redux [nfc]: Simplify combined reducer (6/n): unroll the main loop.

The list of our reducers is already right here in the same source
file, so we don't really need all this metaprogramming on it.  By
unrolling the loop and unpacking what it does, we make everything
more explicit, and in particular make it explicit to the type-checker
so that it's able to check the types and we no longer need any fixme
comments to turn it off.

src/boot/reducers.js

@@ -32,35 +32,6 @@ import timing from '../utils/timing';
 
 const migrations = (state: MigrationsState = NULL_OBJECT): MigrationsState => state;
 
-const reducers = {
-  migrations,
-  accounts,
-  alertWords,
-  caughtUp,
-  drafts,
-  fetching,
-  flags,
-  messages,
-  narrows,
-  mute,
-  outbox,
-  pmConversations,
-  presence,
-  realm,
-  session,
-  settings,
-  streams,
-  subscriptions,
-  topics,
-  typing,
-  unread,
-  userGroups,
-  userStatus,
-  users,
-};
-
-const reducerKeys = Object.keys(reducers);
-
 const { enableReduxSlowReducerWarnings, slowReducersThreshold } = config;
 
 function maybeLogSlowReducer(action, key, startMs, endMs) {
@@ -93,21 +64,39 @@ function applyReducer<Key: $Keys<GlobalState>, State>(
 // Inlined just now from Redux upstream.
 // We'll clean this up in the next few commits.
 const combinedReducer = (state: void | GlobalState, action: Action): GlobalState => {
-  let hasChanged = false;
-  const nextState = {};
-  for (let i = 0; i < reducerKeys.length; i++) {
-    const key = reducerKeys[i];
-    const reducer = reducers[key];
-    const previousStateForKey = state?.[key];
+  // prettier-ignore
+  const nextState = {
+    migrations: applyReducer('migrations', migrations, state?.migrations, action),
+    accounts: applyReducer('accounts', accounts, state?.accounts, action),
+    alertWords: applyReducer('alertWords', alertWords, state?.alertWords, action),
+    caughtUp: applyReducer('caughtUp', caughtUp, state?.caughtUp, action),
+    drafts: applyReducer('drafts', drafts, state?.drafts, action),
+    fetching: applyReducer('fetching', fetching, state?.fetching, action),
+    flags: applyReducer('flags', flags, state?.flags, action),
+    messages: applyReducer('messages', messages, state?.messages, action),
+    narrows: applyReducer('narrows', narrows, state?.narrows, action),
+    mute: applyReducer('mute', mute, state?.mute, action),
+    outbox: applyReducer('outbox', outbox, state?.outbox, action),
+    pmConversations: applyReducer('pmConversations', pmConversations, state?.pmConversations, action),
+    presence: applyReducer('presence', presence, state?.presence, action),
+    realm: applyReducer('realm', realm, state?.realm, action),
+    session: applyReducer('session', session, state?.session, action),
+    settings: applyReducer('settings', settings, state?.settings, action),
+    streams: applyReducer('streams', streams, state?.streams, action),
+    subscriptions: applyReducer('subscriptions', subscriptions, state?.subscriptions, action),
+    topics: applyReducer('topics', topics, state?.topics, action),
+    typing: applyReducer('typing', typing, state?.typing, action),
+    unread: applyReducer('unread', unread, state?.unread, action),
+    userGroups: applyReducer('userGroups', userGroups, state?.userGroups, action),
+    userStatus: applyReducer('userStatus', userStatus, state?.userStatus, action),
+    users: applyReducer('users', users, state?.users, action),
+  };
 
-    // $FlowFixMe -- works because reducer and previousStateForKey are from same key
-    const nextStateForKey = applyReducer(key, reducer, previousStateForKey, action);
-
-    nextState[key] = nextStateForKey;
-    hasChanged = hasChanged || nextStateForKey !== previousStateForKey;
+  if (state && Object.keys(nextState).every(key => nextState[key] === state[key])) {
+    return state;
   }
-  // $FlowFixMe -- works because we didn't mix up keys above
-  return hasChanged ? nextState : state;
+
+  return nextState;
 };
 
 export default enableBatching(combinedReducer);