diff --git a/package.json b/package.json
index 7956c4b47f..bb703c33f0 100644
--- a/package.json
+++ b/package.json
@@ -123,6 +123,7 @@
     "@sinonjs/fake-timers": "^7.0.2",
     "@types/classnames": "^2.2.11",
     "@types/counterpart": "^0.18.1",
+    "@types/diff-match-patch": "^1.0.5",
     "@types/flux": "^3.1.9",
     "@types/jest": "^26.0.20",
     "@types/linkifyjs": "^2.1.3",
diff --git a/src/@types/diff-dom.ts b/src/@types/diff-dom.ts
new file mode 100644
index 0000000000..884ee6126d
--- /dev/null
+++ b/src/@types/diff-dom.ts
@@ -0,0 +1,50 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+declare module "diff-dom" {
+    enum Action {
+        AddElement = "addElement",
+        AddTextElement = "addTextElement",
+        RemoveTextElement = "removeTextElement",
+        RemoveElement = "removeElement",
+        ReplaceElement = "replaceElement",
+        ModifyTextElement = "modifyTextElement",
+        AddAttribute = "addAttribute",
+        RemoveAttribute = "removeAttribute",
+        ModifyAttribute = "modifyAttribute",
+    }
+
+    export interface IDiff {
+        action: Action;
+        name: string;
+        text?: string;
+        route: number[];
+        value: string;
+        element: unknown;
+        oldValue: string;
+        newValue: string;
+    }
+
+    interface IOpts {
+    }
+
+    export class DiffDOM {
+        public constructor(opts?: IOpts);
+        public apply(tree: unknown, diffs: IDiff[]): unknown;
+        public undo(tree: unknown, diffs: IDiff[]): unknown;
+        public diff(a: HTMLElement | string, b: HTMLElement | string): IDiff[];
+    }
+}
diff --git a/src/DecryptionFailureTracker.js b/src/DecryptionFailureTracker.ts
similarity index 79%
rename from src/DecryptionFailureTracker.js
rename to src/DecryptionFailureTracker.ts
index b02a5e937b..07c0c546fe 100644
--- a/src/DecryptionFailureTracker.js
+++ b/src/DecryptionFailureTracker.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2018 New Vector Ltd
+Copyright 2018 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,34 +14,40 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { MatrixError } from "matrix-js-sdk/src/http-api";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+
 export class DecryptionFailure {
-    constructor(failedEventId, errorCode) {
-        this.failedEventId = failedEventId;
-        this.errorCode = errorCode;
+    public readonly ts: number;
+
+    constructor(public readonly failedEventId: string, public readonly errorCode: string) {
         this.ts = Date.now();
     }
 }
 
+type TrackingFn = (count: number, trackedErrCode: string) => void;
+type ErrCodeMapFn = (errcode: string) => string;
+
 export class DecryptionFailureTracker {
     // Array of items of type DecryptionFailure. Every `CHECK_INTERVAL_MS`, this list
     // is checked for failures that happened > `GRACE_PERIOD_MS` ago. Those that did
     // are accumulated in `failureCounts`.
-    failures = [];
+    public failures: DecryptionFailure[] = [];
 
     // A histogram of the number of failures that will be tracked at the next tracking
     // interval, split by failure error code.
-    failureCounts = {
+    public failureCounts: Record<string, number> = {
         // [errorCode]: 42
     };
 
     // Event IDs of failures that were tracked previously
-    trackedEventHashMap = {
+    public trackedEventHashMap: Record<string, boolean> = {
         // [eventId]: true
     };
 
     // Set to an interval ID when `start` is called
-    checkInterval = null;
-    trackInterval = null;
+    public checkInterval: NodeJS.Timeout = null;
+    public trackInterval: NodeJS.Timeout = null;
 
     // Spread the load on `Analytics` by tracking at a low frequency, `TRACK_INTERVAL_MS`.
     static TRACK_INTERVAL_MS = 60000;
@@ -67,7 +73,7 @@ export class DecryptionFailureTracker {
      * @param {function?} errorCodeMapFn The function used to map error codes to the
      * trackedErrorCode. If not provided, the `.code` of errors will be used.
      */
-    constructor(fn, errorCodeMapFn) {
+    constructor(private readonly fn: TrackingFn, private readonly errorCodeMapFn?: ErrCodeMapFn) {
         if (!fn || typeof fn !== 'function') {
             throw new Error('DecryptionFailureTracker requires tracking function');
         }
@@ -75,9 +81,6 @@ export class DecryptionFailureTracker {
         if (errorCodeMapFn && typeof errorCodeMapFn !== 'function') {
             throw new Error('DecryptionFailureTracker second constructor argument should be a function');
         }
-
-        this._trackDecryptionFailure = fn;
-        this._mapErrorCode = errorCodeMapFn;
     }
 
     // loadTrackedEventHashMap() {
@@ -88,7 +91,7 @@ export class DecryptionFailureTracker {
     //     localStorage.setItem('mx-decryption-failure-event-id-hashes', JSON.stringify(this.trackedEventHashMap));
     // }
 
-    eventDecrypted(e, err) {
+    public eventDecrypted(e: MatrixEvent, err: MatrixError | Error): void {
         if (err) {
             this.addDecryptionFailure(new DecryptionFailure(e.getId(), err.code));
         } else {
@@ -97,18 +100,18 @@ export class DecryptionFailureTracker {
         }
     }
 
-    addDecryptionFailure(failure) {
+    public addDecryptionFailure(failure: DecryptionFailure): void {
         this.failures.push(failure);
     }
 
-    removeDecryptionFailuresForEvent(e) {
+    public removeDecryptionFailuresForEvent(e: MatrixEvent): void {
         this.failures = this.failures.filter((f) => f.failedEventId !== e.getId());
     }
 
     /**
      * Start checking for and tracking failures.
      */
-    start() {
+    public start(): void {
         this.checkInterval = setInterval(
             () => this.checkFailures(Date.now()),
             DecryptionFailureTracker.CHECK_INTERVAL_MS,
@@ -123,7 +126,7 @@ export class DecryptionFailureTracker {
     /**
      * Clear state and stop checking for and tracking failures.
      */
-    stop() {
+    public stop(): void {
         clearInterval(this.checkInterval);
         clearInterval(this.trackInterval);
 
@@ -132,11 +135,11 @@ export class DecryptionFailureTracker {
     }
 
     /**
-     * Mark failures that occured before nowTs - GRACE_PERIOD_MS as failures that should be
+     * Mark failures that occurred before nowTs - GRACE_PERIOD_MS as failures that should be
      * tracked. Only mark one failure per event ID.
      * @param {number} nowTs the timestamp that represents the time now.
      */
-    checkFailures(nowTs) {
+    public checkFailures(nowTs: number): void {
         const failuresGivenGrace = [];
         const failuresNotReady = [];
         while (this.failures.length > 0) {
@@ -175,10 +178,10 @@ export class DecryptionFailureTracker {
 
         const dedupedFailures = dedupedFailuresMap.values();
 
