diff --git a/lib/components/thread-draft-updater.react.js b/lib/components/thread-draft-updater.react.js
--- a/lib/components/thread-draft-updater.react.js
+++ b/lib/components/thread-draft-updater.react.js
@@ -18,7 +18,7 @@
 
     const cachedThreadIDsRef = React.useRef<?Set<string>>();
     if (!cachedThreadIDsRef.current) {
-      const newCachedThreadIDs = new Set();
+      const newCachedThreadIDs = new Set<string>();
       for (const realizedThreadID of pendingToRealizedThreadIDs.values()) {
         newCachedThreadIDs.add(realizedThreadID);
       }
diff --git a/lib/hooks/child-threads.js b/lib/hooks/child-threads.js
--- a/lib/hooks/child-threads.js
+++ b/lib/hooks/child-threads.js
@@ -33,9 +33,9 @@
   const childThreads = useSelector(state => childThreadInfos(state)[threadID]);
   const subchannelIDs = React.useMemo(() => {
     if (!childThreads) {
-      return new Set();
+      return new Set<string>();
     }
-    return new Set(
+    return new Set<string>(
       childThreads.filter(predicate).map(threadInfo => threadInfo.id),
     );
   }, [childThreads, predicate]);
diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -1528,13 +1528,13 @@
 
     // Store message IDs already contained within threads so that we
     // do not insert duplicates
-    const existingMessageIDs = new Set();
+    const existingMessageIDs = new Set<string>();
     for (const threadID in threads) {
       threads[threadID].messageIDs.forEach(msgID => {
         existingMessageIDs.add(msgID);
       });
     }
-    const threadsNeedMsgIDsResorting = new Set();
+    const threadsNeedMsgIDsResorting = new Set<string>();
     const actionPayloadMessages =
       messageStoreOpsHandlers.translateClientDBData(payloadMessages);
 
diff --git a/lib/reducers/thread-reducer.js b/lib/reducers/thread-reducer.js
--- a/lib/reducers/thread-reducer.js
+++ b/lib/reducers/thread-reducer.js
@@ -195,7 +195,7 @@
       threadStoreOperations,
     };
   } else if (action.type === saveMessagesActionType) {
-    const threadIDToMostRecentTime = new Map();
+    const threadIDToMostRecentTime = new Map<string, number>();
     for (const messageInfo of action.payload.rawMessageInfos) {
       const current = threadIDToMostRecentTime.get(messageInfo.threadID);
       if (!current || current < messageInfo.time) {
diff --git a/lib/selectors/nav-selectors.js b/lib/selectors/nav-selectors.js
--- a/lib/selectors/nav-selectors.js
+++ b/lib/selectors/nav-selectors.js
@@ -19,6 +19,7 @@
 import type { BaseNavInfo } from '../types/nav-types.js';
 import type { BaseAppState } from '../types/redux-types.js';
 import type { RawThreadInfo, ThreadInfo } from '../types/thread-types.js';
+import type { UserInfo } from '../types/user-types.js';
 import { getConfig } from '../utils/config.js';
 import { values } from '../utils/objects.js';
 import { useSelector } from '../utils/redux-utils.js';
@@ -94,7 +95,7 @@
     state => state.currentUserInfo && state.currentUserInfo.id,
   );
   const nonViewerMembers = React.useMemo(() => {
-    const allMembersOfAllThreads = new Map();
+    const allMembersOfAllThreads = new Map<string, UserInfo>();
     for (const threadInfo of threadInfos) {
       for (const member of threadInfo.members) {
         const isParentAdmin = memberHasAdminPowers(member);
@@ -121,7 +122,7 @@
   );
 
   const memberMap = React.useMemo(() => {
-    const result = new Map();
+    const result = new Map<string, UserInfo>();
     for (const userInfo of nonViewerMembersWithENSNames) {
       result.set(userInfo.id, userInfo);
     }
diff --git a/lib/selectors/thread-selectors.js b/lib/selectors/thread-selectors.js
--- a/lib/selectors/thread-selectors.js
+++ b/lib/selectors/thread-selectors.js
@@ -420,7 +420,7 @@
 ) => $ReadOnlyMap<string, string> = createSelector(
   (rawThreadInfos: RawThreadInfos) => rawThreadInfos,
   (rawThreadInfos: RawThreadInfos) => {
-    const result = new Map();
+    const result = new Map<string, string>();
     for (const threadID in rawThreadInfos) {
       const rawThreadInfo = rawThreadInfos[threadID];
       if (
diff --git a/lib/selectors/user-selectors.js b/lib/selectors/user-selectors.js
--- a/lib/selectors/user-selectors.js
+++ b/lib/selectors/user-selectors.js
@@ -209,7 +209,7 @@
   state => state.currentUserInfo && state.currentUserInfo.id,
   state => state.threadStore.threadInfos,
   (viewerID, threadInfos) => {
-    const personalThreadMembers = new Set();
+    const personalThreadMembers = new Set<string>();
 
