nullObjects [nfc]: Annotate NULL_ARRAY.

This will help move us along toward Flow's new "Types-First" mode;
switching entirely is zulip#4907.

And chase through many Flow errors pointing out that various other
object and array types should be marked read-only too. All of them
look like they should be read-only anyway; our read-only markers
haven't generally kept up with all the places we pass around objects
and arrays expecting them not to be mutated.

After adding the annotation, I went through five iterations of

- run `yarn flow`
- resolve each error in the output (and no others) by
  command-clicking to the type that Flow says should be made
  read-only, and making it so

, stopping when Flow didn't show any more errors.

src/api/initialDataTypes.js

@@ -242,7 +242,7 @@ export type HuddlesUnreadItem = {|
   user_ids_string: string,
 
   // Sorted.
-  unread_message_ids: number[],
+  unread_message_ids: $ReadOnlyArray<number>,
 |};
 
 /** The unreads in a 1:1 PM thread, as represented in `unread_msgs`. */
@@ -259,7 +259,7 @@ export type PmsUnreadItem = {|
   sender_id: UserId,
 
   // Sorted.
-  unread_message_ids: number[],
+  unread_message_ids: $ReadOnlyArray<number>,
 |};
 
 /** Initial data for `update_message_flags` events. */

src/chat/narrowsSelectors.js

@@ -33,7 +33,7 @@ import { shouldBeMuted } from '../utils/message';
 import { NULL_ARRAY, NULL_SUBSCRIPTION } from '../nullObjects';
 import * as logging from '../utils/logging';
 
-export const outboxMessagesForNarrow: Selector<Outbox[], Narrow> = createSelector(
+export const outboxMessagesForNarrow: Selector<$ReadOnlyArray<Outbox>, Narrow> = createSelector(
   (state, narrow) => narrow,
   getCaughtUpForNarrow,
   state => getOutbox(state),
@@ -57,8 +57,10 @@ export const outboxMessagesForNarrow: Selector<Outbox[], Narrow> = createSelecto
   },
 );
 
-export const getFetchedMessageIdsForNarrow = (state: GlobalState, narrow: Narrow): number[] =>
-  getAllNarrows(state).get(keyFromNarrow(narrow)) || NULL_ARRAY;
+export const getFetchedMessageIdsForNarrow = (
+  state: GlobalState,
+  narrow: Narrow,
+): $ReadOnlyArray<number> => getAllNarrows(state).get(keyFromNarrow(narrow)) || NULL_ARRAY;
 
 const getFetchedMessagesForNarrow: Selector<Message[], Narrow> = createSelector(
   getFetchedMessageIdsForNarrow,

src/message/messageActionSheet.js

@@ -43,7 +43,7 @@ type TopicArgs = {
   auth: Auth,
   streamId: number,
   topic: string,
-  subscriptions: Subscription[],
+  subscriptions: $ReadOnlyArray<Subscription>,
   streams: Map<number, Stream>,
   dispatch: Dispatch,
   _: GetText,
@@ -240,7 +240,7 @@ export const constructTopicActionButtons = ({
   backgroundData: $ReadOnly<{
     mute: MuteState,
     streams: Map<number, Stream>,
-    subscriptions: Subscription[],
+    subscriptions: $ReadOnlyArray<Subscription>,
     unread: UnreadState,
     ownUser: User,
     ...
@@ -380,7 +380,7 @@ export const showMessageActionSheet = ({
   |},
   backgroundData: $ReadOnly<{
     auth: Auth,
-    subscriptions: Subscription[],
+    subscriptions: $ReadOnlyArray<Subscription>,
     ownUser: User,
     flags: FlagsState,
     ...
@@ -419,7 +419,7 @@ export const showTopicActionSheet = ({
     auth: Auth,
     mute: MuteState,
     streams: Map<number, Stream>,
-    subscriptions: Subscription[],
+    subscriptions: $ReadOnlyArray<Subscription>,
     unread: UnreadState,
     ownUser: User,
     flags: FlagsState,