-        this._aggregateFailures(dedupedFailures);
+        this.aggregateFailures(dedupedFailures);
     }
 
-    _aggregateFailures(failures) {
+    private aggregateFailures(failures: DecryptionFailure[]): void {
         for (const failure of failures) {
             const errorCode = failure.errorCode;
             this.failureCounts[errorCode] = (this.failureCounts[errorCode] || 0) + 1;
@@ -189,12 +192,12 @@ export class DecryptionFailureTracker {
      * If there are failures that should be tracked, call the given trackDecryptionFailure
      * function with the number of failures that should be tracked.
      */
-    trackFailures() {
+    public trackFailures(): void {
         for (const errorCode of Object.keys(this.failureCounts)) {
             if (this.failureCounts[errorCode] > 0) {
-                const trackedErrorCode = this._mapErrorCode ? this._mapErrorCode(errorCode) : errorCode;
+                const trackedErrorCode = this.errorCodeMapFn ? this.errorCodeMapFn(errorCode) : errorCode;
 
-                this._trackDecryptionFailure(this.failureCounts[errorCode], trackedErrorCode);
+                this.fn(this.failureCounts[errorCode], trackedErrorCode);
                 this.failureCounts[errorCode] = 0;
             }
         }
diff --git a/src/HtmlUtils.tsx b/src/HtmlUtils.tsx
index ef5ac383e3..983538d65b 100644
--- a/src/HtmlUtils.tsx
+++ b/src/HtmlUtils.tsx
@@ -17,11 +17,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React from 'react';
+import React, { ReactNode } from 'react';
 import sanitizeHtml from 'sanitize-html';
-import { IExtendedSanitizeOptions } from './@types/sanitize-html';
+import cheerio from 'cheerio';
 import * as linkify from 'linkifyjs';
-import linkifyMatrix from './linkify-matrix';
 import _linkifyElement from 'linkifyjs/element';
 import _linkifyString from 'linkifyjs/string';
 import classNames from 'classnames';
@@ -29,13 +28,15 @@ import EMOJIBASE_REGEX from 'emojibase-regex';
 import url from 'url';
 import katex from 'katex';
 import { AllHtmlEntities } from 'html-entities';
-import SettingsStore from './settings/SettingsStore';
-import cheerio from 'cheerio';
+import { IContent } from 'matrix-js-sdk/src/models/event';
 
-import {tryTransformPermalinkToLocalHref} from "./utils/permalinks/Permalinks";
-import {SHORTCODE_TO_EMOJI, getEmojiFromUnicode} from "./emoji";
+import { IExtendedSanitizeOptions } from './@types/sanitize-html';
+import linkifyMatrix from './linkify-matrix';
+import SettingsStore from './settings/SettingsStore';
+import { tryTransformPermalinkToLocalHref } from "./utils/permalinks/Permalinks";
+import { SHORTCODE_TO_EMOJI, getEmojiFromUnicode } from "./emoji";
 import ReplyThread from "./components/views/elements/ReplyThread";
-import {mediaFromMxc} from "./customisations/Media";
+import { mediaFromMxc } from "./customisations/Media";
 
 linkifyMatrix(linkify);
 
@@ -66,7 +67,7 @@ export const PERMITTED_URL_SCHEMES = ['http', 'https', 'ftp', 'mailto', 'magnet'
  * need emojification.
  * unicodeToImage uses this function.
  */
-function mightContainEmoji(str: string) {
+function mightContainEmoji(str: string): boolean {
     return SURROGATE_PAIR_PATTERN.test(str) || SYMBOL_PATTERN.test(str);
 }
 
@@ -76,7 +77,7 @@ function mightContainEmoji(str: string) {
  * @param {String} char The emoji character
  * @return {String} The shortcode (such as :thumbup:)
  */
-export function unicodeToShortcode(char: string) {
+export function unicodeToShortcode(char: string): string {
     const data = getEmojiFromUnicode(char);
     return (data && data.shortcodes ? `:${data.shortcodes[0]}:` : '');
 }
@@ -87,7 +88,7 @@ export function unicodeToShortcode(char: string) {
  * @param {String} shortcode The shortcode (such as :thumbup:)
  * @return {String} The emoji character; null if none exists
  */
-export function shortcodeToUnicode(shortcode: string) {
+export function shortcodeToUnicode(shortcode: string): string {
     shortcode = shortcode.slice(1, shortcode.length - 1);
     const data = SHORTCODE_TO_EMOJI.get(shortcode);
     return data ? data.unicode : null;
@@ -124,13 +125,13 @@ export function processHtmlForSending(html: string): string {
  * Given an untrusted HTML string, return a React node with an sanitized version
  * of that HTML.
  */
-export function sanitizedHtmlNode(insaneHtml: string) {
+export function sanitizedHtmlNode(insaneHtml: string): ReactNode {
     const saneHtml = sanitizeHtml(insaneHtml, sanitizeHtmlParams);
 
     return <div dangerouslySetInnerHTML={{ __html: saneHtml }} dir="auto" />;
 }
 
-export function getHtmlText(insaneHtml: string) {
+export function getHtmlText(insaneHtml: string): string {
     return sanitizeHtml(insaneHtml, {
         allowedTags: [],
         allowedAttributes: {},
@@ -148,7 +149,7 @@ export function getHtmlText(insaneHtml: string) {
  * other places we need to sanitise URLs.
  * @return true if permitted, otherwise false
  */
-export function isUrlPermitted(inputUrl: string) {
+export function isUrlPermitted(inputUrl: string): boolean {
     try {
         const parsed = url.parse(inputUrl);
         if (!parsed.protocol) return false;
@@ -351,13 +352,6 @@ class HtmlHighlighter extends BaseHighlighter<string> {
     }
 }
 
-interface IContent {
-    format?: string;
-    // eslint-disable-next-line camelcase
-    formatted_body?: string;
-    body: string;
-}
-
 interface IOpts {
     highlightLink?: string;
     disableBigEmoji?: boolean;
@@ -367,6 +361,14 @@ interface IOpts {
     ref?: React.Ref<any>;
 }
 
+export interface IOptsReturnNode extends IOpts {
+    returnString: false;
+}
+
+export interface IOptsReturnString extends IOpts {
+    returnString: true;
+}
+
 /* turn a matrix event body into html
  *
  * content: 'content' of the MatrixEvent
@@ -380,6 +382,8 @@ interface IOpts {
  * opts.forComposerQuote: optional param to lessen the url rewriting done by sanitization, for quoting into composer
  * opts.ref: React ref to attach to any React components returned (not compatible with opts.returnString)
  */
+export function bodyToHtml(content: IContent, highlights: string[], opts: IOptsReturnString): string;
+export function bodyToHtml(content: IContent, highlights: string[], opts: IOptsReturnNode): ReactNode;
 export function bodyToHtml(content: IContent, highlights: string[], opts: IOpts = {}) {
     const isHtmlMessage = content.format === "org.matrix.custom.html" && content.formatted_body;
     let bodyHasEmoji = false;
@@ -501,7 +505,7 @@ export function bodyToHtml(content: IContent, highlights: string[], opts: IOpts
  * @param {object} [options] Options for linkifyString. Default: linkifyMatrix.options
  * @returns {string} Linkified string
  */
-export function linkifyString(str: string, options = linkifyMatrix.options) {
+export function linkifyString(str: string, options = linkifyMatrix.options): string {
     return _linkifyString(str, options);
 }
 