     for (const threadID in threadInfos) {
       const thread = threadInfos[threadID];
diff --git a/lib/shared/message-utils.js b/lib/shared/message-utils.js
--- a/lib/shared/message-utils.js
+++ b/lib/shared/message-utils.js
@@ -201,7 +201,7 @@
 function usersInMessageInfos(
   messageInfos: $ReadOnlyArray<MessageInfo | RawMessageInfo>,
 ): string[] {
-  const userIDs = new Set();
+  const userIDs = new Set<string>();
   for (const messageInfo of messageInfos) {
     if (messageInfo.creatorID) {
       userIDs.add(messageInfo.creatorID);
diff --git a/lib/shared/radix-tree.js b/lib/shared/radix-tree.js
--- a/lib/shared/radix-tree.js
+++ b/lib/shared/radix-tree.js
@@ -91,7 +91,7 @@
   }
 
   getAllMatchingPrefix(prefix: string): V[] {
-    const result = new Set();
+    const result = new Set<V>();
     const stack = [{ node: this.root, partLeft: prefix }];
     while (stack.length > 0) {
       const { node, partLeft: prevPartLeft } = stack.pop();
diff --git a/lib/shared/search-utils.js b/lib/shared/search-utils.js
--- a/lib/shared/search-utils.js
+++ b/lib/shared/search-utils.js
@@ -20,8 +20,9 @@
 import type {
   ChatMessageInfoItem,
   MessageListData,
-} from '../selectors/chat-selectors';
-import type { MessageInfo, RawMessageInfo } from '../types/message-types';
+  ChatMessageItem,
+} from '../selectors/chat-selectors.js';
+import type { MessageInfo, RawMessageInfo } from '../types/message-types.js';
 import { isComposableMessageType } from '../types/message-types.js';
 import type { MinimallyEncodedThreadInfo } from '../types/minimally-encoded-thread-permissions-types.js';
 import { userRelationshipStatus } from '../types/relationship-types.js';
@@ -320,7 +321,7 @@
       isComposableMessageType(item.messageInfo.type),
   );
 
-  const uniqueChatMessageInfoItemsMap = new Map();
+  const uniqueChatMessageInfoItemsMap = new Map<string, ChatMessageItem>();
   chatMessageInfoItems.forEach(
     item =>
       item.messageInfo &&
diff --git a/lib/shared/thread-utils.js b/lib/shared/thread-utils.js
--- a/lib/shared/thread-utils.js
+++ b/lib/shared/thread-utils.js
@@ -541,7 +541,7 @@
     RelativeMemberInfo | MinimallyEncodedRelativeMemberInfo,
   >,
 ): Map<string, AccountUserInfo> {
-  const memberMap = new Map();
+  const memberMap = new Map<string, AccountUserInfo>();
   for (const member of members) {
     const { id, role, username } = member;
     if (!role || !username) {
@@ -560,7 +560,7 @@
   const memberMap = memberLowercaseUsernameMap(threadInfo.members);
   const mentions = extractUserMentionsFromText(text);
 
-  const mentionedMembers = new Map();
+  const mentionedMembers = new Map<string, AccountUserInfo>();
   for (const mention of mentions) {
     const userInfo = memberMap.get(mention.toLowerCase());
     if (userInfo) {
@@ -611,7 +611,7 @@
   const { color, type: parentThreadType } = parentThreadInfo;
   const threadName = trimText(messageTitle, 30);
 
