diff --git a/src/Avatar.ts b/src/Avatar.ts
index 8a3f10a22c..986ae6306a 100644
--- a/src/Avatar.ts
+++ b/src/Avatar.ts
@@ -31,7 +31,7 @@ export function avatarUrlForMember(
     height: number,
     resizeMethod: ResizeMethod,
 ): string {
-    let url: string;
+    let url: string | null | undefined;
     if (member?.getMxcAvatarUrl()) {
         url = mediaFromMxc(member.getMxcAvatarUrl()).getThumbnailOfSourceHttp(width, height, resizeMethod);
     }
@@ -118,7 +118,7 @@ export function defaultAvatarUrlForString(s: string): string {
  * @param {string} name
  * @return {string} the first letter
  */
-export function getInitialLetter(name: string): string {
+export function getInitialLetter(name: string): string | undefined {
     if (!name) {
         // XXX: We should find out what causes the name to sometimes be falsy.
         console.trace("`name` argument to `getInitialLetter` not supplied");
@@ -146,7 +146,7 @@ export function avatarUrlForRoom(
     if (!room) return null; // null-guard
 
     if (room.getMxcAvatarUrl()) {
-        return mediaFromMxc(room.getMxcAvatarUrl()).getThumbnailOfSourceHttp(width, height, resizeMethod);
+        return mediaFromMxc(room.getMxcAvatarUrl() || undefined).getThumbnailOfSourceHttp(width, height, resizeMethod);
     }
 
     // space rooms cannot be DMs so skip the rest
diff --git a/src/BasePlatform.ts b/src/BasePlatform.ts
index ab8bca4b5f..46f964995a 100644
--- a/src/BasePlatform.ts
+++ b/src/BasePlatform.ts
@@ -130,7 +130,7 @@ export default abstract class BasePlatform {
         if (MatrixClientPeg.userRegisteredWithinLastHours(24)) return false;
 
         try {
-            const [version, deferUntil] = JSON.parse(localStorage.getItem(UPDATE_DEFER_KEY));
+            const [version, deferUntil] = JSON.parse(localStorage.getItem(UPDATE_DEFER_KEY)!);
             return newVersion !== version || Date.now() > deferUntil;
         } catch (e) {
             return true;
@@ -211,7 +211,7 @@ export default abstract class BasePlatform {
                 metricsTrigger: "Notification",
             };
 
-            if (ev.getThread()) {
+            if (ev?.getThread()) {
                 payload.event_id = ev.getId();
             }
 
@@ -255,7 +255,7 @@ export default abstract class BasePlatform {
         return false;
     }
 
-    public getSettingValue(settingName: string): Promise<any> {
+    public async getSettingValue(settingName: string): Promise<any> {
         return undefined;
     }
 
@@ -278,7 +278,7 @@ export default abstract class BasePlatform {
     public setSpellCheckEnabled(enabled: boolean): void {}
 
     public async getSpellCheckEnabled(): Promise<boolean> {
-        return null;
+        return false;
     }
 
     public setSpellCheckLanguages(preferredLangs: string[]): void {}
@@ -333,7 +333,7 @@ export default abstract class BasePlatform {
         // persist hs url and is url for when the user is returned to the app with the login token
         localStorage.setItem(SSO_HOMESERVER_URL_KEY, mxClient.getHomeserverUrl());
         if (mxClient.getIdentityServerUrl()) {
-            localStorage.setItem(SSO_ID_SERVER_URL_KEY, mxClient.getIdentityServerUrl());
+            localStorage.setItem(SSO_ID_SERVER_URL_KEY, mxClient.getIdentityServerUrl()!);
         }
         if (idpId) {
             localStorage.setItem(SSO_IDP_ID_KEY, idpId);
diff --git a/src/ContentMessages.ts b/src/ContentMessages.ts
index 1381e9431e..85ca90067d 100644
--- a/src/ContentMessages.ts
+++ b/src/ContentMessages.ts
@@ -372,7 +372,7 @@ export default class ContentMessages {
         const replyToEvent = SdkContextClass.instance.roomViewStore.getQuotingEvent();
         if (!this.mediaConfig) {
             // hot-path optimization to not flash a spinner if we don't need to
-            const modal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+            const modal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
             await this.ensureMediaConfigFetched(matrixClient);
             modal.close();
         }
diff --git a/src/DecryptionFailureTracker.ts b/src/DecryptionFailureTracker.ts
index c275a176eb..7329c665bc 100644
--- a/src/DecryptionFailureTracker.ts
+++ b/src/DecryptionFailureTracker.ts
@@ -83,8 +83,8 @@ export class DecryptionFailureTracker {
     public trackedEvents: Set<string> = new Set();
 
     // Set to an interval ID when `start` is called
-    public checkInterval: number = null;
-    public trackInterval: number = null;
+    public checkInterval: number | null = null;
+    public trackInterval: number | null = null;
 
     // Spread the load on `Analytics` by tracking at a low frequency, `TRACK_INTERVAL_MS`.
     public static TRACK_INTERVAL_MS = 60000;
diff --git a/src/DeviceListener.ts b/src/DeviceListener.ts
index be48717415..eccfc1c0e3 100644
--- a/src/DeviceListener.ts
+++ b/src/DeviceListener.ts
@@ -58,12 +58,12 @@ export default class DeviceListener {
     private dismissedThisDeviceToast = false;
     // cache of the key backup info
     private keyBackupInfo: IKeyBackupInfo | null = null;
-    private keyBackupFetchedAt: number = null;
+    private keyBackupFetchedAt: number | null = null;
     private keyBackupStatusChecked = false;
     // We keep a list of our own device IDs so we can batch ones that were already
     // there the last time the app launched into a single toast, but display new
     // ones in their own toasts.
-    private ourDeviceIdsAtStart: Set<string> = null;
+    private ourDeviceIdsAtStart: Set<string> | null = null;
     // The set of device IDs we're currently displaying toasts for
     private displayingToastsForDeviceIds = new Set<string>();
     private running = false;
@@ -203,7 +203,7 @@ export default class DeviceListener {
         }
     };
 
-    private onSync = (state: SyncState, prevState?: SyncState): void => {
+    private onSync = (state: SyncState, prevState: SyncState | null): void => {
         if (state === "PREPARED" && prevState === null) {
             this.recheck();
         }
diff --git a/src/ImageUtils.ts b/src/ImageUtils.ts
index 42db71ebab..e8564fb017 100644
--- a/src/ImageUtils.ts
+++ b/src/ImageUtils.ts
@@ -28,7 +28,12 @@ limitations under the License.
  * consume in the timeline, when performing scroll offset calculations
  * (e.g. scroll locking)
  */
-export function thumbHeight(fullWidth: number, fullHeight: number, thumbWidth: number, thumbHeight: number): number {
+export function thumbHeight(
+    fullWidth: number,
+    fullHeight: number,
+    thumbWidth: number,
+    thumbHeight: number,
+): number | null {
     if (!fullWidth || !fullHeight) {
         // Cannot calculate thumbnail height for image: missing w/h in metadata. We can't even
         // log this because it's spammy
diff --git a/src/accessibility/KeyboardShortcutUtils.ts b/src/accessibility/KeyboardShortcutUtils.ts
index 394daf7ef9..bb42f7c1ce 100644
--- a/src/accessibility/KeyboardShortcutUtils.ts
+++ b/src/accessibility/KeyboardShortcutUtils.ts
@@ -72,7 +72,7 @@ const getUIOnlyShortcuts = (): IKeyboardShortcuts => {
         },
     };
 