@@ -512,7 +516,7 @@ export function linkifyString(str: string, options = linkifyMatrix.options) {
  * @param {object} [options] Options for linkifyElement. Default: linkifyMatrix.options
  * @returns {object}
  */
-export function linkifyElement(element: HTMLElement, options = linkifyMatrix.options) {
+export function linkifyElement(element: HTMLElement, options = linkifyMatrix.options): HTMLElement {
     return _linkifyElement(element, options);
 }
 
@@ -523,7 +527,7 @@ export function linkifyElement(element: HTMLElement, options = linkifyMatrix.opt
  * @param {object} [options] Options for linkifyString. Default: linkifyMatrix.options
  * @returns {string}
  */
-export function linkifyAndSanitizeHtml(dirtyHtml: string, options = linkifyMatrix.options) {
+export function linkifyAndSanitizeHtml(dirtyHtml: string, options = linkifyMatrix.options): string {
     return sanitizeHtml(linkifyString(dirtyHtml, options), sanitizeHtmlParams);
 }
 
@@ -534,7 +538,7 @@ export function linkifyAndSanitizeHtml(dirtyHtml: string, options = linkifyMatri
  * @param {Node} node
  * @returns {bool}
  */
-export function checkBlockNode(node: Node) {
+export function checkBlockNode(node: Node): boolean {
     switch (node.nodeName) {
         case "H1":
         case "H2":
diff --git a/src/Rooms.js b/src/Rooms.ts
similarity index 89%
rename from src/Rooms.js
rename to src/Rooms.ts
index 955498faaa..19d1c9ee05 100644
--- a/src/Rooms.js
+++ b/src/Rooms.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2015 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,7 +14,9 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {MatrixClientPeg} from './MatrixClientPeg';
+import { Room } from "matrix-js-sdk/src/models/room";
+
+import { MatrixClientPeg } from './MatrixClientPeg';
 
 /**
  * Given a room object, return the alias we should use for it,
@@ -25,11 +27,11 @@ import {MatrixClientPeg} from './MatrixClientPeg';
  * @param {Object} room The room object
  * @returns {string} A display alias for the given room
  */
-export function getDisplayAliasForRoom(room) {
+export function getDisplayAliasForRoom(room: Room): string {
     return room.getCanonicalAlias() || room.getAltAliases()[0];
 }
 
-export function looksLikeDirectMessageRoom(room, myUserId) {
+export function looksLikeDirectMessageRoom(room: Room, myUserId: string): boolean {
     const myMembership = room.getMyMembership();
     const me = room.getMember(myUserId);
 
@@ -48,7 +50,7 @@ export function looksLikeDirectMessageRoom(room, myUserId) {
     return false;
 }
 
-export function guessAndSetDMRoom(room, isDirect) {
+export function guessAndSetDMRoom(room: Room, isDirect: boolean): Promise<void> {
     let newTarget;
     if (isDirect) {
         const guessedUserId = guessDMRoomTargetId(
@@ -70,7 +72,7 @@ export function guessAndSetDMRoom(room, isDirect) {
                    this room as a DM room
  * @returns {object} A promise
  */
-export function setDMRoom(roomId, userId) {
+export function setDMRoom(roomId: string, userId: string): Promise<void> {
     if (MatrixClientPeg.get().isGuest()) {
         return Promise.resolve();
     }
@@ -114,7 +116,7 @@ export function setDMRoom(roomId, userId) {
  * @param {string} myUserId User ID of the current user
  * @returns {string} User ID of the user that the room is probably a DM with
  */
-function guessDMRoomTargetId(room, myUserId) {
+function guessDMRoomTargetId(room: Room, myUserId: string): string {
     let oldestTs;
     let oldestUser;
 
diff --git a/src/Unread.js b/src/Unread.ts
similarity index 75%
rename from src/Unread.js
rename to src/Unread.ts
index 25c425aa9a..b733f4175a 100644
--- a/src/Unread.js
+++ b/src/Unread.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2015 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,9 +14,13 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {MatrixClientPeg} from "./MatrixClientPeg";
+import { Room } from "matrix-js-sdk/src/models/room";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { EventType, MsgType } from "matrix-js-sdk/src/@types/event";
+
+import { MatrixClientPeg } from "./MatrixClientPeg";
 import shouldHideEvent from './shouldHideEvent';
-import {haveTileForEvent} from "./components/views/rooms/EventTile";
+import { haveTileForEvent } from "./components/views/rooms/EventTile";
 
 /**
  * Returns true iff this event arriving in a room should affect the room's
@@ -25,28 +29,33 @@ import {haveTileForEvent} from "./components/views/rooms/EventTile";
  * @param {Object} ev The event
  * @returns {boolean} True if the given event should affect the unread message count
  */
-export function eventTriggersUnreadCount(ev) {
+export function eventTriggersUnreadCount(ev: MatrixEvent): boolean {
     if (ev.sender && ev.sender.userId == MatrixClientPeg.get().credentials.userId) {
         return false;
-    } else if (ev.getType() == 'm.room.member') {
-        return false;
-    } else if (ev.getType() == 'm.room.third_party_invite') {
-        return false;
-    } else if (ev.getType() == 'm.call.answer' || ev.getType() == 'm.call.hangup') {
-        return false;
-    } else if (ev.getType() == 'm.room.message' && ev.getContent().msgtype == 'm.notify') {
-        return false;
-    } else if (ev.getType() == 'm.room.aliases' || ev.getType() == 'm.room.canonical_alias') {
-        return false;
-    } else if (ev.getType() == 'm.room.server_acl') {
-        return false;
-    } else if (ev.isRedacted()) {
-        return false;
     }
+
+    switch (ev.getType()) {
+        case EventType.RoomMember:
+        case EventType.RoomThirdPartyInvite:
+        case EventType.CallAnswer:
+        case EventType.CallHangup:
+        case EventType.RoomAliases:
+        case EventType.RoomCanonicalAlias:
+        case EventType.RoomServerAcl:
+            return false;
+
+        case EventType.RoomMessage:
+            if (ev.getContent().msgtype === MsgType.Notice) {
+                return false;
+            }
+            break;
+    }
+
+    if (ev.isRedacted()) return false;
     return haveTileForEvent(ev);
 }
 