-  const initialMembers = new Map();
+  const initialMembers = new Map<string, UserIDAndUsername>();
 
   const { id: viewerID, username: viewerUsername } = loggedInUserInfo;
   initialMembers.set(viewerID, { id: viewerID, username: viewerUsername });
@@ -1208,7 +1208,7 @@
 function identifyInvalidatedThreads(
   updateInfos: $ReadOnlyArray<ClientUpdateInfo>,
 ): Set<string> {
-  const invalidated = new Set();
+  const invalidated = new Set<string>();
   for (const updateInfo of updateInfos) {
     if (updateInfo.type === updateTypes.DELETE_THREAD) {
       invalidated.add(updateInfo.threadID);
@@ -1640,7 +1640,7 @@
   );
 
   const [threadSearchResults, setThreadSearchResults] = React.useState(
-    new Set(),
+    new Set<string>(),
   );
   const [usersSearchResults, setUsersSearchResults] = React.useState<
     $ReadOnlyArray<GlobalAccountUserInfo>,
@@ -1649,7 +1649,7 @@
   React.useEffect(() => {
     (async () => {
       const results = threadSearchIndex.getSearchResults(searchText);
-      setThreadSearchResults(new Set(results));
+      setThreadSearchResults(new Set<string>(results));
       const usersResults = await searchUsers(searchText);
       setUsersSearchResults(usersResults);
     })();
@@ -1836,7 +1836,7 @@
     Object.keys(threadInfo.roles).forEach(roleID => {
       const roleName = threadInfo.roles[roleID].name;
       const rolePermissions = Object.keys(threadInfo.roles[roleID].permissions);
-      const setOfUserSurfacedPermissions = new Set();
+      const setOfUserSurfacedPermissions = new Set<UserSurfacedPermission>();
 
       rolePermissions.forEach(rolePermission => {
         const userSurfacedPermission = Object.keys(
diff --git a/lib/socket/api-request-handler.react.js b/lib/socket/api-request-handler.react.js
--- a/lib/socket/api-request-handler.react.js
+++ b/lib/socket/api-request-handler.react.js
@@ -11,6 +11,7 @@
   serverSocketMessageTypes,
   type ClientSocketMessageWithoutID,
   type ConnectionInfo,
+  type APIResponseServerSocketMessage,
 } from '../types/socket-types.js';
 import { registerActiveSocket } from '../utils/action-utils.js';
 import { SocketOffline } from '../utils/errors.js';
@@ -84,10 +85,11 @@
       type: clientSocketMessageTypes.API_REQUEST,
       payload: request,
     });
-    const response = await inflightRequests.fetchResponse(
-      messageID,
-      serverSocketMessageTypes.API_RESPONSE,
-    );
+    const response =
+      await inflightRequests.fetchResponse<APIResponseServerSocketMessage>(
+        messageID,
+        serverSocketMessageTypes.API_RESPONSE,
+      );
     return response.payload;
   };
 }
diff --git a/lib/socket/socket.react.js b/lib/socket/socket.react.js
--- a/lib/socket/socket.react.js
+++ b/lib/socket/socket.react.js
@@ -62,6 +62,9 @@
   type ConnectionStatus,
   setLateResponseActionType,
   type CommTransportLayer,
+  type ActivityUpdateResponseServerSocketMessage,
+  type ClientStateSyncServerSocketMessage,
+  type PongServerSocketMessage,
 } from '../types/socket-types.js';
 import { actionLogger } from '../utils/action-logger.js';
 import type { DispatchActionPromise } from '../utils/action-utils.js';
@@ -570,10 +573,11 @@
         type: serverRequestTypes.INITIAL_ACTIVITY_UPDATES,
         activityUpdates: queuedActivityUpdates,
       });
-      activityUpdatePromise = inflightRequests.fetchResponse(
-        messageID,
-        serverSocketMessageTypes.ACTIVITY_UPDATE_RESPONSE,
-      );
+      activityUpdatePromise =
+        inflightRequests.fetchResponse<ActivityUpdateResponseServerSocketMessage>(
+          messageID,
+          serverSocketMessageTypes.ACTIVITY_UPDATE_RESPONSE,
+        );
     }
 
     const sessionState = this.props.sessionStateFunc();
@@ -590,10 +594,11 @@
     this.initializedWithUserState = this.props.preRequestUserState;
     this.sendMessage(initialMessage);
 
-    const stateSyncPromise = inflightRequests.fetchResponse(
-      messageID,
-      serverSocketMessageTypes.STATE_SYNC,
-    );
+    const stateSyncPromise =
+      inflightRequests.fetchResponse<ClientStateSyncServerSocketMessage>(
+        messageID,
+        serverSocketMessageTypes.STATE_SYNC,
+      );
 
