diff --git a/web/app.react.js b/web/app.react.js
--- a/web/app.react.js
+++ b/web/app.react.js
@@ -287,7 +287,7 @@
 
   getMainContentWithSwitcher(): React.Node {
     const { tab, settingsSection } = this.props.navInfo;
-    let mainContent;
+    let mainContent: React.Node;
 
     if (tab === 'settings') {
       if (settingsSection === 'account') {
diff --git a/web/calendar/filter-panel.react.js b/web/calendar/filter-panel.react.js
--- a/web/calendar/filter-panel.react.js
+++ b/web/calendar/filter-panel.react.js
@@ -85,7 +85,7 @@
       this.inCurrentCommunity(item.threadInfo.id),
     );
 
-    let filters = [];
+    let filters: React.Node[] = [];
     if (!this.state.query || filterThreadInfosInCurrentCommunity.length > 0) {
       filters.push(
         <Category
diff --git a/web/chat/chat-input-bar.react.js b/web/chat/chat-input-bar.react.js
--- a/web/chat/chat-input-bar.react.js
+++ b/web/chat/chat-input-bar.react.js
@@ -642,7 +642,7 @@
 
     const suggestions = React.useMemo(() => {
       if (!typeaheadMatchedStrings) {
-        return [];
+        return ([]: $ReadOnlyArray<MentionTypeaheadSuggestionItem>);
       }
       const suggestedUsers = getMentionTypeaheadUserSuggestions(
         userSearchIndex,
@@ -655,7 +655,10 @@
         props.inputState.typeaheadState.frozenChatMentionsCandidates,
         typeaheadMatchedStrings.query,
       );
-      return [...suggestedUsers, ...suggestedChats];
+      return ([
+        ...suggestedUsers,
+        ...suggestedChats,
+      ]: $ReadOnlyArray<MentionTypeaheadSuggestionItem>);
     }, [
       typeaheadMatchedStrings,
       userSearchIndex,
diff --git a/web/chat/text-message.react.js b/web/chat/text-message.react.js
--- a/web/chat/text-message.react.js
+++ b/web/chat/text-message.react.js
@@ -31,7 +31,7 @@
     creator: { isViewer },
   } = props.item.messageInfo;
 
-  const messageStyle = {};
+  const messageStyle: { backgroundColor?: string } = {};
   let darkColor = true;
   if (isViewer) {
     const threadColor = props.threadInfo.color;
diff --git a/web/components/community-actions-menu.react.js b/web/components/community-actions-menu.react.js
--- a/web/components/community-actions-menu.react.js
+++ b/web/components/community-actions-menu.react.js
@@ -12,6 +12,7 @@
 
 import css from './community-actions-menu.css';
 import MenuItem from '../components/menu-item.react.js';
+import type { MenuItemProps } from '../components/menu-item.react.js';
 import Menu from '../components/menu.react.js';
 import ManageInviteLinksModal from '../invite-links/manage-invite-links-modal.react.js';
 import ViewInviteLinkModal from '../invite-links/view-invite-link-modal.react.js';
@@ -59,7 +60,7 @@
   );
 
   const items = React.useMemo(() => {
-    const itemSpecs = [];
+    const itemSpecs: MenuItemProps[] = [];
 
     if (canManageLinks) {
       itemSpecs.push({
diff --git a/web/components/menu-item.react.js b/web/components/menu-item.react.js
--- a/web/components/menu-item.react.js
+++ b/web/components/menu-item.react.js
@@ -15,7 +15,7 @@
   +text: string,
   +dangerous?: boolean,
 };
-type MenuItemProps =
+export type MenuItemProps =
   | {
       ...MenuItemPropsBase,
       +icon: Icon,
diff --git a/web/input/input-state-container.react.js b/web/input/input-state-container.react.js
--- a/web/input/input-state-container.react.js
+++ b/web/input/input-state-container.react.js
@@ -147,13 +147,14 @@
   +unregisterSendCallback: (() => mixed) => void,
   +textMessageCreationSideEffectsFunc: CreationSideEffectsFunc<RawTextMessageInfo>,
 };
-type State = {
-  +pendingUploads: {
+type WritableState = {
+  pendingUploads: {
     [threadID: string]: { [localUploadID: string]: PendingMultimediaUpload },
   },
-  +textCursorPositions: { [threadID: string]: number },
-  +typeaheadState: TypeaheadState,
+  textCursorPositions: { [threadID: string]: number },
+  typeaheadState: TypeaheadState,
 };
+type State = $ReadOnly<WritableState>;
 