src/message/messageSelectors.js

@@ -21,7 +21,7 @@ import { NULL_ARRAY } from '../nullObjects';
  * Returns something which may or may not be an array, but is at least JSONable
  * and human-readable.
  */
-function truncateForLogging<T: JSONable>(arr: Array<T>, len = 10): JSONable {
+function truncateForLogging<T: JSONable>(arr: $ReadOnlyArray<T>, len = 10): JSONable {
   if (arr.length <= 2 * len) {
     return arr;
   }

src/nav/getInitialRouteInfo.js

@@ -6,7 +6,7 @@ import type { Account } from '../types';
 
 export default (args: {|
   hasAuth: boolean,
-  accounts: Account[],
+  accounts: $ReadOnlyArray<Account>,
 |}): {| initialRouteName: string, initialRouteParams?: ScreenParams |} => {
   const { hasAuth, accounts } = args;
 

src/nullObjects.js

@@ -3,7 +3,7 @@ import type { Subscription } from './types';
 
 export const NULL_OBJECT: {||} = Object.freeze({});
 
-export const NULL_ARRAY = Object.freeze([]);
+export const NULL_ARRAY: $ReadOnlyArray<empty> = Object.freeze([]);
 
 /*
  * All the below objects are DEPRECATED; rather than using one, choose the

src/reduxTypes.js

@@ -47,9 +47,9 @@ export type * from './actionTypes';
  *    the app.
  *  * `Account` for details on the properties of each account object.
  */
-export type AccountsState = Account[];
+export type AccountsState = $ReadOnlyArray<Account>;
 
-export type AlertWordsState = string[];
+export type AlertWordsState = $ReadOnlyArray<string>;
 
 /**
  * Info about how complete our knowledge is of the messages in some narrow.
@@ -174,7 +174,7 @@ export type MigrationsState = {|
   version?: string,
 |};
 
-export type MuteState = MuteTuple[];
+export type MuteState = $ReadOnlyArray<MuteTuple>;
 
 /** A map from user IDs to the Unix timestamp in seconds when they were muted. */
 export type MutedUsersState = Immutable.Map<UserId, number>;
@@ -199,7 +199,7 @@ export type MutedUsersState = Immutable.Map<UserId, number>;
  */
 export type NarrowsState = Immutable.Map<string, number[]>;
 
-export type OutboxState = Outbox[];
+export type OutboxState = $ReadOnlyArray<Outbox>;
 
 /**
  * The `presence` subtree of our Redux state.
@@ -300,9 +300,9 @@ export type SettingsState = {|
   doNotMarkMessagesAsRead: boolean,
 |};
 
-export type StreamsState = Stream[];
+export type StreamsState = $ReadOnlyArray<Stream>;
 
-export type SubscriptionsState = Subscription[];
+export type SubscriptionsState = $ReadOnlyArray<Subscription>;
 
 export type TopicsState = {|
   [number]: Topic[],
@@ -315,7 +315,7 @@ export type TypingState = {|
   |},
 |};
 
-export type UserGroupsState = UserGroup[];
+export type UserGroupsState = $ReadOnlyArray<UserGroup>;
 
 export type UserStatusState = UserStatusMapObject;
 
@@ -325,7 +325,7 @@ export type UserStatusState = UserStatusMapObject;
  * This contains all users except deactivated users and cross-realm bots.
  * For those, see RealmState.
  */
-export type UsersState = User[];
+export type UsersState = $ReadOnlyArray<User>;
 
 /**
  * Our complete Redux state tree.

src/subscriptions/StreamListCard.js

@@ -33,8 +33,8 @@ type Props = $ReadOnly<{|
   dispatch: Dispatch,
   auth: Auth,
   canCreateStreams: boolean,
-  streams: Stream[],
-  subscriptions: Subscription[],
+  streams: $ReadOnlyArray<Stream>,
+  subscriptions: $ReadOnlyArray<Subscription>,
 |}>;
 
 class StreamListCard extends PureComponent<Props> {

src/topics/topicSelectors.js

@@ -16,7 +16,7 @@ import { getStreamsById } from '../subscriptions/subscriptionSelectors';
 import { NULL_ARRAY } from '../nullObjects';
 import { isStreamNarrow, streamNameOfNarrow } from '../utils/narrow';
 