     // https://flow.org/try/#1N4Igxg9gdgZglgcxALlAJwKYEMwBcD6aArlLnALYYrgA2WAzvXGCADQgYAeOBARgJ74AJhhhYiNXClzEM7DFCLl602QF92kEdQb8oYAAQwSeONAMAHNBHJx6GAII0aAHgAqyA8AMBqANoA1hj8nvQycFAIALqetpwYQgZqAHwAFAA6UAYGERZEuJ4AJABK2EIA8lA0-O7JrJkAlJ4ACta29i6F5bwAVgCyWBburAa4-BYYEDAGhVgA7lhwuMnJXpnZkFBhlm12GPQGALwGflEA3OsGm9tB-AfH3T0YeAB0t-SpufkNF1lGEGgDKkaBhcDkjgYAAxncEuAzvF4gyK4AAWMLgPh8DTWfw20BuwQh7z8cHOlzxWzBVhsewhX1wgWCZNxOxp9noLzy9BRqWp7QwP0uaku1zBmHoElw9wM80WYNabIwLywzl5u3Zgr+ooMAgAclhKJ5gH4wmgItFYnB4kI1BDgGpftkYACgSCwXAIdDYfDghykQhUejMdjgOSrviwbcib6Sczstk9QaMIz+FEIeLJfRY46kpdMLgiGgsonKL9hVBMrp9EYTGRzPYoEIAJJQJZwFV9fb0LAIDCpEOXN2jfa4BX8nNwaYZEAojDOCDpEAvMJYNBSgDqSx5i4Ci4aA5ZuBHY9pxxP9he4ogNAAbn2ZEQBTny5dZUtWfynDRUt4j2FzxgSSamobAgHeaBMNA1A3pCLwAEwAIwACwvJCIBqEAA
     // $FlowFixMe fixed in Flow 0.214
@@ -759,7 +764,7 @@
         this.state.inflightRequests,
         'inflightRequests falsey inside sendPing',
       );
-      await this.state.inflightRequests.fetchResponse(
+      await this.state.inflightRequests.fetchResponse<PongServerSocketMessage>(
         messageID,
         serverSocketMessageTypes.PONG,
       );
diff --git a/lib/types/filter-types.js b/lib/types/filter-types.js
--- a/lib/types/filter-types.js
+++ b/lib/types/filter-types.js
@@ -18,14 +18,15 @@
   +type: 'threads',
   +threadIDs: $ReadOnlyArray<string>,
 };
-export type CalendarFilter = { +type: 'not_deleted' } | CalendarThreadFilter;
+type NotDeletedFilter = { +type: 'not_deleted' };
+export type CalendarFilter = NotDeletedFilter | CalendarThreadFilter;
 
 export const calendarFilterValidator: TUnion<CalendarFilter> = t.union([
   tShape<CalendarThreadFilter>({
     type: tString('threads'),
     threadIDs: t.list(tID),
   }),
-  tShape({ type: tString('not_deleted') }),
+  tShape<NotDeletedFilter>({ type: tString('not_deleted') }),
 ]);
 
 export const defaultCalendarFilters: $ReadOnlyArray<CalendarFilter> = [
diff --git a/lib/types/request-types.js b/lib/types/request-types.js
--- a/lib/types/request-types.js
+++ b/lib/types/request-types.js
@@ -96,37 +96,41 @@
   ...EntryInconsistencyReportShape,
 };
 
+type FailUnmentioned = Shape<{
+  +threadInfos: boolean,
+  +entryInfos: boolean,
+  +userInfos: boolean,
+}>;
+
+type StateChanges = Shape<{
+  +rawThreadInfos: RawThreadInfo[],
+  +rawEntryInfos: RawEntryInfo[],
+  +currentUserInfo: CurrentUserInfo,
+  +userInfos: AccountUserInfo[],
+  +deleteThreadIDs: string[],
+  +deleteEntryIDs: string[],
+  +deleteUserInfoIDs: string[],
+}>;
+
 export type ServerCheckStateServerRequest = {
   +type: 6,
   +hashesToCheck: { +[key: string]: number },
-  +failUnmentioned?: Shape<{
-    +threadInfos: boolean,
-    +entryInfos: boolean,
-    +userInfos: boolean,
-  }>,
-  +stateChanges?: Shape<{
-    +rawThreadInfos: RawThreadInfo[],
-    +rawEntryInfos: RawEntryInfo[],
-    +currentUserInfo: CurrentUserInfo,
-    +userInfos: AccountUserInfo[],
-    +deleteThreadIDs: string[],
-    +deleteEntryIDs: string[],
-    +deleteUserInfoIDs: string[],
-  }>,
+  +failUnmentioned?: FailUnmentioned,
+  +stateChanges?: StateChanges,
 };
 const serverCheckStateServerRequestValidator =
   tShape<ServerCheckStateServerRequest>({
     type: tNumber(serverRequestTypes.CHECK_STATE),
     hashesToCheck: t.dict(t.String, t.Number),
     failUnmentioned: t.maybe(
-      tShape({
+      tShape<FailUnmentioned>({
         threadInfos: t.maybe(t.Boolean),
         entryInfos: t.maybe(t.Boolean),
         userInfos: t.maybe(t.Boolean),
       }),
     ),
     stateChanges: t.maybe(
-      tShape({
+      tShape<StateChanges>({
         rawThreadInfos: t.maybe(t.list(rawThreadInfoValidator)),
         rawEntryInfos: t.maybe(t.list(rawEntryInfoValidator)),
         currentUserInfo: t.maybe(currentUserInfoValidator),
diff --git a/lib/types/socket-types.js b/lib/types/socket-types.js
--- a/lib/types/socket-types.js
+++ b/lib/types/socket-types.js
@@ -304,18 +304,19 @@
     payload: serverStateSyncSocketPayloadValidator,
   });
 
