg_content = message.content;
             const p_index = msg_content.indexOf("</p>");
-            message_container.status_message =
+            return {
+                status_message:
                     msg_content.slice("<p>/me ".length, p_index) +
-                msg_content.slice(p_index + "</p>".length);
-            message_container.include_sender = true;
-        } else {
-            message_container.status_message = false;
+                    msg_content.slice(p_index + "</p>".length),
+                include_sender: true,
+            };
         }
+        return {
+            status_message: false,
+        };
     }
 
     /* This function exist for two purposes:

web/tests/message_list_view.test.js

@@ -168,7 +168,10 @@ test("msg_moved_var", () => {
         const list = build_list([message_group]);
 
         for (const message_container of messages) {
-            list._maybe_format_me_message(message_container);
+            Object.assign(
+                message_container,
+                list._maybe_get_me_message(message_container.is_hidden, message_container.msg),
+            );
             list._add_msg_edited_vars(message_container);
         }
 
@@ -277,7 +280,10 @@ test("msg_edited_vars", () => {
         const list = build_list([message_group]);
 
         for (const message_container of messages) {
-            list._maybe_format_me_message(message_container);
+            Object.assign(
+                message_container,
+                list._maybe_get_me_message(message_container.is_hidden, message_container.msg),
+            );
             list._add_msg_edited_vars(message_container);
         }