-    if (PlatformPeg.get().overrideBrowserShortcuts()) {
+    if (PlatformPeg.get()?.overrideBrowserShortcuts()) {
         // XXX: This keyboard shortcut isn't manually added to
         // KeyBindingDefaults as it can't be easily handled by the
         // KeyBindingManager
@@ -92,7 +92,7 @@ const getUIOnlyShortcuts = (): IKeyboardShortcuts => {
  * This function gets keyboard shortcuts that can be consumed by the KeyBindingDefaults.
  */
 export const getKeyboardShortcuts = (): IKeyboardShortcuts => {
-    const overrideBrowserShortcuts = PlatformPeg.get().overrideBrowserShortcuts();
+    const overrideBrowserShortcuts = PlatformPeg.get()?.overrideBrowserShortcuts();
 
     return Object.keys(KEYBOARD_SHORTCUTS)
         .filter((k: KeyBindingAction) => {
@@ -120,11 +120,11 @@ export const getKeyboardShortcutsForUI = (): IKeyboardShortcuts => {
     }, {} as IKeyboardShortcuts);
 };
 
-export const getKeyboardShortcutValue = (name: string): KeyCombo => {
+export const getKeyboardShortcutValue = (name: string): KeyCombo | undefined => {
     return getKeyboardShortcutsForUI()[name]?.default;
 };
 
-export const getKeyboardShortcutDisplayName = (name: string): string | null => {
+export const getKeyboardShortcutDisplayName = (name: string): string | undefined => {
     const keyboardShortcutDisplayName = getKeyboardShortcutsForUI()[name]?.displayName;
     return keyboardShortcutDisplayName && _t(keyboardShortcutDisplayName);
 };
diff --git a/src/accessibility/RovingTabIndex.tsx b/src/accessibility/RovingTabIndex.tsx
index e90aed87a9..605ffb1f5b 100644
--- a/src/accessibility/RovingTabIndex.tsx
+++ b/src/accessibility/RovingTabIndex.tsx
@@ -56,7 +56,7 @@ export function checkInputableElement(el: HTMLElement): boolean {
 }
 
 export interface IState {
-    activeRef: Ref;
+    activeRef?: Ref;
     refs: Ref[];
 }
 
@@ -67,7 +67,6 @@ interface IContext {
 
 export const RovingTabIndexContext = createContext<IContext>({
     state: {
-        activeRef: null,
         refs: [], // list of refs in DOM order
     },
     dispatch: () => {},
@@ -102,7 +101,7 @@ export const reducer: Reducer<IState, IAction> = (state: IState, action: IAction
                     return 0;
                 }
 
-                const position = a.current.compareDocumentPosition(b.current);
+                const position = a.current!.compareDocumentPosition(b.current!);
 
                 if (position & Node.DOCUMENT_POSITION_FOLLOWING || position & Node.DOCUMENT_POSITION_CONTAINED_BY) {
                     return -1;
@@ -167,7 +166,7 @@ export const findSiblingElement = (
     refs: RefObject<HTMLElement>[],
     startIndex: number,
     backwards = false,
-): RefObject<HTMLElement> => {
+): RefObject<HTMLElement> | undefined => {
     if (backwards) {
         for (let i = startIndex; i < refs.length && i >= 0; i--) {
             if (refs[i].current?.offsetParent !== null) {
@@ -191,7 +190,6 @@ export const RovingTabIndexProvider: React.FC<IProps> = ({
     onKeyDown,
 }) => {
     const [state, dispatch] = useReducer<Reducer<IState, IAction>>(reducer, {
-        activeRef: null,
         refs: [],
     });
 
@@ -208,7 +206,7 @@ export const RovingTabIndexProvider: React.FC<IProps> = ({
 
             let handled = false;
             const action = getKeyBindingsManager().getAccessibilityAction(ev);
-            let focusRef: RefObject<HTMLElement>;
+            let focusRef: RefObject<HTMLElement> | undefined;
             // Don't interfere with input default keydown behaviour
             // but allow people to move focus from it with Tab.
             if (checkInputableElement(ev.target as HTMLElement)) {
@@ -216,7 +214,7 @@ export const RovingTabIndexProvider: React.FC<IProps> = ({
                     case KeyBindingAction.Tab:
                         handled = true;
                         if (context.state.refs.length > 0) {
-                            const idx = context.state.refs.indexOf(context.state.activeRef);
+                            const idx = context.state.refs.indexOf(context.state.activeRef!);
                             focusRef = findSiblingElement(
                                 context.state.refs,
                                 idx + (ev.shiftKey ? -1 : 1),
@@ -252,7 +250,7 @@ export const RovingTabIndexProvider: React.FC<IProps> = ({
                         ) {
                             handled = true;
                             if (context.state.refs.length > 0) {
-                                const idx = context.state.refs.indexOf(context.state.activeRef);
+                                const idx = context.state.refs.indexOf(context.state.activeRef!);
                                 focusRef = findSiblingElement(context.state.refs, idx + 1);
                             }
                         }
@@ -266,7 +264,7 @@ export const RovingTabIndexProvider: React.FC<IProps> = ({
                         ) {
                             handled = true;
                             if (context.state.refs.length > 0) {
-                                const idx = context.state.refs.indexOf(context.state.activeRef);
+                                const idx = context.state.refs.indexOf(context.state.activeRef!);
                                 focusRef = findSiblingElement(context.state.refs, idx - 1, true);
                             }
                         }
diff --git a/src/actions/MatrixActionCreators.ts b/src/actions/MatrixActionCreators.ts
index 88de8b3d17..bb40a463ba 100644
--- a/src/actions/MatrixActionCreators.ts
+++ b/src/actions/MatrixActionCreators.ts
@@ -221,7 +221,7 @@ function createRoomTimelineAction(
         action: "MatrixActions.Room.timeline",
         event: timelineEvent,
         isLiveEvent: data.liveEvent,
-        isLiveUnfilteredRoomTimelineEvent: room && data.timeline.getTimelineSet() === room.getUnfilteredTimelineSet(),
+        isLiveUnfilteredRoomTimelineEvent: data.timeline.getTimelineSet() === room?.getUnfilteredTimelineSet(),
         room,
     };
 }
diff --git a/src/audio/PlaybackQueue.ts b/src/audio/PlaybackQueue.ts
index 7c521b9ca6..0828a3df1d 100644
--- a/src/audio/PlaybackQueue.ts
+++ b/src/audio/PlaybackQueue.ts
@@ -45,7 +45,7 @@ export class PlaybackQueue {
     private playbacks = new Map<string, Playback>(); // keyed by event ID
     private clockStates = new Map<string, number>(); // keyed by event ID
     private playbackIdOrder: string[] = []; // event IDs, last == current
-    private currentPlaybackId: string; // event ID, broken out from above for ease of use
+    private currentPlaybackId: string | null = null; // event ID, broken out from above for ease of use
     private recentFullPlays = new Set<string>(); // event IDs
 