+type ServerRequestsServerSocketMessagePayload = {
+  +serverRequests: $ReadOnlyArray<ServerServerRequest>,
+};
 export type ServerRequestsServerSocketMessage = {
   +type: 1,
   +responseTo?: number,
-  +payload: {
-    +serverRequests: $ReadOnlyArray<ServerServerRequest>,
-  },
+  +payload: ServerRequestsServerSocketMessagePayload,
 };
 export const serverRequestsServerSocketMessageValidator: TInterface<ServerRequestsServerSocketMessage> =
   tShape<ServerRequestsServerSocketMessage>({
     type: tNumber(serverSocketMessageTypes.REQUESTS),
     responseTo: t.maybe(t.Number),
-    payload: tShape({
+    payload: tShape<ServerRequestsServerSocketMessagePayload>({
       serverRequests: t.list(serverServerRequestValidator),
     }),
   });
@@ -334,14 +335,16 @@
     payload: t.maybe(t.Object),
   });
 
+type SessionChange = {
+  +cookie: string,
+  +currentUserInfo: LoggedOutUserInfo,
+};
+
 export type AuthErrorServerSocketMessage = {
   +type: 3,
   +responseTo: number,
   +message: string,
-  +sessionChange: {
-    +cookie: string,
-    +currentUserInfo: LoggedOutUserInfo,
-  },
+  +sessionChange: SessionChange,
 };
 export const authErrorServerSocketMessageValidator: TInterface<AuthErrorServerSocketMessage> =
   tShape<AuthErrorServerSocketMessage>({
@@ -349,7 +352,10 @@
     responseTo: t.Number,
     message: t.String,
     sessionChange: t.maybe(
-      tShape({ cookie: t.String, currentUserInfo: loggedOutUserInfoValidator }),
+      tShape<SessionChange>({
+        cookie: t.String,
+        currentUserInfo: loggedOutUserInfoValidator,
+      }),
     ),
   });
 
diff --git a/lib/utils/comm-config.js b/lib/utils/comm-config.js
--- a/lib/utils/comm-config.js
+++ b/lib/utils/comm-config.js
@@ -19,7 +19,7 @@
   return `${configName.folder}/${configName.name}.json`;
 }
 
-const cachedJSON = new Map();
+const cachedJSON = new Map<string, mixed>();
 
 // This function checks for an env var named COMM_JSONCONFIG_{folder}_{name}
 // If it doesn't find one, it then looks for keyserver/{folder}/{name}.json
@@ -28,13 +28,13 @@
   const key = getKeyForConfigName(configName);
   const cached = cachedJSON.get(key);
   if (cached !== undefined) {
-    return cached;
+    return (cached: any);
   }
-  const json = await getJSON(configName);
+  const json = await getJSON<T>(configName);
   if (!cachedJSON.has(key)) {
     cachedJSON.set(key, json);
   }
-  return cachedJSON.get(key);
+  return (cachedJSON.get(key): any);
 }
 
 async function getJSON<T>(configName: ConfigName): Promise<?T> {
diff --git a/lib/utils/conversion-utils.test.js b/lib/utils/conversion-utils.test.js
--- a/lib/utils/conversion-utils.test.js
+++ b/lib/utils/conversion-utils.test.js
@@ -9,9 +9,11 @@
 } from './conversion-utils.js';
 import { tShape, tID, idSchemaRegex } from './validation-utils.js';
 
