messagesActions [nfc]: Rearrange getSingleMessage logic slightly

Now that this is its own function, it can be simplified slightly
with an early return, and by dropping some internal comments that
are now covered by its jsdoc.

Author: gnprice

src/message/messagesActions.js

@@ -48,40 +48,39 @@ const getSingleMessage =
   (messageId: number): ThunkAction<Promise<Message | null>> =>
   async (dispatch, getState) => {
     const state = getState();
-
-    const auth = getAuth(state);
     const messages = getMessages(state);
-    const zulipFeatureLevel = getZulipFeatureLevel(state);
-    const allowEditHistory = getRealm(state).allowEditHistory;
-
-    // Try to get it from our local data to avoid a server round-trip…
     let message = messages.get(messageId);
+    if (message) {
+      return message;
+    }
 
-    // …but if we have to, go and ask the server.
     // TODO: Give feedback when the server round trip takes longer than
     //   expected.
     // TODO: Let the user cancel the request so we don't force a doNarrow
     //   after they've given up on tapping the link, and perhaps forgotten
     //   about it. Like any request, this might take well over a minute to
     //   resolve, or never resolve.
     // TODO: When these are fixed, remove warning in jsdoc.
-    if (!message) {
+
+    const auth = getAuth(state);
+    const zulipFeatureLevel = getZulipFeatureLevel(state);
+    const allowEditHistory = getRealm(state).allowEditHistory;
+
+    if (zulipFeatureLevel < 120) {
       // TODO(server-5.0): Simplify; simplify jsdoc to match.
-      if (zulipFeatureLevel < 120) {
-        // api.getSingleMessage won't give us the message's stream and
-        // topic; see there.
-        return null;
-      }
-      try {
-        message = await api.getSingleMessage(
-          auth,
-          { message_id: messageId },
-          zulipFeatureLevel,
-          allowEditHistory,
-        );
-      } catch {
-        return null;
-      }
+      // api.getSingleMessage won't give us the message's stream and
+      // topic; see there.
+      return null;
+    }
+    try {
+      message = await api.getSingleMessage(
+        auth,
+        { message_id: messageId },
+        zulipFeatureLevel,
+        allowEditHistory,
+      );
+    } catch {
+      return null;
     }
 
     // The FL 120 condition on calling api.getSingleMessage should ensure
@@ -91,7 +90,6 @@ const getSingleMessage =
       logging.error('`message` from api.getSingleMessage unexpectedly falsy');
       return null;
     }
-
     return message;
   };