     public constructor(private room: Room) {
@@ -68,7 +68,7 @@ export class PlaybackQueue {
         const room = cli.getRoom(roomId);
         if (!room) throw new Error("Unknown room");
         if (PlaybackQueue.queues.has(room.roomId)) {
-            return PlaybackQueue.queues.get(room.roomId);
+            return PlaybackQueue.queues.get(room.roomId)!;
         }
         const queue = new PlaybackQueue(room);
         PlaybackQueue.queues.set(room.roomId, queue);
@@ -101,7 +101,7 @@ export class PlaybackQueue {
         const wasLastPlaying = this.currentPlaybackId === mxEvent.getId();
         if (newState === PlaybackState.Stopped && this.clockStates.has(mxEvent.getId()) && !wasLastPlaying) {
             // noinspection JSIgnoredPromiseFromCall
-            playback.skipTo(this.clockStates.get(mxEvent.getId()));
+            playback.skipTo(this.clockStates.get(mxEvent.getId())!);
         } else if (newState === PlaybackState.Stopped) {
             // Remove the now-useless clock for some space savings
             this.clockStates.delete(mxEvent.getId());
diff --git a/src/autocomplete/AutocompleteProvider.tsx b/src/autocomplete/AutocompleteProvider.tsx
index 546e052f58..32c2d80c6f 100644
--- a/src/autocomplete/AutocompleteProvider.tsx
+++ b/src/autocomplete/AutocompleteProvider.tsx
@@ -70,7 +70,7 @@ export default abstract class AutocompleteProvider {
      * @param {boolean} force True if the user is forcing completion
      * @return {object} { command, range } where both objects fields are null if no match
      */
-    public getCurrentCommand(query: string, selection: ISelectionRange, force = false): ICommand {
+    public getCurrentCommand(query: string, selection: ISelectionRange, force = false): ICommand | null {
         let commandRegex = this.commandRegex;
 
         if (force && this.shouldForceComplete()) {
@@ -83,7 +83,7 @@ export default abstract class AutocompleteProvider {
 
         commandRegex.lastIndex = 0;
 
-        let match: RegExpExecArray;
+        let match: RegExpExecArray | null;
         while ((match = commandRegex.exec(query)) !== null) {
             const start = match.index;
             const end = start + match[0].length;
diff --git a/src/autocomplete/Autocompleter.ts b/src/autocomplete/Autocompleter.ts
index b609f265f1..c769faf155 100644
--- a/src/autocomplete/Autocompleter.ts
+++ b/src/autocomplete/Autocompleter.ts
@@ -87,7 +87,7 @@ export default class Autocompleter {
          to predict whether an action will actually do what is intended
         */
         // list of results from each provider, each being a list of completions or null if it times out
-        const completionsList: ICompletion[][] = await Promise.all(
+        const completionsList: Array<ICompletion[] | null> = await Promise.all(
             this.providers.map(async (provider): Promise<ICompletion[] | null> => {
                 return timeout(
                     provider.getCompletions(query, selection, force, limit),
@@ -113,6 +113,6 @@ export default class Autocompleter {
                     command: this.providers[i].getCurrentCommand(query, selection, force),
                 };
             })
-            .filter(Boolean);
+            .filter(Boolean) as IProviderCompletions[];
     }
 }
diff --git a/src/autocomplete/CommandProvider.tsx b/src/autocomplete/CommandProvider.tsx
index caafe98f08..113c928790 100644
--- a/src/autocomplete/CommandProvider.tsx
+++ b/src/autocomplete/CommandProvider.tsx
@@ -56,10 +56,10 @@ export default class CommandProvider extends AutocompleteProvider {
         if (command[0] !== command[1]) {
             // The input looks like a command with arguments, perform exact match
             const name = command[1].slice(1); // strip leading `/`
-            if (CommandMap.has(name) && CommandMap.get(name).isEnabled()) {
+            if (CommandMap.has(name) && CommandMap.get(name)!.isEnabled()) {
                 // some commands, namely `me` don't suit having the usage shown whilst typing their arguments
-                if (CommandMap.get(name).hideCompletionAfterSpace) return [];
-                matches = [CommandMap.get(name)];
+                if (CommandMap.get(name)!.hideCompletionAfterSpace) return [];
+                matches = [CommandMap.get(name)!];
             }
         } else {
             if (query === "/") {
diff --git a/src/autocomplete/RoomProvider.tsx b/src/autocomplete/RoomProvider.tsx
index bf102d55bc..9cde501365 100644
--- a/src/autocomplete/RoomProvider.tsx
+++ b/src/autocomplete/RoomProvider.tsx
@@ -39,12 +39,12 @@ function canonicalScore(displayedAlias: string, room: Room): number {
 
 function matcherObject(
     room: Room,
-    displayedAlias: string,
+    displayedAlias: string | null,
     matchName = "",
 ): {
     room: Room;
     matchName: string;
-    displayedAlias: string;
+    displayedAlias: string | null;
 } {
     return {
         room,
@@ -58,7 +58,7 @@ export default class RoomProvider extends AutocompleteProvider {
 
     public constructor(room: Room, renderingType?: TimelineRenderingType) {
         super({ commandRegex: ROOM_REGEX, renderingType });
-        this.matcher = new QueryMatcher([], {
+        this.matcher = new QueryMatcher<ReturnType<typeof matcherObject>>([], {
             keys: ["displayedAlias", "matchName"],
         });
     }
@@ -79,7 +79,7 @@ export default class RoomProvider extends AutocompleteProvider {
         const { command, range } = this.getCurrentCommand(query, selection, force);
         if (command) {
             // the only reason we need to do this is because Fuse only matches on properties
-            let matcherObjects = this.getRooms().reduce((aliases, room) => {
+            let matcherObjects = this.getRooms().reduce<ReturnType<typeof matcherObject>[]>((aliases, room) => {
                 if (room.getCanonicalAlias()) {
                     aliases = aliases.concat(matcherObject(room, room.getCanonicalAlias(), room.name));
                 }
diff --git a/src/components/structures/MatrixChat.tsx b/src/components/structures/MatrixChat.tsx
index 44424619a7..cc07f28e4a 100644
--- a/src/components/structures/MatrixChat.tsx
+++ b/src/components/structures/MatrixChat.tsx
@@ -648,7 +648,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
                     onFinished: (confirm) => {
                         if (confirm) {
                             // FIXME: controller shouldn't be loading a view :(
-                            const modal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+                            const modal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
 
                             MatrixClientPeg.get()
                                 .leave(payload.room_id)
diff --git a/src/components/views/messages/MKeyVerificationConclusion.tsx b/src/components/views/messages/MKeyVerificationConclusion.tsx
index 656745997c..24b925544e 100644
--- a/src/components/views/messages/MKeyVerificationConclusion.tsx
+++ b/src/components/views/messages/MKeyVerificationConclusion.tsx
@@ -72,7 +72,7 @@ export default class MKeyVerificationConclusion extends React.Component<IProps>
         this.forceUpdate();
     };
 
-    public static shouldRender(mxEvent: MatrixEvent, request: VerificationRequest): boolean {
+    public static shouldRender(mxEvent: MatrixEvent, request?: VerificationRequest): boolean {
         // normally should not happen
         if (!request) {
             return false;
@@ -99,9 +99,9 @@ export default class MKeyVerificationConclusion extends React.Component<IProps>
         return true;
     }
 
-    public render(): JSX.Element {
+    public render(): JSX.Element | null {
         const { mxEvent } = this.props;
-        const request = mxEvent.verificationRequest;
+        const request = mxEvent.verificationRequest!;
 
         if (!MKeyVerificationConclusion.shouldRender(mxEvent, request)) {
             return null;
@@ -110,7 +110,7 @@ export default class MKeyVerificationConclusion extends React.Component<IProps>
         const client = MatrixClientPeg.get();
         const myUserId = client.getUserId();
 