 type PropsAndState = {
   ...Props,
@@ -193,7 +194,7 @@
     state: { +[threadID: string]: T },
     props: Props,
   ): ?{ [threadID: string]: T } {
-    const newState = {};
+    const newState: { [string]: T } = {};
     let updated = false;
     for (const threadID in state) {
       const newThreadID =
@@ -220,7 +221,7 @@
       return null;
     }
 
-    const stateUpdate = {};
+    const stateUpdate: Partial<WritableState> = {};
     if (pendingUploads) {
       stateUpdate.pendingUploads = pendingUploads;
     }
@@ -518,7 +519,7 @@
       this.setState(prevState => {
         const newThreadID = this.getRealizedOrPendingThreadID(threadID);
         const prevUploads = prevState.pendingUploads[newThreadID];
-        const newUploads = {};
+        const newUploads: { [string]: PendingMultimediaUpload } = {};
         for (const localUploadID in prevUploads) {
           const upload = prevUploads[localUploadID];
           if (upload.messageID !== localID) {
@@ -588,8 +589,11 @@
           draft: ?string,
           textCursorPosition: ?number,
         ) => {
-          let threadPendingUploads = [];
-          const assignedUploads = {};
+          let threadPendingUploads: $ReadOnlyArray<PendingMultimediaUpload> =
+            [];
+          const assignedUploads: {
+            [string]: $ReadOnlyArray<PendingMultimediaUpload>,
+          } = {};
           if (pendingUploads) {
             const [uploadsWithMessageIDs, uploadsWithoutMessageIDs] =
               _partition('messageID')(pendingUploads);
@@ -740,7 +744,7 @@
       | MediaMissionFailure
       | { success: true, pendingUpload: PendingMultimediaUpload },
   }> {
-    const steps = [
+    const steps: MediaMissionStep[] = [
       {
         step: 'web_selection',
         filename: file.name,
@@ -1164,7 +1168,7 @@
   cancelPendingUpload(threadID: ?string, localUploadID: string) {
     invariant(threadID, 'threadID should be set in cancelPendingUpload');
 
-    let revokeURL, abortRequest;
+    let revokeURL: ?string, abortRequest: ?() => void;
     this.setState(
       prevState => {
         const newThreadID = this.getRealizedOrPendingThreadID(threadID);
@@ -1375,7 +1379,7 @@
       if (!currentPendingUploads) {
         return {};
       }
-      const newPendingUploads = {};
+      const newPendingUploads: { [string]: PendingMultimediaUpload } = {};
       let uploadAssigned = false;
       for (const localUploadID in currentPendingUploads) {
         const upload = currentPendingUploads[localUploadID];
@@ -1543,7 +1547,7 @@
       if (!prevPendingUploads) {
         return {};
       }
-      const newPendingUploads = {};
+      const newPendingUploads: { [string]: PendingMultimediaUpload } = {};
       let pendingUploadChanged = false;
       for (const localID in prevPendingUploads) {
         const pendingUpload = prevPendingUploads[localID];
diff --git a/web/loading-indicator.react.js b/web/loading-indicator.react.js
--- a/web/loading-indicator.react.js
+++ b/web/loading-indicator.react.js
@@ -24,7 +24,7 @@
   const size = props.size ? props.size : 'small';
   const color = props.color ? props.color : 'white';
   if (props.status === 'loading') {
-    const classNameInput = {
+    const classNameInput: { [string]: boolean } = {
       [css['loading-indicator-loading']]: true,
       [css['loading-indicator-loading-medium']]:
         hasRendered && size === 'medium',
@@ -38,7 +38,7 @@
     }
     return <span className={classNames(classNameInput)} />;
   } else if (props.status === 'error') {
-    const classNameInput = {
+    const classNameInput: { [string]: boolean } = {
       [css['loading-indicator-error']]: true,
       [css['loading-indicator-error-black']]: tinycolor.equals(color, 'black'),
     };
diff --git a/web/media/blob-utils.js b/web/media/blob-utils.js
--- a/web/media/blob-utils.js
+++ b/web/media/blob-utils.js
@@ -38,7 +38,7 @@
   steps: $ReadOnlyArray<MediaMissionStep>,
   result: MediaMissionFailure | ProbeFileSuccess,
 }> {
-  const steps = [];
+  const steps: MediaMissionStep[] = [];
 