-export function doesRoomHaveUnreadMessages(room) {
+export function doesRoomHaveUnreadMessages(room: Room): boolean {
     const myUserId = MatrixClientPeg.get().getUserId();
 
     // get the most recent read receipt sent by our account.
diff --git a/src/components/structures/MatrixChat.tsx b/src/components/structures/MatrixChat.tsx
index 0af2d3d635..27d628fecf 100644
--- a/src/components/structures/MatrixChat.tsx
+++ b/src/components/structures/MatrixChat.tsx
@@ -1461,7 +1461,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
         });
 
         const dft = new DecryptionFailureTracker((total, errorCode) => {
-            Analytics.trackEvent('E2E', 'Decryption failure', errorCode, total);
+            Analytics.trackEvent('E2E', 'Decryption failure', errorCode, String(total));
             CountlyAnalytics.instance.track("decryption_failure", { errorCode }, null, { sum: total });
         }, (errorCode) => {
             // Map JS-SDK error codes to tracker codes for aggregation
diff --git a/src/components/views/dialogs/ForwardDialog.tsx b/src/components/views/dialogs/ForwardDialog.tsx
index a83f3f177c..6fbed6fc8b 100644
--- a/src/components/views/dialogs/ForwardDialog.tsx
+++ b/src/components/views/dialogs/ForwardDialog.tsx
@@ -14,30 +14,31 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React, {useMemo, useState, useEffect} from "react";
+import React, { useMemo, useState, useEffect } from "react";
 import classnames from "classnames";
-import {MatrixEvent} from "matrix-js-sdk/src/models/event";
-import {Room} from "matrix-js-sdk/src/models/room";
-import {MatrixClient} from "matrix-js-sdk/src/client";
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+import { Room } from "matrix-js-sdk/src/models/room";
+import { MatrixClient } from "matrix-js-sdk/src/client";
+import { RoomMember } from "matrix-js-sdk/src/models/room-member";
 
-import {_t} from "../../../languageHandler";
+import { _t } from "../../../languageHandler";
 import dis from "../../../dispatcher/dispatcher";
-import {useSettingValue, useFeatureEnabled} from "../../../hooks/useSettings";
-import {UIFeature} from "../../../settings/UIFeature";
-import {Layout} from "../../../settings/Layout";
-import {IDialogProps} from "./IDialogProps";
+import { useSettingValue, useFeatureEnabled } from "../../../hooks/useSettings";
+import { UIFeature } from "../../../settings/UIFeature";
+import { Layout } from "../../../settings/Layout";
+import { IDialogProps } from "./IDialogProps";
 import BaseDialog from "./BaseDialog";
-import {avatarUrlForUser} from "../../../Avatar";
+import { avatarUrlForUser } from "../../../Avatar";
 import EventTile from "../rooms/EventTile";
 import SearchBox from "../../structures/SearchBox";
 import DecoratedRoomAvatar from "../avatars/DecoratedRoomAvatar";
-import {Alignment} from '../elements/Tooltip';
+import { Alignment } from '../elements/Tooltip';
 import AccessibleTooltipButton from "../elements/AccessibleTooltipButton";
 import AutoHideScrollbar from "../../structures/AutoHideScrollbar";
-import {StaticNotificationState} from "../../../stores/notifications/StaticNotificationState";
+import { StaticNotificationState } from "../../../stores/notifications/StaticNotificationState";
 import NotificationBadge from "../rooms/NotificationBadge";
-import {RoomPermalinkCreator} from "../../../utils/permalinks/Permalinks";
-import {sortRooms} from "../../../stores/room-list/algorithms/tag-sorting/RecentAlgorithm";
+import { RoomPermalinkCreator } from "../../../utils/permalinks/Permalinks";
+import { sortRooms } from "../../../stores/room-list/algorithms/tag-sorting/RecentAlgorithm";
 import QueryMatcher from "../../../autocomplete/QueryMatcher";
 
 const AVATAR_SIZE = 30;
@@ -171,7 +172,7 @@ const ForwardDialog: React.FC<IProps> = ({ matrixClient: cli, event, permalinkCr
             );
         },
         getMxcAvatarUrl: () => profileInfo.avatar_url,
-    };
+    } as RoomMember;
 
     const [query, setQuery] = useState("");
     const lcQuery = query.toLowerCase();
diff --git a/src/components/views/elements/EventTilePreview.tsx b/src/components/views/elements/EventTilePreview.tsx
index d39557c9bb..332f3ac333 100644
--- a/src/components/views/elements/EventTilePreview.tsx
+++ b/src/components/views/elements/EventTilePreview.tsx
@@ -17,13 +17,14 @@ limitations under the License.
 import React from 'react';
 import classnames from 'classnames';
 import { MatrixEvent } from 'matrix-js-sdk/src/models/event';
+import { RoomMember } from 'matrix-js-sdk/src/models/room-member';
 
 import * as Avatar from '../../../Avatar';
 import EventTile from '../rooms/EventTile';
 import SettingsStore from "../../../settings/SettingsStore";
-import {Layout} from "../../../settings/Layout";
-import {UIFeature} from "../../../settings/UIFeature";
-import {replaceableComponent} from "../../../utils/replaceableComponent";
+import { Layout } from "../../../settings/Layout";
+import { UIFeature } from "../../../settings/UIFeature";
+import { replaceableComponent } from "../../../utils/replaceableComponent";
 
 interface IProps {
     /**
@@ -111,7 +112,7 @@ export default class EventTilePreview extends React.Component<IProps, IState> {
                 );
             },
             getMxcAvatarUrl: () => this.props.avatarUrl,
-        };
+        } as RoomMember;
 
         return event;
     }
diff --git a/src/components/views/rooms/AuxPanel.tsx b/src/components/views/rooms/AuxPanel.tsx
index f6cc0f4d45..04c7a57048 100644
--- a/src/components/views/rooms/AuxPanel.tsx
+++ b/src/components/views/rooms/AuxPanel.tsx
@@ -25,7 +25,7 @@ import RateLimitedFunc from '../../../ratelimitedfunc';
 import SettingsStore from "../../../settings/SettingsStore";
 import AutoHideScrollbar from "../../structures/AutoHideScrollbar";
 import { UIFeature } from "../../../settings/UIFeature";
-import { ResizeNotifier } from "../../../utils/ResizeNotifier";
+import ResizeNotifier from "../../../utils/ResizeNotifier";
 import CallViewForRoom from '../voip/CallViewForRoom';
 import { objectHasDiff } from "../../../utils/objects";
 import { replaceableComponent } from "../../../utils/replaceableComponent";
diff --git a/src/components/views/rooms/EventTile.tsx b/src/components/views/rooms/EventTile.tsx
index 0099bf73fb..3d674efe04 100644
--- a/src/components/views/rooms/EventTile.tsx
+++ b/src/components/views/rooms/EventTile.tsx
@@ -17,7 +17,6 @@ limitations under the License.
 