-export const getTopicsForNarrow: Selector<string[], Narrow> = createSelector(
+export const getTopicsForNarrow: Selector<$ReadOnlyArray<string>, Narrow> = createSelector(
   (state, narrow) => narrow,
   state => getTopics(state),
   state => getStreams(state),

src/typing/typingSelectors.js

@@ -12,7 +12,7 @@ export const getCurrentTypingUsers: Selector<$ReadOnlyArray<UserOrBot>, Narrow>
   (state, narrow) => narrow,
   state => getTyping(state),
   state => getAllUsersById(state),
-  (narrow, typing, allUsersById): UserOrBot[] => {
+  (narrow, typing, allUsersById): $ReadOnlyArray<UserOrBot> => {
     if (!isPmNarrow(narrow)) {
       return NULL_ARRAY;
     }

src/unread/unreadHelpers.js

@@ -2,9 +2,12 @@
 import type { HuddlesUnreadItem, PmsUnreadItem, UserId } from '../types';
 import { addItemsToArray, removeItemsFromArray, filterArray } from '../utils/immutability';
 
-type SomeUnreadItem = { unread_message_ids: number[], ... };
+type SomeUnreadItem = { unread_message_ids: $ReadOnlyArray<number>, ... };
 
-export function removeItemsDeeply<T: SomeUnreadItem>(objArray: T[], messageIds: number[]): T[] {
+export function removeItemsDeeply<T: SomeUnreadItem>(
+  objArray: $ReadOnlyArray<T>,
+  messageIds: number[],
+): $ReadOnlyArray<T> {
   let changed = false;
   const objWithAddedUnreadIds = objArray.map(obj => {
     const filteredIds = removeItemsFromArray(obj.unread_message_ids, messageIds);
@@ -29,7 +32,11 @@ export function removeItemsDeeply<T: SomeUnreadItem>(objArray: T[], messageIds:
   );
 }
 
-function addItemsDeeply<T: SomeUnreadItem>(input: T[], itemsToAdd: number[], index: number): T[] {
+function addItemsDeeply<T: SomeUnreadItem>(
+  input: $ReadOnlyArray<T>,
+  itemsToAdd: number[],
+  index: number,
+): $ReadOnlyArray<T> {
   const item = input[index];
 
   const unreadMessageIds = addItemsToArray(item.unread_message_ids, itemsToAdd);
@@ -48,10 +55,10 @@ function addItemsDeeply<T: SomeUnreadItem>(input: T[], itemsToAdd: number[], ind
 }
 
 export const addItemsToPmArray = (
-  input: PmsUnreadItem[],
+  input: $ReadOnlyArray<PmsUnreadItem>,
   itemsToAdd: number[],
   senderId: UserId,
-): PmsUnreadItem[] => {
+): $ReadOnlyArray<PmsUnreadItem> => {
   const index = input.findIndex(sender => sender.sender_id === senderId);
 
   if (index !== -1) {
@@ -68,10 +75,10 @@ export const addItemsToPmArray = (
 };
 
 export const addItemsToHuddleArray = (
-  input: HuddlesUnreadItem[],
+  input: $ReadOnlyArray<HuddlesUnreadItem>,
   itemsToAdd: number[],
   userIds: string,
-): HuddlesUnreadItem[] => {
+): $ReadOnlyArray<HuddlesUnreadItem> => {
   const index = input.findIndex(recipients => recipients.user_ids_string === userIds);
 
   if (index !== -1) {

src/unread/unreadModelTypes.js

@@ -29,7 +29,7 @@ export type UnreadStreamsState =
  *
  * Part of `UnreadState`; see there for more.
  */
-export type UnreadHuddlesState = HuddlesUnreadItem[];
+export type UnreadHuddlesState = $ReadOnlyArray<HuddlesUnreadItem>;
 
 /**
  * A summary of (almost) all unread 1:1 PMs.
@@ -41,7 +41,7 @@ export type UnreadHuddlesState = HuddlesUnreadItem[];
  * Part of `UnreadState`; see there for more.
  * See in particular `UnreadHuddlesState` for group PMs.
  */
-export type UnreadPmsState = PmsUnreadItem[];
+export type UnreadPmsState = $ReadOnlyArray<PmsUnreadItem>;
 
