diff --git a/lib/components/integrity-handler.react.js b/lib/components/integrity-handler.react.js
--- a/lib/components/integrity-handler.react.js
+++ b/lib/components/integrity-handler.react.js
@@ -17,8 +17,9 @@
   const threadInfos = useSelector(state => state.threadStore.threadInfos);
   const integrityStore = useSelector(state => state.integrityStore);
 
-  const [batches, setBatches] = React.useState(null);
-  const timeout = React.useRef(null);
+  const [batches, setBatches] =
+    React.useState<?$ReadOnlyArray<$ReadOnlyArray<string>>>(null);
+  const timeout = React.useRef<?TimeoutID>(null);
 
   React.useEffect(() => {
     if (integrityStore.threadHashingStatus === 'starting') {
diff --git a/lib/components/modal-overlay.react.js b/lib/components/modal-overlay.react.js
--- a/lib/components/modal-overlay.react.js
+++ b/lib/components/modal-overlay.react.js
@@ -23,8 +23,8 @@
     backgroundColor = 'var(--modal-overlay-background-90)',
   } = props;
 
-  const overlayRef = React.useRef();
-  const firstClickRef = React.useRef(null);
+  const overlayRef = React.useRef<?HTMLDivElement>();
+  const firstClickRef = React.useRef<?HTMLElement>(null);
 
   React.useLayoutEffect(() => {
     if (overlayRef.current) {
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
@@ -17,7 +17,7 @@
     );
     const dispatch = useDispatch();
 
-    const cachedThreadIDsRef = React.useRef();
+    const cachedThreadIDsRef = React.useRef<?Set<string>>();
     if (!cachedThreadIDsRef.current) {
       const newCachedThreadIDs = new Set();
       for (const realizedThreadID of pendingToRealizedThreadIDs.values()) {
diff --git a/lib/hooks/disconnected-bar.js b/lib/hooks/disconnected-bar.js
--- a/lib/hooks/disconnected-bar.js
+++ b/lib/hooks/disconnected-bar.js
@@ -5,7 +5,10 @@
 import { useDispatch } from 'react-redux';
 
 import { connectionSelector } from '../selectors/keyserver-selectors.js';
-import { updateDisconnectedBarActionType } from '../types/socket-types.js';
+import {
+  updateDisconnectedBarActionType,
+  type ConnectionStatus,
+} from '../types/socket-types.js';
 import { useSelector } from '../utils/redux-utils.js';
 
 function useDisconnectedBarVisibilityHandler(networkConnected: boolean): void {
@@ -27,7 +30,7 @@
     [disconnected, dispatch],
   );
 
-  const networkActiveRef = React.useRef(true);
+  const networkActiveRef = React.useRef<boolean>(true);
   React.useEffect(() => {
     networkActiveRef.current = networkConnected;
     if (!networkConnected) {
@@ -35,7 +38,7 @@
     }
   }, [setDisconnected, networkConnected]);
 
-  const prevConnectionStatusRef = React.useRef();
+  const prevConnectionStatusRef = React.useRef<?ConnectionStatus>();
   const connectionStatus = connection.status;
   const someRequestIsLate = connection.lateResponses.length !== 0;
   React.useEffect(() => {
@@ -84,7 +87,7 @@
   const { disconnected, shouldShowDisconnectedBar } =
     useShouldShowDisconnectedBar();
 
-  const prevShowDisconnectedBar = React.useRef();
+  const prevShowDisconnectedBar = React.useRef<?boolean>();
   React.useEffect(() => {
     const wasShowing = prevShowDisconnectedBar.current;
     if (shouldShowDisconnectedBar && wasShowing === false) {
diff --git a/lib/hooks/search-threads.js b/lib/hooks/search-threads.js
--- a/lib/hooks/search-threads.js
+++ b/lib/hooks/search-threads.js
@@ -34,7 +34,7 @@
   threadInfo: ThreadInfo,
   childThreadInfos: $ReadOnlyArray<U>,
 ): SearchThreadsResult<U> {
-  const [searchState, setSearchState] = React.useState({
+  const [searchState, setSearchState] = React.useState<ThreadSearchState>({
     text: '',
     results: new Set<string>(),
   });
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
@@ -1413,7 +1413,9 @@
   const [threadSearchResults, setThreadSearchResults] = React.useState(
     new Set(),
   );
-  const [usersSearchResults, setUsersSearchResults] = React.useState([]);
+  const [usersSearchResults, setUsersSearchResults] = React.useState<
+    $ReadOnlyArray<GlobalAccountUserInfo>,
+  >([]);
   const threadSearchIndex = useGlobalThreadSearchIndex();
   React.useEffect(() => {
     (async () => {
diff --git a/lib/socket/activity-handler.react.js b/lib/socket/activity-handler.react.js
--- a/lib/socket/activity-handler.react.js
+++ b/lib/socket/activity-handler.react.js
@@ -12,6 +12,7 @@
 import { getMostRecentNonLocalMessageID } from '../shared/message-utils.js';
 import { threadIsPending } from '../shared/thread-utils.js';
 import { queueActivityUpdatesActionType } from '../types/activity-types.js';
+import type { ConnectionStatus } from '../types/socket-types.js';
 import {
   useServerCall,
   useDispatchActionPromise,
@@ -25,7 +26,7 @@
 function ActivityHandler(props: Props): React.Node {
   const { activeThread, frozen } = props;
 
-  const prevActiveThreadRef = React.useRef();
+  const prevActiveThreadRef = React.useRef<?string>();
   React.useEffect(() => {
     prevActiveThreadRef.current = activeThread;
   }, [activeThread]);
@@ -34,7 +35,7 @@
   const connection = useSelector(connectionSelector);
   invariant(connection, 'keyserver missing from keyserverStore');
   const connectionStatus = connection.status;
-  const prevConnectionStatusRef = React.useRef();
+  const prevConnectionStatusRef = React.useRef<?ConnectionStatus>();
   React.useEffect(() => {
     prevConnectionStatusRef.current = connectionStatus;
   }, [connectionStatus]);
@@ -46,7 +47,7 @@
     }
     return getMostRecentNonLocalMessageID(activeThread, state.messageStore);
   });
-  const prevActiveThreadLatestMessageRef = React.useRef();
+  const prevActiveThreadLatestMessageRef = React.useRef<?string>();
   React.useEffect(() => {
     prevActiveThreadLatestMessageRef.current = activeThreadLatestMessage;
   }, [activeThreadLatestMessage]);
@@ -54,7 +55,7 @@
     prevActiveThreadLatestMessageRef.current;
 
   const canSend = connectionStatus === 'connected' && !frozen;
-  const prevCanSendRef = React.useRef();
+  const prevCanSendRef = React.useRef<?boolean>();
   React.useEffect(() => {
     prevCanSendRef.current = canSend;
   }, [canSend]);