   let arrayBuffer, arrayBufferExceptionMessage;
   const arrayBufferStart = Date.now();
diff --git a/web/media/image-utils.js b/web/media/image-utils.js
--- a/web/media/image-utils.js
+++ b/web/media/image-utils.js
@@ -61,7 +61,7 @@
   +steps: $ReadOnlyArray<MediaMissionStep>,
   +result: GenerateThumbhashResult | MediaMissionFailure,
 }> {
-  const steps = [];
+  const steps: MediaMissionStep[] = [];
   const initialURI = URL.createObjectURL(file);
   const { steps: preloadSteps, result: image } = await preloadImage(initialURI);
   steps.push(...preloadSteps);
diff --git a/web/media/media-utils.js b/web/media/media-utils.js
--- a/web/media/media-utils.js
+++ b/web/media/media-utils.js
@@ -123,7 +123,7 @@
   ]);
   const { steps: preloadSteps, result: image } = preloadResponse;
 
-  const steps = [...preloadSteps, orientationStep];
+  const steps: MediaMissionStep[] = [...preloadSteps, orientationStep];
 
   if (!image) {
     return {
diff --git a/web/modals/chat/message-results-modal.react.js b/web/modals/chat/message-results-modal.react.js
--- a/web/modals/chat/message-results-modal.react.js
+++ b/web/modals/chat/message-results-modal.react.js
@@ -7,7 +7,10 @@
   useFetchPinnedMessages,
 } from 'lib/actions/message-actions.js';
 import { useModalContext } from 'lib/components/modal-provider.react.js';
-import { messageListData } from 'lib/selectors/chat-selectors.js';
+import {
+  messageListData,
+  type ChatMessageInfoItem,
+} from 'lib/selectors/chat-selectors.js';
 import { createLoadingStatusSelector } from 'lib/selectors/loading-selectors.js';
 import {
   createMessageInfo,
@@ -73,7 +76,7 @@
 
   const sortedUniqueChatMessageInfoItems = React.useMemo(() => {
     if (!chatMessageInfos) {
-      return [];
+      return ([]: ChatMessageInfoItem[]);
     }
 
     const chatMessageInfoItems = chatMessageInfos.filter(
diff --git a/web/modals/threads/settings/thread-settings-modal.react.js b/web/modals/threads/settings/thread-settings-modal.react.js
--- a/web/modals/threads/settings/thread-settings-modal.react.js
+++ b/web/modals/threads/settings/thread-settings-modal.react.js
@@ -16,6 +16,7 @@
   getSingleOtherUser,
   threadUIName,
 } from 'lib/shared/thread-utils.js';
+import type { RelationshipButton } from 'lib/types/relationship-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
 import { threadTypes } from 'lib/types/thread-types-enum.js';
 import { type ThreadInfo, type ThreadChanges } from 'lib/types/thread-types.js';
@@ -88,7 +89,7 @@
 
     const availableRelationshipActions = React.useMemo(() => {
       if (!otherUserInfo) {
-        return [];
+        return ([]: RelationshipButton[]);
       }
       return getAvailableRelationshipButtons(otherUserInfo);
     }, [otherUserInfo]);
diff --git a/web/olm/olm-utils.js b/web/olm/olm-utils.js
--- a/web/olm/olm-utils.js
+++ b/web/olm/olm-utils.js
@@ -14,7 +14,7 @@
   return await olm.init({ locateFile });
 }
 