 import React from 'react';
 import classNames from "classnames";
-
 import { EventType } from "matrix-js-sdk/src/@types/event";
 import { EventStatus, MatrixEvent } from "matrix-js-sdk/src/models/event";
 import { Relations } from "matrix-js-sdk/src/models/relations";
@@ -29,24 +28,24 @@ import { hasText } from "../../../TextForEvent";
 import * as sdk from "../../../index";
 import dis from '../../../dispatcher/dispatcher';
 import SettingsStore from "../../../settings/SettingsStore";
-import {Layout} from "../../../settings/Layout";
-import {formatTime} from "../../../DateUtils";
-import {MatrixClientPeg} from '../../../MatrixClientPeg';
-import {ALL_RULE_TYPES} from "../../../mjolnir/BanList";
+import { Layout } from "../../../settings/Layout";
+import { formatTime } from "../../../DateUtils";
+import { MatrixClientPeg } from '../../../MatrixClientPeg';
+import { ALL_RULE_TYPES } from "../../../mjolnir/BanList";
 import MatrixClientContext from "../../../contexts/MatrixClientContext";
-import {E2E_STATE} from "./E2EIcon";
-import {toRem} from "../../../utils/units";
-import {WidgetType} from "../../../widgets/WidgetType";
+import { E2E_STATE } from "./E2EIcon";
+import { toRem } from "../../../utils/units";
+import { WidgetType } from "../../../widgets/WidgetType";
 import RoomAvatar from "../avatars/RoomAvatar";
-import {WIDGET_LAYOUT_EVENT_TYPE} from "../../../stores/widgets/WidgetLayoutStore";
-import {objectHasDiff} from "../../../utils/objects";
-import {replaceableComponent} from "../../../utils/replaceableComponent";
+import { WIDGET_LAYOUT_EVENT_TYPE } from "../../../stores/widgets/WidgetLayoutStore";
+import { objectHasDiff } from "../../../utils/objects";
+import { replaceableComponent } from "../../../utils/replaceableComponent";
 import Tooltip from "../elements/Tooltip";
-import { EditorStateTransfer } from "../../../utils/EditorStateTransfer";
+import EditorStateTransfer from "../../../utils/EditorStateTransfer";
 import { RoomPermalinkCreator } from '../../../utils/permalinks/Permalinks';
-import {StaticNotificationState} from "../../../stores/notifications/StaticNotificationState";
+import { StaticNotificationState } from "../../../stores/notifications/StaticNotificationState";
 import NotificationBadge from "./NotificationBadge";
-import {ComposerInsertPayload} from "../../../dispatcher/payloads/ComposerInsertPayload";
+import { ComposerInsertPayload } from "../../../dispatcher/payloads/ComposerInsertPayload";
 import { Action } from '../../../dispatcher/actions';
 
 const eventTileTypes = {
diff --git a/src/components/views/rooms/RoomList.tsx b/src/components/views/rooms/RoomList.tsx
index 5a1c3a24b3..d277a69907 100644
--- a/src/components/views/rooms/RoomList.tsx
+++ b/src/components/views/rooms/RoomList.tsx
@@ -22,7 +22,7 @@ import { EventType } from "matrix-js-sdk/src/@types/event";
 
 import { _t, _td } from "../../../languageHandler";
 import { RovingTabIndexProvider } from "../../../accessibility/RovingTabIndex";
-import { ResizeNotifier } from "../../../utils/ResizeNotifier";
+import ResizeNotifier from "../../../utils/ResizeNotifier";
 import RoomListStore, { LISTS_UPDATE_EVENT } from "../../../stores/room-list/RoomListStore";
 import RoomViewStore from "../../../stores/RoomViewStore";
 import { ITagMap } from "../../../stores/room-list/algorithms/models";
diff --git a/src/components/views/rooms/RoomSublist.tsx b/src/components/views/rooms/RoomSublist.tsx
index ba8bbffbcc..61166b4230 100644
--- a/src/components/views/rooms/RoomSublist.tsx
+++ b/src/components/views/rooms/RoomSublist.tsx
@@ -45,7 +45,7 @@ import { ActionPayload } from "../../../dispatcher/payloads";
 import { Enable, Resizable } from "re-resizable";
 import { Direction } from "re-resizable/lib/resizer";
 import { polyfillTouchEvent } from "../../../@types/polyfill";
-import { ResizeNotifier } from "../../../utils/ResizeNotifier";
+import ResizeNotifier from "../../../utils/ResizeNotifier";
 import { RoomNotificationStateStore } from "../../../stores/notifications/RoomNotificationStateStore";
 import RoomListLayoutStore from "../../../stores/room-list/RoomListLayoutStore";
 import { arrayFastClone, arrayHasOrderChange } from "../../../utils/arrays";
diff --git a/src/languageHandler.tsx b/src/languageHandler.tsx
index 16950dc008..cc129fb6b9 100644
--- a/src/languageHandler.tsx
+++ b/src/languageHandler.tsx
@@ -112,7 +112,7 @@ export interface IVariables {
     [key: string]: SubstitutionValue;
 }
 
-type Tags = Record<string, SubstitutionValue>;
+export type Tags = Record<string, SubstitutionValue>;
 
 export type TranslatedString = string | React.ReactNode;
 
diff --git a/src/utils/EditorStateTransfer.js b/src/utils/EditorStateTransfer.ts
similarity index 52%
rename from src/utils/EditorStateTransfer.js
rename to src/utils/EditorStateTransfer.ts
index c7782a9ea8..ba303f9b73 100644
--- a/src/utils/EditorStateTransfer.js
+++ b/src/utils/EditorStateTransfer.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The Matrix.org Foundation C.I.C.
+Copyright 2019 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,36 +14,40 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+
+import { SerializedPart } from "../editor/parts";
+import { Caret } from "../editor/caret";
+
 /**
  * Used while editing, to pass the event, and to preserve editor state
  * from one editor instance to another when remounting the editor
  * upon receiving the remote echo for an unsent event.
  */
 export default class EditorStateTransfer {
-    constructor(event) {
-        this._event = event;
-        this._serializedParts = null;
-        this.caret = null;
+    private serializedParts: SerializedPart[] = null;
+    private caret: Caret = null;
+
+    constructor(private readonly event: MatrixEvent) {}
+
+    public setEditorState(caret: Caret, serializedParts: SerializedPart[]) {
+        this.caret = caret;
+        this.serializedParts = serializedParts;
     }
 
-    setEditorState(caret, serializedParts) {
-        this._caret = caret;
-        this._serializedParts = serializedParts;
+    public hasEditorState() {
+        return !!this.serializedParts;
     }
 
-    hasEditorState() {
-        return !!this._serializedParts;
+    public getSerializedParts() {
+        return this.serializedParts;
     }
 
-    getSerializedParts() {
-        return this._serializedParts;
+    public getCaret() {
+        return this.caret;
     }
 