-        let title;
+        let title: string | undefined;
 
         if (request.done) {
             title = _t("You verified %(name)s", {
diff --git a/src/createRoom.ts b/src/createRoom.ts
index 2bd49a4bb6..f4d9769d90 100644
--- a/src/createRoom.ts
+++ b/src/createRoom.ts
@@ -268,7 +268,7 @@ export default async function createRoom(opts: IOpts): Promise<string | null> {
     }
 
     let modal;
-    if (opts.spinner) modal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+    if (opts.spinner) modal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
 
     let roomId: string;
     let room: Promise<Room>;
@@ -417,9 +417,9 @@ export async function ensureVirtualRoomExists(
     client: MatrixClient,
     userId: string,
     nativeRoomId: string,
-): Promise<string> {
+): Promise<string | null> {
     const existingDMRoom = findDMForUser(client, userId);
-    let roomId;
+    let roomId: string | null;
     if (existingDMRoom) {
         roomId = existingDMRoom.roomId;
     } else {
@@ -440,13 +440,13 @@ export async function ensureVirtualRoomExists(
     return roomId;
 }
 
-export async function ensureDMExists(client: MatrixClient, userId: string): Promise<string> {
+export async function ensureDMExists(client: MatrixClient, userId: string): Promise<string | null> {
     const existingDMRoom = findDMForUser(client, userId);
-    let roomId;
+    let roomId: string | null;
     if (existingDMRoom) {
         roomId = existingDMRoom.roomId;
     } else {
-        let encryption: boolean = undefined;
+        let encryption: boolean | undefined;
         if (privateShouldBeEncrypted()) {
             encryption = await canEncryptToAllUsers(client, [userId]);
         }
diff --git a/src/customisations/Media.ts b/src/customisations/Media.ts
index 7c9ca139a2..c3debef86f 100644
--- a/src/customisations/Media.ts
+++ b/src/customisations/Media.ts
@@ -75,7 +75,7 @@ export class Media {
     /**
      * The HTTP URL for the source media.
      */
-    public get srcHttp(): string {
+    public get srcHttp(): string | null {
         // eslint-disable-next-line no-restricted-properties
         return this.client.mxcUrlToHttp(this.srcMxc);
     }
@@ -87,7 +87,7 @@ export class Media {
     public get thumbnailHttp(): string | undefined | null {
         if (!this.hasThumbnail) return null;
         // eslint-disable-next-line no-restricted-properties
-        return this.client.mxcUrlToHttp(this.thumbnailMxc);
+        return this.client.mxcUrlToHttp(this.thumbnailMxc!);
     }
 
     /**
@@ -98,13 +98,13 @@ export class Media {
      * @param {"scale"|"crop"} mode The desired thumbnailing mode. Defaults to scale.
      * @returns {string} The HTTP URL which points to the thumbnail.
      */
-    public getThumbnailHttp(width: number, height: number, mode: ResizeMethod = "scale"): string | null | undefined {
+    public getThumbnailHttp(width: number, height: number, mode: ResizeMethod = "scale"): string | null {
         if (!this.hasThumbnail) return null;
         // scale using the device pixel ratio to keep images clear
         width = Math.floor(width * window.devicePixelRatio);
         height = Math.floor(height * window.devicePixelRatio);
         // eslint-disable-next-line no-restricted-properties
-        return this.client.mxcUrlToHttp(this.thumbnailMxc, width, height, mode);
+        return this.client.mxcUrlToHttp(this.thumbnailMxc!, width, height, mode);
     }
 
     /**
@@ -114,7 +114,7 @@ export class Media {
      * @param {"scale"|"crop"} mode The desired thumbnailing mode. Defaults to scale.
      * @returns {string} The HTTP URL which points to the thumbnail.
      */
-    public getThumbnailOfSourceHttp(width: number, height: number, mode: ResizeMethod = "scale"): string {
+    public getThumbnailOfSourceHttp(width: number, height: number, mode: ResizeMethod = "scale"): string | null {
         // scale using the device pixel ratio to keep images clear
         width = Math.floor(width * window.devicePixelRatio);
         height = Math.floor(height * window.devicePixelRatio);
@@ -128,7 +128,7 @@ export class Media {
      * @param {number} dim The desired width and height.
      * @returns {string} An HTTP URL for the thumbnail.
      */
-    public getSquareThumbnailHttp(dim: number): string {
+    public getSquareThumbnailHttp(dim: number): string | null {
         dim = Math.floor(dim * window.devicePixelRatio); // scale using the device pixel ratio to keep images clear
         if (this.hasThumbnail) {
             return this.getThumbnailHttp(dim, dim, "crop");
@@ -161,6 +161,6 @@ export function mediaFromContent(content: Partial<IMediaEventContent>, client?:
  * @param {MatrixClient} client? Optional client to use.
  * @returns {Media} The media object.
  */
-export function mediaFromMxc(mxc: string, client?: MatrixClient): Media {
+export function mediaFromMxc(mxc?: string, client?: MatrixClient): Media {
     return mediaFromContent({ url: mxc }, client);
 }
diff --git a/src/customisations/Security.ts b/src/customisations/Security.ts
index bff7fc19f6..bfc2caaddd 100644
--- a/src/customisations/Security.ts
+++ b/src/customisations/Security.ts
@@ -30,19 +30,19 @@ function persistCredentials(credentials: IMatrixClientCreds): void {
 }
 
 /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
-function createSecretStorageKey(): Uint8Array {
+function createSecretStorageKey(): Uint8Array | null {
     // E.g. generate or retrieve secret storage key somehow
     return null;
 }
 
 /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
-function getSecretStorageKey(): Uint8Array {
+function getSecretStorageKey(): Uint8Array | null {
     // E.g. retrieve secret storage key from some other place
     return null;
 }
 
 /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
-function getDehydrationKey(keyInfo: ISecretStorageKeyInfo): Promise<Uint8Array> {
+function getDehydrationKey(keyInfo: ISecretStorageKeyInfo): Promise<Uint8Array | null> {
     return Promise.resolve(null);
 }
 
diff --git a/src/customisations/models/IMediaEventContent.ts b/src/customisations/models/IMediaEventContent.ts
index 299432eaaa..bd17ba2204 100644
--- a/src/customisations/models/IMediaEventContent.ts
+++ b/src/customisations/models/IMediaEventContent.ts
@@ -72,7 +72,7 @@ export interface IMediaObject {
  * @throws Throws if the given content cannot be packaged into a prepared media object.
  */
 export function prepEventContentAsMedia(content: Partial<IMediaEventContent>): IPreparedMedia {
-    let thumbnail: IMediaObject = null;
+    let thumbnail: IMediaObject | undefined;
     if (content?.info?.thumbnail_url) {
         thumbnail = {
             mxc: content.info.thumbnail_url,
diff --git a/src/editor/caret.ts b/src/editor/caret.ts
index ea6777c057..61c6a85640 100644
--- a/src/editor/caret.ts
+++ b/src/editor/caret.ts
@@ -32,7 +32,7 @@ export function setSelection(editor: HTMLDivElement, model: EditorModel, selecti
 }
 
 function setDocumentRangeSelection(editor: HTMLDivElement, model: EditorModel, range: Range): void {
-    const sel = document.getSelection();
+    const sel = document.getSelection()!;
     sel.removeAllRanges();
     const selectionRange = document.createRange();
     const start = getNodeAndOffsetForPosition(editor, model, range.start);
@@ -50,7 +50,7 @@ export function setCaretPosition(editor: HTMLDivElement, model: EditorModel, car
     range.setStart(node, offset);
     range.collapse(true);
 