+type ComplexType = { +ids: { +[string]: $ReadOnlyArray<string> } };
+
 describe('id conversion', () => {
   it('should convert string id', () => {
-    const validator = tShape({ id: tID });
+    const validator = tShape<{ +id: string }>({ id: tID });
     const serverData = { id: '1' };
     const clientData = { id: '0|1' };
 
@@ -24,7 +26,7 @@
   });
 
   it('should convert a complex type', () => {
-    const validator = tShape({ ids: t.dict(tID, t.list(tID)) });
+    const validator = tShape<ComplexType>({ ids: t.dict(tID, t.list(tID)) });
     const serverData = { ids: { '1': ['11', '12'], '2': [], '3': ['13'] } };
     const clientData = {
       ids: { '0|1': ['0|11', '0|12'], '0|2': [], '0|3': ['0|13'] },
diff --git a/lib/utils/ens-cache.js b/lib/utils/ens-cache.js
--- a/lib/utils/ens-cache.js
+++ b/lib/utils/ens-cache.js
@@ -200,10 +200,10 @@
         ensNames = raceResult;
       } catch (e) {
         console.log(e);
-        ensNames = new Array(needFetch.length).fill(null);
+        ensNames = new Array<?string>(needFetch.length).fill(null);
       }
 
-      const resultMap = new Map();
+      const resultMap = new Map<string, ?string>();
       for (let i = 0; i < needFetch.length; i++) {
         const ethAddress = needFetch[i];
         let ensName = ensNames[i];
diff --git a/lib/utils/ens-helpers.js b/lib/utils/ens-helpers.js
--- a/lib/utils/ens-helpers.js
+++ b/lib/utils/ens-helpers.js
@@ -40,7 +40,7 @@
     })
     .filter(Boolean);
 
-  const ensNames = new Map();
+  const ensNames = new Map<string, string>();
   if (needFetch.length > 0) {
     const results = await ensCache.getNamesForAddresses(needFetch);
     for (let i = 0; i < needFetch.length; i++) {
diff --git a/lib/utils/pkcs7-padding.test.js b/lib/utils/pkcs7-padding.test.js
--- a/lib/utils/pkcs7-padding.test.js
+++ b/lib/utils/pkcs7-padding.test.js
@@ -114,7 +114,7 @@
     // 2 blocks of data + 2 blocks of padding = 4 blocks total (1 superblock)
     const padded = new Uint8Array([
       ...generateRandomArray(2 * 16),
-      ...new Array(2 * 16).fill(2),
+      ...new Array<number>(2 * 16).fill(2),
     ]);
 
     const unpadded = superblockUnpad(padded, blockSizeBytes);
@@ -127,7 +127,7 @@
     const blockSizeBytes = 16;
     const padded = new Uint8Array([
       ...generateRandomArray(2 * 16),
-      ...new Array(2 * 16).fill(0),
+      ...new Array<number>(2 * 16).fill(0),
     ]);
     expect(() => superblockUnpad(padded, blockSizeBytes)).toThrow();
   });
@@ -137,7 +137,7 @@
     // 4 blocks total, but filled with 5s
     const padded = new Uint8Array([
       ...generateRandomArray(2 * 16),
-      ...new Array(2 * 16).fill(5),
+      ...new Array<number>(2 * 16).fill(5),
     ]);
     expect(() => superblockUnpad(padded, blockSizeBytes)).toThrow();
   });
@@ -243,5 +243,5 @@
 }
 
 function generateRandomArray(length: number): Array<number> {
-  return new Array(length).map(() => Math.floor(Math.random() * 256));
+  return new Array<void>(length).map(() => Math.floor(Math.random() * 256));
 }
diff --git a/lib/utils/role-utils.js b/lib/utils/role-utils.js
--- a/lib/utils/role-utils.js
+++ b/lib/utils/role-utils.js
@@ -80,7 +80,7 @@
     community ? threadInfoSelector(state)[community] : null,
   );
   const topMostThreadInfo = communityThreadInfo || threadInfo;
-  const roleMap = new Map();
+  const roleMap = new Map<string, ?RoleInfo | ?MinimallyEncodedRoleInfo>();
 
   if (topMostThreadInfo.type === threadTypes.GENESIS) {
     memberInfos.forEach(memberInfo =>