-let olmUtilityInstance;
+let olmUtilityInstance: ?olm.Utility | Promise<olm.Utility>;
 function olmUtility(): Promise<Utility> {
   if (!olmUtilityInstance) {
     olmUtilityInstance = (async () => {
diff --git a/web/redux/action-types.js b/web/redux/action-types.js
--- a/web/redux/action-types.js
+++ b/web/redux/action-types.js
@@ -25,7 +25,7 @@
     allKeyserverIDs: $ReadOnlyArray<string>,
   ): ((input: URLInfo) => Promise<InitialReduxState>) =>
   async urlInfo => {
-    const requests = {};
+    const requests: { [string]: URLInfo } = {};
     const { thread, inviteSecret, ...rest } = urlInfo;
     const threadKeyserverID = thread ? extractKeyserverIDFromID(thread) : null;
 
diff --git a/web/redux/persist.js b/web/redux/persist.js
--- a/web/redux/persist.js
+++ b/web/redux/persist.js
@@ -273,7 +273,7 @@
 };
 const keyserverStoreTransform: Transform = createTransform(
   (state: KeyserverStore): PersistedKeyserverStore => {
-    const keyserverInfos = {};
+    const keyserverInfos: { [string]: PersistedKeyserverInfo } = {};
     for (const key in state.keyserverInfos) {
       const { connection, updatesCurrentAsOf, sessionID, ...rest } =
         state.keyserverInfos[key];
@@ -285,7 +285,7 @@
     };
   },
   (state: PersistedKeyserverStore): KeyserverStore => {
-    const keyserverInfos = {};
+    const keyserverInfos: { [string]: KeyserverInfo } = {};
     for (const key in state.keyserverInfos) {
       keyserverInfos[key] = {
         ...state.keyserverInfos[key],
diff --git a/web/settings/relationship/add-users-list.react.js b/web/settings/relationship/add-users-list.react.js
--- a/web/settings/relationship/add-users-list.react.js
+++ b/web/settings/relationship/add-users-list.react.js
@@ -14,7 +14,10 @@
   UserRelationshipStatus,
   RelationshipAction,
 } from 'lib/types/relationship-types.js';
-import type { GlobalAccountUserInfo } from 'lib/types/user-types.js';
+import type {
+  GlobalAccountUserInfo,
+  AccountUserInfo,
+} from 'lib/types/user-types.js';
 import {
   useDispatchActionPromise,
   useServerCall,
@@ -80,7 +83,8 @@
   const searchTextPresent = searchText.length > 0;
   const userInfos = useSelector(state => state.userStore.userInfos);
   const mergedUserInfos = React.useMemo(() => {
-    const mergedInfos = {};
+    const mergedInfos: { [string]: GlobalAccountUserInfo | AccountUserInfo } =
+      {};
 
     for (const userInfo of serverSearchResults) {
       mergedInfos[userInfo.id] = userInfo;
diff --git a/web/settings/tunnelbroker-message-list.react.js b/web/settings/tunnelbroker-message-list.react.js
--- a/web/settings/tunnelbroker-message-list.react.js
+++ b/web/settings/tunnelbroker-message-list.react.js
@@ -27,7 +27,7 @@
     return () => removeListener(listener);
   }, [addListener, listener, removeListener]);
 
-  let messageList = (
+  let messageList: React.Node = (
     <div className={css.messageRow}>
       <div className={css.messageCol}>No messages</div>
     </div>
diff --git a/web/url-utils.js b/web/url-utils.js
--- a/web/url-utils.js
+++ b/web/url-utils.js
@@ -90,7 +90,7 @@
   const { navInfo } = backupInfo;
   const now = backupInfo.now ? backupInfo.now : new Date();
 
-  let year = urlInfo.year;
+  let year: ?number = urlInfo.year;
   if (!year && navInfo) {
     year = yearExtractor(navInfo.startDate, navInfo.endDate);
   }
@@ -98,7 +98,7 @@
     year = now.getFullYear();
   }
 
-  let month = urlInfo.month;
+  let month: ?number = urlInfo.month;
   if (!month && navInfo) {
     month = monthExtractor(navInfo.startDate, navInfo.endDate);
   }