-    const sel = document.getSelection();
+    const sel = document.getSelection()!;
     if (sel.rangeCount === 1) {
         const existingRange = sel.getRangeAt(0);
         if (
@@ -124,7 +124,7 @@ function findNodeInLineForPart(parts: Part[], partIndex: number): { lineIndex: n
     let lineIndex = 0;
     let nodeIndex = -1;
 
-    let prevPart = null;
+    let prevPart: Part | undefined;
     // go through to parts up till (and including) the index
     // to find newline parts
     for (let i = 0; i <= partIndex; ++i) {
@@ -132,7 +132,7 @@ function findNodeInLineForPart(parts: Part[], partIndex: number): { lineIndex: n
         if (part.type === Type.Newline) {
             lineIndex += 1;
             nodeIndex = -1;
-            prevPart = null;
+            prevPart = undefined;
         } else {
             nodeIndex += 1;
             if (needsCaretNodeBefore(part, prevPart)) {
diff --git a/src/editor/deserialize.ts b/src/editor/deserialize.ts
index f8ecd19c16..d4d0de6ed8 100644
--- a/src/editor/deserialize.ts
+++ b/src/editor/deserialize.ts
@@ -51,7 +51,7 @@ export function longestBacktickSequence(text: string): number {
 }
 
 function isListChild(n: Node): boolean {
-    return LIST_TYPES.includes(n.parentNode?.nodeName);
+    return LIST_TYPES.includes(n.parentNode?.nodeName || "");
 }
 
 function parseAtRoomMentions(text: string, pc: PartCreator, opts: IParseOptions): Part[] {
diff --git a/src/editor/history.ts b/src/editor/history.ts
index dd9bd19b26..2f0260025d 100644
--- a/src/editor/history.ts
+++ b/src/editor/history.ts
@@ -31,7 +31,7 @@ export default class HistoryManager {
     private newlyTypedCharCount = 0;
     private currentIndex = -1;
     private changedSinceLastPush = false;
-    private lastCaret: Caret = null;
+    private lastCaret: Caret | null = null;
     private nonWordBoundarySinceLastPush = false;
     private addedSinceLastPush = false;
     private removedSinceLastPush = false;
@@ -65,7 +65,7 @@ export default class HistoryManager {
             // as long as you've only been adding or removing since the last push
             if (this.addedSinceLastPush !== this.removedSinceLastPush) {
                 // add steps by word boundary, up to MAX_STEP_LENGTH characters
-                const str = diff.added ? diff.added : diff.removed;
+                const str = diff.added ? diff.added : diff.removed!;
                 const isWordBoundary = str === " " || str === "\t" || str === "\n";
                 if (this.nonWordBoundarySinceLastPush && isWordBoundary) {
                     return true;
diff --git a/src/editor/model.ts b/src/editor/model.ts
index 4ec76bf008..f4c59e3665 100644
--- a/src/editor/model.ts
+++ b/src/editor/model.ts
@@ -51,13 +51,13 @@ type ManualTransformCallback = () => Caret;
 export default class EditorModel {
     private _parts: Part[];
     private readonly _partCreator: PartCreator;
-    private activePartIdx: number = null;
-    private _autoComplete: AutocompleteWrapperModel = null;
-    private autoCompletePartIdx: number = null;
+    private activePartIdx: number | null = null;
+    private _autoComplete: AutocompleteWrapperModel | null = null;
+    private autoCompletePartIdx: number | null = null;
     private autoCompletePartCount = 0;
-    private transformCallback: TransformCallback = null;
+    private transformCallback: TransformCallback | null = null;
 
-    public constructor(parts: Part[], partCreator: PartCreator, private updateCallback: UpdateCallback = null) {
+    public constructor(parts: Part[], partCreator: PartCreator, private updateCallback: UpdateCallback | null = null) {
         this._parts = parts;
         this._partCreator = partCreator;
         this.transformCallback = null;
diff --git a/src/editor/parts.ts b/src/editor/parts.ts
index 2c8596b492..8202f765d3 100644
--- a/src/editor/parts.ts
+++ b/src/editor/parts.ts
@@ -455,7 +455,7 @@ class AtRoomPillPart extends RoomPillPart {
 }
 
 class UserPillPart extends PillPart {
-    public constructor(userId, displayName, private member: RoomMember) {
+    public constructor(userId, displayName, private member?: RoomMember) {
         super(userId, displayName);
     }
 
@@ -625,7 +625,7 @@ export class PartCreator {
 
     public userPill(displayName: string, userId: string): UserPillPart {
         const member = this.room.getMember(userId);
-        return new UserPillPart(userId, displayName, member);
+        return new UserPillPart(userId, displayName, member || undefined);
     }
 
     private static isRegionalIndicator(c: string): boolean {
diff --git a/src/editor/position.ts b/src/editor/position.ts
index e2a08752b2..3f23016a8a 100644
--- a/src/editor/position.ts
+++ b/src/editor/position.ts
@@ -79,6 +79,8 @@ export default class DocumentPosition implements IPosition {
                 offset = 0;
             }
         }
+
+        return this; // impossible but Typescript doesn't believe us
     }
 
     public backwardsWhile(model: EditorModel, predicate: Predicate): DocumentPosition {
@@ -104,6 +106,8 @@ export default class DocumentPosition implements IPosition {
                 offset = parts[index].text.length;
             }
         }
+
+        return this; // impossible but Typescript doesn't believe us
     }
 
     public asOffset(model: EditorModel): DocumentOffset {
diff --git a/src/editor/render.ts b/src/editor/render.ts
index 317218cd8a..bb7b088780 100644
--- a/src/editor/render.ts
+++ b/src/editor/render.ts
@@ -18,7 +18,7 @@ limitations under the License.
 import { Part, Type } from "./parts";
 import EditorModel from "./model";
 
-export function needsCaretNodeBefore(part: Part, prevPart: Part): boolean {
+export function needsCaretNodeBefore(part: Part, prevPart?: Part): boolean {
     const isFirst = !prevPart || prevPart.type === Type.Newline;
     return !part.acceptsCaret && (isFirst || !prevPart.acceptsCaret);
 }
@@ -30,9 +30,9 @@ export function needsCaretNodeAfter(part: Part, isLastOfLine: boolean): boolean
 function insertAfter(node: HTMLElement, nodeToInsert: HTMLElement): void {
     const next = node.nextSibling;
     if (next) {
-        node.parentElement.insertBefore(nodeToInsert, next);
+        node.parentElement!.insertBefore(nodeToInsert, next);
     } else {
-        node.parentElement.appendChild(nodeToInsert);
+        node.parentElement!.appendChild(nodeToInsert);
     }
 }
 
@@ -58,11 +58,11 @@ function updateCaretNode(node: HTMLElement): void {
     }
 }
 
-export function isCaretNode(node: HTMLElement): boolean {
-    return node && node.tagName === "SPAN" && node.className === "caretNode";
+export function isCaretNode(node?: Node | null): node is HTMLElement {
+    return !!node && node instanceof HTMLElement && node.tagName === "SPAN" && node.className === "caretNode";
 }
 