-    getCaret() {
-        return this._caret;
-    }
-
-    getEvent() {
-        return this._event;
+    public getEvent() {
+        return this.event;
     }
 }
diff --git a/src/utils/ErrorUtils.js b/src/utils/ErrorUtils.tsx
similarity index 84%
rename from src/utils/ErrorUtils.js
rename to src/utils/ErrorUtils.tsx
index b5bd5b0af0..c39ee21f09 100644
--- a/src/utils/ErrorUtils.js
+++ b/src/utils/ErrorUtils.tsx
@@ -1,5 +1,5 @@
 /*
-Copyright 2018 New Vector Ltd
+Copyright 2018 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,7 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { _t, _td } from '../languageHandler';
+import React, { ReactNode } from "react";
+import { MatrixError } from "matrix-js-sdk/src/http-api";
+
+import { _t, _td, Tags, TranslatedString } from '../languageHandler';
 
 /**
  * Produce a translated error message for a
@@ -30,7 +33,12 @@ import { _t, _td } from '../languageHandler';
  *     for any tags in the strings apart from 'a'
  * @returns {*} Translated string or react component
  */
-export function messageForResourceLimitError(limitType, adminContact, strings, extraTranslations) {
+export function messageForResourceLimitError(
+    limitType: string,
+    adminContact: string,
+    strings: Record<string, string>,
+    extraTranslations?: Tags,
+): TranslatedString {
     let errString = strings[limitType];
     if (errString === undefined) errString = strings[''];
 
@@ -49,7 +57,7 @@ export function messageForResourceLimitError(limitType, adminContact, strings, e
     }
 }
 
-export function messageForSyncError(err) {
+export function messageForSyncError(err: MatrixError | Error): ReactNode {
     if (err.errcode === 'M_RESOURCE_LIMIT_EXCEEDED') {
         const limitError = messageForResourceLimitError(
             err.data.limit_type,
diff --git a/src/utils/EventUtils.js b/src/utils/EventUtils.ts
similarity index 85%
rename from src/utils/EventUtils.js
rename to src/utils/EventUtils.ts
index be21896417..3d9c60d9cd 100644
--- a/src/utils/EventUtils.js
+++ b/src/utils/EventUtils.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 New Vector Ltd
+Copyright 2019 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,9 +14,12 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { EventStatus } from 'matrix-js-sdk/src/models/event';
-import {MatrixClientPeg} from '../MatrixClientPeg';
+import { Room } from 'matrix-js-sdk/src/models/room';
+import { MatrixEvent, EventStatus } from 'matrix-js-sdk/src/models/event';
+
+import { MatrixClientPeg } from '../MatrixClientPeg';
 import shouldHideEvent from "../shouldHideEvent";
+
 /**
  * Returns whether an event should allow actions like reply, reactions, edit, etc.
  * which effectively checks whether it's a regular message that has been sent and that we
@@ -25,7 +28,7 @@ import shouldHideEvent from "../shouldHideEvent";
  * @param {MatrixEvent} mxEvent The event to check
  * @returns {boolean} true if actionable
  */
-export function isContentActionable(mxEvent) {
+export function isContentActionable(mxEvent: MatrixEvent): boolean {
     const { status: eventStatus } = mxEvent;
 
     // status is SENT before remote-echo, null after
@@ -45,7 +48,7 @@ export function isContentActionable(mxEvent) {
     return false;
 }
 
-export function canEditContent(mxEvent) {
+export function canEditContent(mxEvent: MatrixEvent): boolean {
     if (mxEvent.status === EventStatus.CANCELLED || mxEvent.getType() !== "m.room.message" || mxEvent.isRedacted()) {
         return false;
     }
@@ -56,7 +59,7 @@ export function canEditContent(mxEvent) {
         mxEvent.getSender() === MatrixClientPeg.get().getUserId();
 }
 
-export function canEditOwnEvent(mxEvent) {
+export function canEditOwnEvent(mxEvent: MatrixEvent): boolean {
     // for now we only allow editing
     // your own events. So this just call through
     // In the future though, moderators will be able to
@@ -67,7 +70,7 @@ export function canEditOwnEvent(mxEvent) {
 }
 
 const MAX_JUMP_DISTANCE = 100;
-export function findEditableEvent(room, isForward, fromEventId = undefined) {
+export function findEditableEvent(room: Room, isForward: boolean, fromEventId: string = undefined): MatrixEvent {
     const liveTimeline = room.getLiveTimeline();
     const events = liveTimeline.getEvents().concat(room.getPendingEvents());
     const maxIdx = events.length - 1;
diff --git a/src/utils/IdentityServerUtils.js b/src/utils/IdentityServerUtils.ts
similarity index 82%
rename from src/utils/IdentityServerUtils.js
rename to src/utils/IdentityServerUtils.ts
index 5ece308954..2476adca19 100644
--- a/src/utils/IdentityServerUtils.js
+++ b/src/utils/IdentityServerUtils.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The Matrix.org Foundation C.I.C.
+Copyright 2019 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -15,14 +15,15 @@ limitations under the License.
 */
 
 import { SERVICE_TYPES } from 'matrix-js-sdk/src/service-types';
+
 import SdkConfig from '../SdkConfig';
 import {MatrixClientPeg} from '../MatrixClientPeg';
 
-export function getDefaultIdentityServerUrl() {
+export function getDefaultIdentityServerUrl(): string {
     return SdkConfig.get()['validated_server_config']['isUrl'];
 }
 
-export function useDefaultIdentityServer() {
+export function useDefaultIdentityServer(): void {
     const url = getDefaultIdentityServerUrl();
     // Account data change will update localstorage, client, etc through dispatcher
     MatrixClientPeg.get().setAccountData("m.identity_server", {
@@ -30,7 +31,7 @@ export function useDefaultIdentityServer() {
     });
 }
 
-export async function doesIdentityServerHaveTerms(fullUrl) {
+export async function doesIdentityServerHaveTerms(fullUrl: string): Promise<boolean> {
     let terms;
     try {
         terms = await MatrixClientPeg.get().getTerms(SERVICE_TYPES.IS, fullUrl);
@@ -46,7 +47,7 @@ export async function doesIdentityServerHaveTerms(fullUrl) {
     return terms && terms["policies"] && (Object.keys(terms["policies"]).length > 0);
 }
 