 /**
  * A summary of (almost) all unread messages with @-mentions.
@@ -54,7 +54,7 @@ export type UnreadPmsState = PmsUnreadItem[];
  *
  * Part of `UnreadState`; see there for more.
  */
-export type UnreadMentionsState = number[];
+export type UnreadMentionsState = $ReadOnlyArray<number>;
 
 /**
  * A summary of (almost) all unread messages, even those we don't have.

src/users/userHelpers.js

@@ -145,7 +145,7 @@ export const getAutocompleteSuggestion = (
 };
 
 export const getAutocompleteUserGroupSuggestions = (
-  userGroups: UserGroup[],
+  userGroups: $ReadOnlyArray<UserGroup>,
   filter: string = '',
 ): UserGroup[] =>
   userGroups.filter(

src/utils/immutability.js

@@ -1,16 +1,22 @@
 /* @flow strict-local */
 
-export const removeItemsFromArray = (input: number[], itemsToRemove: number[]): number[] => {
+export const removeItemsFromArray = (
+  input: $ReadOnlyArray<number>,
+  itemsToRemove: number[],
+): $ReadOnlyArray<number> => {
   const output = input.filter((item: number) => !itemsToRemove.includes(item));
   return input.length === output.length ? input : output;
 };
 
-export function addItemsToArray<T>(input: T[], itemsToAdd: T[]): T[] {
+export function addItemsToArray<T>(input: $ReadOnlyArray<T>, itemsToAdd: T[]): $ReadOnlyArray<T> {
   const newItems = itemsToAdd.filter(item => !input.includes(item));
   return newItems.length > 0 ? [...input, ...itemsToAdd] : input;
 }
 
-export function filterArray<T>(input: T[], predicate: T => boolean): T[] {
+export function filterArray<T>(
+  input: $ReadOnlyArray<T>,
+  predicate: T => boolean,
+): $ReadOnlyArray<T> {
   const filteredList = input.filter(predicate);
   return filteredList.length === input.length ? input : filteredList;
 }

src/utils/message.js

@@ -9,7 +9,7 @@ export const isTopicMuted = (stream: string, topic: string, mute: MuteState = []
 export const shouldBeMuted = (
   message: Message | Outbox,
   narrow: Narrow,
-  subscriptions: Subscription[] = [],
+  subscriptions: $ReadOnlyArray<Subscription> = [],
   mutes: MuteState = [],
 ): boolean => {
   if (message.type === 'private') {

src/webview/MessageList.js

@@ -88,7 +88,7 @@ export type BackgroundData = $ReadOnly<{|
   ownUser: User,
   streams: Map<number, Stream>,
   streamsByName: Map<string, Stream>,
-  subscriptions: Subscription[],
+  subscriptions: $ReadOnlyArray<Subscription>,
   unread: UnreadState,
   theme: ThemeName,
   twentyFourHourTime: boolean,

src/webview/html/processAlertWords.js

@@ -21,7 +21,7 @@ function escape_user_regex(value) {
 
 /* Helper borrowed near-verbatim from webapp; see comment above. */
 // prettier-ignore
-const process_message = function (words: string[],
+const process_message = function (words: $ReadOnlyArray<string>,
                                   message: {| alerted: boolean, content: string |}) {
     // Parsing for alert words is expensive, so we rely on the host
     // to tell us there any alert words to even look for.
@@ -58,7 +58,12 @@ const process_message = function (words: string[],
 };
 
 /** Mark any alert words in `content` with an appropriate span. */
-export default (content: string, id: number, alertWords: string[], flags: FlagsState): string => {
+export default (
+  content: string,
+  id: number,
+  alertWords: $ReadOnlyArray<string>,
+  flags: FlagsState,
+): string => {
   // This is kind of funny style, but lets us borrow the webapp's code near
   // verbatim, inside `process_message`.
   let message = { content, alerted: flags.has_alert_word[id] };