-function removeNextSiblings(node: ChildNode): void {
+function removeNextSiblings(node: ChildNode | null): void {
     if (!node) {
         return;
     }
@@ -83,13 +83,13 @@ function removeChildren(parent: HTMLElement): void {
 }
 
 function reconcileLine(lineContainer: ChildNode, parts: Part[]): void {
-    let currentNode;
-    let prevPart;
+    let currentNode: ChildNode | null = null;
+    let prevPart: Part | undefined;
     const lastPart = parts[parts.length - 1];
 
     for (const part of parts) {
         const isFirst = !prevPart;
-        currentNode = isFirst ? lineContainer.firstChild : currentNode.nextSibling;
+        currentNode = isFirst ? lineContainer.firstChild : currentNode!.nextSibling;
 
         if (needsCaretNodeBefore(part, prevPart)) {
             if (isCaretNode(currentNode)) {
@@ -109,18 +109,18 @@ function reconcileLine(lineContainer: ChildNode, parts: Part[]): void {
         if (currentNode && part) {
             part.updateDOMNode(currentNode);
         } else if (part) {
-            currentNode = part.toDOMNode();
+            currentNode = part.toDOMNode() as ChildNode;
             // hooks up nextSibling for next iteration
             lineContainer.appendChild(currentNode);
         }
 
         if (needsCaretNodeAfter(part, part === lastPart)) {
-            if (isCaretNode(currentNode.nextSibling)) {
-                currentNode = currentNode.nextSibling;
-                updateCaretNode(currentNode);
+            if (isCaretNode(currentNode?.nextSibling)) {
+                currentNode = currentNode!.nextSibling;
+                updateCaretNode(currentNode as HTMLElement);
             } else {
                 const caretNode = createCaretNode();
-                insertAfter(currentNode, caretNode);
+                insertAfter(currentNode as HTMLElement, caretNode);
                 currentNode = caretNode;
             }
         }
@@ -150,7 +150,7 @@ function reconcileEmptyLine(lineContainer: HTMLElement): void {
 }
 
 export function renderModel(editor: HTMLDivElement, model: EditorModel): void {
-    const lines = model.parts.reduce(
+    const lines = model.parts.reduce<Part[][]>(
         (linesArr, part) => {
             if (part.type === Type.Newline) {
                 linesArr.push([]);
diff --git a/src/stores/OwnProfileStore.ts b/src/stores/OwnProfileStore.ts
index 1aad5fac9a..1ad90b9ee4 100644
--- a/src/stores/OwnProfileStore.ts
+++ b/src/stores/OwnProfileStore.ts
@@ -50,8 +50,8 @@ export class OwnProfileStore extends AsyncStoreWithClient<IState> {
         // round-trip after the client is ready, and we often load widgets in that time, and we'd
         // and up passing them an incorrect display name
         super(defaultDispatcher, {
-            displayName: window.localStorage.getItem(KEY_DISPLAY_NAME),
-            avatarUrl: window.localStorage.getItem(KEY_AVATAR_URL),
+            displayName: window.localStorage.getItem(KEY_DISPLAY_NAME) || undefined,
+            avatarUrl: window.localStorage.getItem(KEY_AVATAR_URL) || undefined,
         });
     }
 
diff --git a/src/stores/ReadyWatchingStore.ts b/src/stores/ReadyWatchingStore.ts
index a233a83ea3..24117bab37 100644
--- a/src/stores/ReadyWatchingStore.ts
+++ b/src/stores/ReadyWatchingStore.ts
@@ -25,7 +25,7 @@ import { IDestroyable } from "../utils/IDestroyable";
 import { Action } from "../dispatcher/actions";
 
 export abstract class ReadyWatchingStore extends EventEmitter implements IDestroyable {
-    protected matrixClient: MatrixClient;
+    protected matrixClient: MatrixClient | null = null;
     private dispatcherRef: string | null = null;
 
     public constructor(protected readonly dispatcher: Dispatcher<ActionPayload>) {
@@ -42,7 +42,7 @@ export abstract class ReadyWatchingStore extends EventEmitter implements IDestro
         }
     }
 
-    public get mxClient(): MatrixClient {
+    public get mxClient(): MatrixClient | null {
         return this.matrixClient; // for external readonly access
     }
 
diff --git a/src/stores/UIStore.ts b/src/stores/UIStore.ts
index f7a6e76d0b..b1cdbae951 100644
--- a/src/stores/UIStore.ts
+++ b/src/stores/UIStore.ts
@@ -21,7 +21,7 @@ export enum UI_EVENTS {
 }
 
 export default class UIStore extends EventEmitter {
-    private static _instance: UIStore = null;
+    private static _instance: UIStore | null = null;
 
     private resizeObserver: ResizeObserver;
 
@@ -58,7 +58,7 @@ export default class UIStore extends EventEmitter {
         }
     }
 
-    public getElementDimensions(name: string): DOMRectReadOnly {
+    public getElementDimensions(name: string): DOMRectReadOnly | undefined {
         return this.uiElementDimensions.get(name);
     }
 
@@ -68,7 +68,7 @@ export default class UIStore extends EventEmitter {
     }
 
     public stopTrackingElementDimensions(name: string): void {
-        let trackedElement: Element;
+        let trackedElement: Element | undefined;
         this.trackedUiElements.forEach((trackedElementName, element) => {
             if (trackedElementName === name) {
                 trackedElement = element;
diff --git a/src/stores/spaces/flattenSpaceHierarchy.ts b/src/stores/spaces/flattenSpaceHierarchy.ts
index 0fc6e4fd31..758681a60d 100644
--- a/src/stores/spaces/flattenSpaceHierarchy.ts
+++ b/src/stores/spaces/flattenSpaceHierarchy.ts
@@ -66,7 +66,7 @@ export const flattenSpaceHierarchyWithCache =
         useCache = true,
     ): Set<string> => {
         if (useCache && cache.has(spaceId)) {
-            return cache.get(spaceId);
+            return cache.get(spaceId)!;
         }
         const result = flattenSpaceHierarchy(spaceEntityMap, spaceDescendantMap, spaceId);
         cache.set(spaceId, result);
diff --git a/src/toasts/UpdateToast.tsx b/src/toasts/UpdateToast.tsx
index c14c57ea79..2783e16943 100644
--- a/src/toasts/UpdateToast.tsx
+++ b/src/toasts/UpdateToast.tsx
@@ -37,12 +37,12 @@ function checkVersion(ver: string): boolean {
 }
 
 function installUpdate(): void {
-    PlatformPeg.get().installUpdate();
+    PlatformPeg.get()?.installUpdate();
 }
 
 export const showToast = (version: string, newVersion: string, releaseNotes?: string): void => {
     function onReject(): void {
-        PlatformPeg.get().deferUpdate(newVersion);
+        PlatformPeg.get()?.deferUpdate(newVersion);
     }
 