-export function doesAccountDataHaveIdentityServer() {
+export function doesAccountDataHaveIdentityServer(): boolean {
     const event = MatrixClientPeg.get().getAccountData("m.identity_server");
     return event && event.getContent() && event.getContent()['base_url'];
 }
diff --git a/src/utils/MessageDiffUtils.js b/src/utils/MessageDiffUtils.tsx
similarity index 84%
rename from src/utils/MessageDiffUtils.js
rename to src/utils/MessageDiffUtils.tsx
index 7398173fdd..b5d5e31432 100644
--- a/src/utils/MessageDiffUtils.js
+++ b/src/utils/MessageDiffUtils.tsx
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The Matrix.org Foundation C.I.C.
+Copyright 2019 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,31 +14,33 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React from 'react';
+import React, { ReactNode } from 'react';
 import classNames from 'classnames';
-import DiffMatchPatch from 'diff-match-patch';
-import {DiffDOM} from "diff-dom";
-import { checkBlockNode, bodyToHtml } from "../HtmlUtils";
+import { diff_match_patch as DiffMatchPatch } from 'diff-match-patch';
+import { Action, DiffDOM, IDiff } from "diff-dom";
+import { IContent } from "matrix-js-sdk/src/models/event";
+
+import { bodyToHtml, checkBlockNode, IOptsReturnString } from "../HtmlUtils";
 
 const decodeEntities = (function() {
     let textarea = null;
-    return function(string) {
+    return function(str: string): string {
         if (!textarea) {
             textarea = document.createElement("textarea");
         }
-        textarea.innerHTML = string;
+        textarea.innerHTML = str;
         return textarea.value;
     };
 })();
 
-function textToHtml(text) {
+function textToHtml(text: string): string {
     const container = document.createElement("div");
     container.textContent = text;
     return container.innerHTML;
 }
 
-function getSanitizedHtmlBody(content) {
-    const opts = {
+function getSanitizedHtmlBody(content: IContent): string {
+    const opts: IOptsReturnString = {
         stripReplyFallback: true,
         returnString: true,
     };
@@ -57,21 +59,21 @@ function getSanitizedHtmlBody(content) {
     }
 }
 
-function wrapInsertion(child) {
+function wrapInsertion(child: Node): HTMLElement {
     const wrapper = document.createElement(checkBlockNode(child) ? "div" : "span");
     wrapper.className = "mx_EditHistoryMessage_insertion";
     wrapper.appendChild(child);
     return wrapper;
 }
 
-function wrapDeletion(child) {
+function wrapDeletion(child: Node): HTMLElement {
     const wrapper = document.createElement(checkBlockNode(child) ? "div" : "span");
     wrapper.className = "mx_EditHistoryMessage_deletion";
     wrapper.appendChild(child);
     return wrapper;
 }
 
-function findRefNodes(root, route, isAddition) {
+function findRefNodes(root: Node, route: number[], isAddition = false) {
     let refNode = root;
     let refParentNode;
     const end = isAddition ? route.length - 1 : route.length;
@@ -79,7 +81,7 @@ function findRefNodes(root, route, isAddition) {
         refParentNode = refNode;
         refNode = refNode.childNodes[route[i]];
     }
-    return {refNode, refParentNode};
+    return { refNode, refParentNode };
 }
 
 function diffTreeToDOM(desc) {
@@ -101,7 +103,7 @@ function diffTreeToDOM(desc) {
     }
 }
 
-function insertBefore(parent, nextSibling, child) {
+function insertBefore(parent: Node, nextSibling: Node | null, child: Node): void {
     if (nextSibling) {
         parent.insertBefore(child, nextSibling);
     } else {
@@ -109,7 +111,7 @@ function insertBefore(parent, nextSibling, child) {
     }
 }
 
-function isRouteOfNextSibling(route1, route2) {
+function isRouteOfNextSibling(route1: number[], route2: number[]): boolean {
     // routes are arrays with indices,
     // to be interpreted as a path in the dom tree
 
@@ -127,7 +129,7 @@ function isRouteOfNextSibling(route1, route2) {
     return route2[lastD1Idx] >= route1[lastD1Idx];
 }
 
-function adjustRoutes(diff, remainingDiffs) {
+function adjustRoutes(diff: IDiff, remainingDiffs: IDiff[]): void {
     if (diff.action === "removeTextElement" || diff.action === "removeElement") {
         // as removed text is not removed from the html, but marked as deleted,
         // we need to readjust indices that assume the current node has been removed.
@@ -140,14 +142,14 @@ function adjustRoutes(diff, remainingDiffs) {
     }
 }
 
-function stringAsTextNode(string) {
+function stringAsTextNode(string: string): Text {
     return document.createTextNode(decodeEntities(string));
 }
 
-function renderDifferenceInDOM(originalRootNode, diff, diffMathPatch) {
+function renderDifferenceInDOM(originalRootNode: Node, diff: IDiff, diffMathPatch: DiffMatchPatch): void {
     const {refNode, refParentNode} = findRefNodes(originalRootNode, diff.route);
     switch (diff.action) {
-        case "replaceElement": {
+        case Action.ReplaceElement: {
             const container = document.createElement("span");
             const delNode = wrapDeletion(diffTreeToDOM(diff.oldValue));
             const insNode = wrapInsertion(diffTreeToDOM(diff.newValue));
@@ -156,22 +158,22 @@ function renderDifferenceInDOM(originalRootNode, diff, diffMathPatch) {
             refNode.parentNode.replaceChild(container, refNode);
             break;
         }
-        case "removeTextElement": {
+        case Action.RemoveTextElement: {
             const delNode = wrapDeletion(stringAsTextNode(diff.value));
             refNode.parentNode.replaceChild(delNode, refNode);
             break;
         }
-        case "removeElement": {
+        case Action.RemoveElement: {
             const delNode = wrapDeletion(diffTreeToDOM(diff.element));
             refNode.parentNode.replaceChild(delNode, refNode);
             break;
         }
-        case "modifyTextElement": {
+        case Action.ModifyTextElement: {
             const textDiffs = diffMathPatch.diff_main(diff.oldValue, diff.newValue);
             diffMathPatch.diff_cleanupSemantic(textDiffs);
             const container = document.createElement("span");
             for (const [modifier, text] of textDiffs) {
-                let textDiffNode = stringAsTextNode(text);
+                let textDiffNode: Node = stringAsTextNode(text);
                 if (modifier < 0) {
                     textDiffNode = wrapDeletion(textDiffNode);
                 } else if (modifier > 0) {
@@ -182,12 +184,12 @@ function renderDifferenceInDOM(originalRootNode, diff, diffMathPatch) {
             refNode.parentNode.replaceChild(container, refNode);
             break;
         }
-        case "addElement": {
+        case Action.AddElement: {
             const insNode = wrapInsertion(diffTreeToDOM(diff.element));
             insertBefore(refParentNode, refNode, insNode);
             break;
         }
-        case "addTextElement": {
+        case Action.AddTextElement: {
             // XXX: sometimes diffDOM says insert a newline when there shouldn't be one
             // but we must insert the node anyway so that we don't break the route child IDs.
             // See https://github.com/fiduswriter/diffDOM/issues/100
@@ -197,11 +199,11 @@ function renderDifferenceInDOM(originalRootNode, diff, diffMathPatch) {
         }
         // e.g. when changing a the href of a link,
         // show the link with old href as removed and with the new href as added
-        case "removeAttribute":
-        case "addAttribute":
-        case "modifyAttribute": {
+        case Action.RemoveAttribute:
+        case Action.AddAttribute:
+        case Action.ModifyAttribute: {
             const delNode = wrapDeletion(refNode.cloneNode(true));
-            const updatedNode = refNode.cloneNode(true);
+            const updatedNode = refNode.cloneNode(true) as HTMLElement;
             if (diff.action === "addAttribute" || diff.action === "modifyAttribute") {
                 updatedNode.setAttribute(diff.name, diff.newValue);
             } else {
@@ -220,12 +222,12 @@ function renderDifferenceInDOM(originalRootNode, diff, diffMathPatch) {
     }
 }
 