     let onAccept;
@@ -55,7 +55,7 @@ export const showToast = (version: string, newVersion: string, releaseNotes?: st
                 button: _t("Update"),
                 onFinished: (update) => {
                     if (update && PlatformPeg.get()) {
-                        PlatformPeg.get().installUpdate();
+                        PlatformPeg.get()!.installUpdate();
                     }
                 },
             });
@@ -67,7 +67,7 @@ export const showToast = (version: string, newVersion: string, releaseNotes?: st
                 newVersion,
                 onFinished: (update) => {
                     if (update && PlatformPeg.get()) {
-                        PlatformPeg.get().installUpdate();
+                        PlatformPeg.get()!.installUpdate();
                     }
                 },
             });
diff --git a/src/utils/DialogOpener.ts b/src/utils/DialogOpener.ts
index ac38eb5018..0121fb7c18 100644
--- a/src/utils/DialogOpener.ts
+++ b/src/utils/DialogOpener.ts
@@ -62,7 +62,7 @@ export class DialogOpener {
                         roomId: payload.room_id || SdkContextClass.instance.roomViewStore.getRoomId(),
                         initialTabId: payload.initial_tab_id,
                     },
-                    /*className=*/ null,
+                    /*className=*/ undefined,
                     /*isPriority=*/ false,
                     /*isStatic=*/ true,
                 );
@@ -90,7 +90,7 @@ export class DialogOpener {
                         initialTabId: payload.initalTabId,
                         space: payload.space,
                     },
-                    null,
+                    undefined,
                     false,
                     true,
                 );
@@ -102,7 +102,7 @@ export class DialogOpener {
                         matrixClient: payload.space.client,
                         space: payload.space,
                     },
-                    /*className=*/ null,
+                    /*className=*/ undefined,
                     /*isPriority=*/ false,
                     /*isStatic=*/ true,
                 );
diff --git a/src/utils/RoomUpgrade.ts b/src/utils/RoomUpgrade.ts
index d534c12e2f..669a8e8bfe 100644
--- a/src/utils/RoomUpgrade.ts
+++ b/src/utils/RoomUpgrade.ts
@@ -62,9 +62,9 @@ export async function upgradeRoom(
     progressCallback?: (progress: IProgress) => void,
 ): Promise<string> {
     const cli = room.client;
-    let spinnerModal: IHandle<any>;
+    let spinnerModal: IHandle<any> | undefined;
     if (!progressCallback) {
-        spinnerModal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+        spinnerModal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
     }
 
     let toInvite: string[] = [];
@@ -78,7 +78,9 @@ export async function upgradeRoom(
     if (updateSpaces) {
         parentsToRelink = Array.from(SpaceStore.instance.getKnownParents(room.roomId))
             .map((roomId) => cli.getRoom(roomId))
-            .filter((parent) => parent?.currentState.maySendStateEvent(EventType.SpaceChild, cli.getUserId()));
+            .filter((parent) =>
+                parent?.currentState.maySendStateEvent(EventType.SpaceChild, cli.getUserId()!),
+            ) as Room[];
     }
 
     const progress: IProgress = {
@@ -117,7 +119,7 @@ export async function upgradeRoom(
     if (toInvite.length > 0) {
         // Errors are handled internally to this function
         await inviteUsersToRoom(newRoomId, toInvite, false, () => {
-            progress.inviteUsersProgress++;
+            progress.inviteUsersProgress!++;
             progressCallback?.(progress);
         });
     }
@@ -137,7 +139,7 @@ export async function upgradeRoom(
                 );
                 await cli.sendStateEvent(parent.roomId, EventType.SpaceChild, {}, room.roomId);
 
-                progress.updateSpacesProgress++;
+                progress.updateSpacesProgress!++;
                 progressCallback?.(progress);
             }
         } catch (e) {
diff --git a/src/utils/ShieldUtils.ts b/src/utils/ShieldUtils.ts
index 6bf57801be..089cf3feeb 100644
--- a/src/utils/ShieldUtils.ts
+++ b/src/utils/ShieldUtils.ts
@@ -51,7 +51,7 @@ export async function shieldStatusForRoom(client: MatrixClient, room: Room): Pro
             !inDMMap && // Don't alarm for self in DMs with other users
             members.length !== 2) || // Don't alarm for self in 1:1 chats with other users
         members.length === 1; // Do alarm for self if we're alone in a room
-    const targets = includeUser ? [...verified, client.getUserId()] : verified;
+    const targets = includeUser ? [...verified, client.getUserId()!] : verified;
     for (const userId of targets) {
         const devices = client.getStoredDevicesForUser(userId);
         const anyDeviceNotVerified = devices.some(({ deviceId }) => {
diff --git a/src/utils/StorageManager.ts b/src/utils/StorageManager.ts
index 5bb8861311..6cab834855 100644
--- a/src/utils/StorageManager.ts
+++ b/src/utils/StorageManager.ts
@@ -181,7 +181,7 @@ export function setCryptoInitialised(cryptoInited: boolean): void {
 /* Simple wrapper functions around IndexedDB.
  */
 
-let idb: IDBDatabase = null;
+let idb: IDBDatabase | null = null;
 
 async function idbInit(): Promise<void> {
     if (!indexedDB) {
@@ -206,7 +206,7 @@ export async function idbLoad(table: string, key: string | string[]): Promise<an
         await idbInit();
     }
     return new Promise((resolve, reject) => {
-        const txn = idb.transaction([table], "readonly");
+        const txn = idb!.transaction([table], "readonly");
         txn.onerror = reject;
 
         const objectStore = txn.objectStore(table);
@@ -223,7 +223,7 @@ export async function idbSave(table: string, key: string | string[], data: any):
         await idbInit();
     }
     return new Promise((resolve, reject) => {
-        const txn = idb.transaction([table], "readwrite");
+        const txn = idb!.transaction([table], "readwrite");
         txn.onerror = reject;
 
         const objectStore = txn.objectStore(table);
@@ -240,7 +240,7 @@ export async function idbDelete(table: string, key: string | string[]): Promise<
         await idbInit();
     }
     return new Promise((resolve, reject) => {
-        const txn = idb.transaction([table], "readwrite");
+        const txn = idb!.transaction([table], "readwrite");
         txn.onerror = reject;
 
         const objectStore = txn.objectStore(table);
diff --git a/src/utils/Timer.ts b/src/utils/Timer.ts
index 1038aed4f9..975a498e96 100644
--- a/src/utils/Timer.ts
+++ b/src/utils/Timer.ts
@@ -26,8 +26,8 @@ Once a timer is finished or aborted, it can't be started again
 a new one through `clone()` or `cloneIfRun()`.
 */
 export default class Timer {
-    private timerHandle: number;
-    private startTs: number;
+    private timerHandle?: number;
+    private startTs?: number;
     private promise: Promise<void>;
     private resolve: () => void;
     private reject: (Error) => void;
@@ -37,19 +37,19 @@ export default class Timer {
     }
 
     private setNotStarted(): void {
-        this.timerHandle = null;
-        this.startTs = null;
+        this.timerHandle = undefined;
+        this.startTs = undefined;
         this.promise = new Promise<void>((resolve, reject) => {
             this.resolve = resolve;
             this.reject = reject;
         }).finally(() => {
-            this.timerHandle = null;
+            this.timerHandle = undefined;
         });
     }
 
     private onTimeout = (): void => {
         const now = Date.now();
-        const elapsed = now - this.startTs;
+        const elapsed = now - this.startTs!;
         if (elapsed >= this.timeout) {
             this.resolve();
             this.setNotStarted();
@@ -124,6 +124,6 @@ export default class Timer {
     }
 
     public isRunning(): boolean {
-        return this.timerHandle !== null;
+        return this.timerHandle !== undefined;
     }
 }
diff --git a/src/utils/UrlUtils.ts b/src/utils/UrlUtils.ts
index bc185b2056..9c4b81ad3e 100644
--- a/src/utils/UrlUtils.ts
+++ b/src/utils/UrlUtils.ts
@@ -31,7 +31,7 @@ export function abbreviateUrl(u: string): string {
 
     if (parsedUrl.path === "/") {
         // we ignore query / hash parts: these aren't relevant for IS server URLs
-        return parsedUrl.host;
+        return parsedUrl.host || "";
     }
 
     return u;
diff --git a/src/utils/arrays.ts b/src/utils/arrays.ts
index 09cc1de3b9..d5436c601c 100644
--- a/src/utils/arrays.ts
+++ b/src/utils/arrays.ts
@@ -266,7 +266,7 @@ export class ArrayUtil<T> {
         const obj = this.a.reduce((rv: Map<K, T[]>, val: T) => {
             const k = fn(val);
             if (!rv.has(k)) rv.set(k, []);
-            rv.get(k).push(val);
+            rv.get(k)!.push(val);
             return rv;
         }, new Map<K, T[]>());
         return new GroupedArray(obj);
@@ -299,7 +299,7 @@ export class GroupedArray<K, T> {
         const a: T[] = [];
         for (const k of keyOrder) {
             if (!this.val.has(k)) continue;
-            a.push(...this.val.get(k));
+            a.push(...this.val.get(k)!);
         }
         return new ArrayUtil(a);
     }
diff --git a/src/utils/leave-behaviour.ts b/src/utils/leave-behaviour.ts
index deaa9f92ae..11047c5fb7 100644
--- a/src/utils/leave-behaviour.ts
+++ b/src/utils/leave-behaviour.ts
@@ -39,7 +39,7 @@ import { SdkContextClass } from "../contexts/SDKContext";
 export async function leaveRoomBehaviour(roomId: string, retry = true, spinner = true): Promise<void> {
     let spinnerModal: IHandle<any>;
     if (spinner) {
-        spinnerModal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+        spinnerModal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
     }
 
     const cli = MatrixClientPeg.get();
diff --git a/src/utils/permalinks/Permalinks.ts b/src/utils/permalinks/Permalinks.ts
index e5ed070c03..eb72a87362 100644
--- a/src/utils/permalinks/Permalinks.ts
+++ b/src/utils/permalinks/Permalinks.ts
@@ -79,14 +79,13 @@ const ANY_REGEX = /.*/;
 // the list and magically have the link work.
 
 export class RoomPermalinkCreator {
-    private room: Room;
     private roomId: string;
-    private highestPlUserId: string;
-    private populationMap: { [serverName: string]: number };
-    private bannedHostsRegexps: RegExp[];
-    private allowedHostsRegexps: RegExp[];
-    private _serverCandidates: string[];
-    private started: boolean;
+    private highestPlUserId: string | null = null;
+    private populationMap: { [serverName: string]: number } | null = null;
+    private bannedHostsRegexps: RegExp[] | null = null;
+    private allowedHostsRegexps: RegExp[] | null = null;
+    private _serverCandidates: string[] | null = null;
+    private started = false;
 
     // We support being given a roomId as a fallback in the event the `room` object
     // doesn't exist or is not healthy for us to rely on. For example, loading a
@@ -94,15 +93,8 @@ export class RoomPermalinkCreator {
     // Some of the tests done by this class are relatively expensive, so normally
     // throttled to not happen on every update. Pass false as the shouldThrottle
     // param to disable this behaviour, eg. for tests.
-    public constructor(room: Room, roomId: string | null = null, shouldThrottle = true) {
-        this.room = room;
-        this.roomId = room ? room.roomId : roomId;
-        this.highestPlUserId = null;
-        this.populationMap = null;
-        this.bannedHostsRegexps = null;
-        this.allowedHostsRegexps = null;
-        this._serverCandidates = null;
-        this.started = false;
+    public constructor(private room: Room, roomId: string | null = null, shouldThrottle = true) {
+        this.roomId = room ? room.roomId : roomId!;
 
         if (!this.roomId) {
             throw new Error("Failed to resolve a roomId for the permalink creator to use");
@@ -316,7 +308,7 @@ export function isPermalinkHost(host: string): boolean {
  * @param {string} entity The entity to transform.
  * @returns {string|null} The transformed permalink or null if unable.
  */
-export function tryTransformEntityToPermalink(entity: string): string {
+export function tryTransformEntityToPermalink(entity: string): string | null {
     if (!entity) return null;
 
     // Check to see if it is a bare entity for starters
@@ -391,7 +383,7 @@ export function tryTransformPermalinkToLocalHref(permalink: string): string {
     return permalink;
 }
 
-export function getPrimaryPermalinkEntity(permalink: string): string {
+export function getPrimaryPermalinkEntity(permalink: string): string | null {
     try {
         let permalinkParts = parsePermalink(permalink);
 
@@ -425,7 +417,7 @@ function getPermalinkConstructor(): PermalinkConstructor {
     return new MatrixToPermalinkConstructor();
 }
 
-export function parsePermalink(fullUrl: string): PermalinkParts {
+export function parsePermalink(fullUrl: string): PermalinkParts | null {
     try {
         const elementPrefix = SdkConfig.get("permalink_prefix");
         if (decodeURIComponent(fullUrl).startsWith(matrixtoBaseUrl)) {
diff --git a/src/utils/space.tsx b/src/utils/space.tsx
index fe062a080c..fc183a3a7d 100644
--- a/src/utils/space.tsx
+++ b/src/utils/space.tsx
@@ -41,7 +41,7 @@ import { OpenAddExistingToSpaceDialogPayload } from "../dispatcher/payloads/Open
 import { SdkContextClass } from "../contexts/SDKContext";
 
 export const shouldShowSpaceSettings = (space: Room): boolean => {
-    const userId = space.client.getUserId();
+    const userId = space.client.getUserId()!;
     return (
         space.getMyMembership() === "join" &&
         (space.currentState.maySendStateEvent(EventType.RoomAvatar, userId) ||
@@ -88,7 +88,7 @@ export const showCreateNewRoom = async (space: Room, type?: RoomType): Promise<b
 };
 
 export const shouldShowSpaceInvite = (space: Room): boolean =>
-    ((space?.getMyMembership() === "join" && space.canInvite(space.client.getUserId())) ||
+    ((space?.getMyMembership() === "join" && space.canInvite(space.client.getUserId()!)) ||
         space.getJoinRule() === JoinRule.Public) &&
     shouldShowComponent(UIComponent.InviteUsers);
 
@@ -149,7 +149,7 @@ export const bulkSpaceBehaviour = async (
     children: Room[],
     fn: (room: Room) => Promise<unknown>,
 ): Promise<void> => {
-    const modal = Modal.createDialog(Spinner, null, "mx_Dialog_spinner");
+    const modal = Modal.createDialog(Spinner, undefined, "mx_Dialog_spinner");
     try {
         for (const room of children) {
             await fn(room);
diff --git a/src/utils/tooltipify.tsx b/src/utils/tooltipify.tsx
index 3cf6ec50a0..e3280f7fe2 100644
--- a/src/utils/tooltipify.tsx
+++ b/src/utils/tooltipify.tsx
@@ -47,9 +47,9 @@ export function tooltipifyLinks(rootNodes: ArrayLike<Element>, ignoredNodes: Ele
         if (
             node.tagName === "A" &&
             node.getAttribute("href") &&
-            node.getAttribute("href") !== node.textContent.trim()
+            node.getAttribute("href") !== node.textContent?.trim()
         ) {
-            let href = node.getAttribute("href");
+            let href = node.getAttribute("href")!;
             try {
                 href = new URL(href, window.location.href).toString();
             } catch (e) {