-function routeIsEqual(r1, r2) {
+function routeIsEqual(r1: number[], r2: number[]): boolean {
     return r1.length === r2.length && !r1.some((e, i) => e !== r2[i]);
 }
 
 // workaround for https://github.com/fiduswriter/diffDOM/issues/90
-function filterCancelingOutDiffs(originalDiffActions) {
+function filterCancelingOutDiffs(originalDiffActions: IDiff[]): IDiff[] {
     const diffActions = originalDiffActions.slice();
 
     for (let i = 0; i < diffActions.length; ++i) {
@@ -252,7 +254,7 @@ function filterCancelingOutDiffs(originalDiffActions) {
  * @param {object} editContent the content for the edit message
  * @return {object} a react element similar to what `bodyToHtml` returns
  */
-export function editBodyDiffToHtml(originalContent, editContent) {
+export function editBodyDiffToHtml(originalContent: IContent, editContent: IContent): ReactNode {
     // wrap the body in a div, DiffDOM needs a root element
     const originalBody = `<div>${getSanitizedHtmlBody(originalContent)}</div>`;
     const editBody = `<div>${getSanitizedHtmlBody(editContent)}</div>`;
diff --git a/src/utils/PinningUtils.js b/src/utils/PinningUtils.ts
similarity index 89%
rename from src/utils/PinningUtils.js
rename to src/utils/PinningUtils.ts
index 90d26cc988..ec1eeccf1f 100644
--- a/src/utils/PinningUtils.js
+++ b/src/utils/PinningUtils.ts
@@ -14,13 +14,15 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+
 export default class PinningUtils {
     /**
      * Determines if the given event may be pinned.
      * @param {MatrixEvent} event The event to check.
      * @return {boolean} True if the event may be pinned, false otherwise.
      */
-    static isPinnable(event) {
+    static isPinnable(event: MatrixEvent): boolean {
         if (!event) return false;
         if (event.getType() !== "m.room.message") return false;
         if (event.isRedacted()) return false;
diff --git a/src/utils/Receipt.js b/src/utils/Receipt.ts
similarity index 83%
rename from src/utils/Receipt.js
rename to src/utils/Receipt.ts
index d88c67fb18..2a626decc4 100644
--- a/src/utils/Receipt.js
+++ b/src/utils/Receipt.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2016 OpenMarket Ltd
+Copyright 2016 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,6 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { MatrixEvent } from "matrix-js-sdk/src/models/event";
+
 /**
  * Given MatrixEvent containing receipts, return the first
  * read receipt from the given user ID, or null if no such
@@ -23,7 +25,7 @@ limitations under the License.
  * @param {string} userId A user ID
  * @returns {Object} Read receipt
  */
-export function findReadReceiptFromUserId(receiptEvent, userId) {
+export function findReadReceiptFromUserId(receiptEvent: MatrixEvent, userId: string): object | null {
     const receiptKeys = Object.keys(receiptEvent.getContent());
     for (let i = 0; i < receiptKeys.length; ++i) {
         const rcpt = receiptEvent.getContent()[receiptKeys[i]];
diff --git a/src/utils/ResizeNotifier.js b/src/utils/ResizeNotifier.ts
similarity index 62%
rename from src/utils/ResizeNotifier.js
rename to src/utils/ResizeNotifier.ts
index 4d46d10f6c..8bb7f52e57 100644
--- a/src/utils/ResizeNotifier.js
+++ b/src/utils/ResizeNotifier.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 New Vector Ltd
+Copyright 2019 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -22,59 +22,58 @@ limitations under the License.
  * Fires when the middle panel has been resized by a pixel.
  * @event module:utils~ResizeNotifier#"middlePanelResizedNoisy"
  */
+
 import { EventEmitter } from "events";
 import { throttle } from "lodash";
 
 export default class ResizeNotifier extends EventEmitter {
-    constructor() {
-        super();
-        // with default options, will call fn once at first call, and then every x ms
-        // if there was another call in that timespan
-        this._throttledMiddlePanel = throttle(() => this.emit("middlePanelResized"), 200);
-        this._isResizing = false;
-    }
+    private _isResizing = false;
 
-    get isResizing() {
+    // with default options, will call fn once at first call, and then every x ms
+    // if there was another call in that timespan
+    private throttledMiddlePanel = throttle(() => this.emit("middlePanelResized"), 200);
+
+    public get isResizing() {
         return this._isResizing;
     }
 
-    startResizing() {
+    public startResizing() {
         this._isResizing = true;
         this.emit("isResizing", true);
     }
 
-    stopResizing() {
+    public stopResizing() {
         this._isResizing = false;
         this.emit("isResizing", false);
     }
 
-    _noisyMiddlePanel() {
+    private noisyMiddlePanel() {
         this.emit("middlePanelResizedNoisy");
     }
 
-    _updateMiddlePanel() {
-        this._throttledMiddlePanel();
-        this._noisyMiddlePanel();
+    private updateMiddlePanel() {
+        this.throttledMiddlePanel();
+        this.noisyMiddlePanel();
     }
 
     // can be called in quick succession
-    notifyLeftHandleResized() {
+    public notifyLeftHandleResized() {
         // don't emit event for own region
-        this._updateMiddlePanel();
+        this.updateMiddlePanel();
     }
 
     // can be called in quick succession
-    notifyRightHandleResized() {
-        this._updateMiddlePanel();
+    public notifyRightHandleResized() {
+        this.updateMiddlePanel();
     }
 
-    notifyTimelineHeightChanged() {
-        this._updateMiddlePanel();
+    public notifyTimelineHeightChanged() {
+        this.updateMiddlePanel();
     }
 
     // can be called in quick succession
-    notifyWindowResized() {
-        this._updateMiddlePanel();
+    public notifyWindowResized() {
+        this.updateMiddlePanel();
     }
 }
 
diff --git a/src/utils/createMatrixClient.js b/src/utils/createMatrixClient.ts
similarity index 76%
rename from src/utils/createMatrixClient.js
rename to src/utils/createMatrixClient.ts
index f5e196d846..caaf75616d 100644
--- a/src/utils/createMatrixClient.js
+++ b/src/utils/createMatrixClient.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2017 Vector Creations Ltd
+Copyright 2017 - 2021 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -14,10 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {createClient} from "matrix-js-sdk/src/matrix";
-import {IndexedDBCryptoStore} from "matrix-js-sdk/src/crypto/store/indexeddb-crypto-store";
-import {WebStorageSessionStore} from "matrix-js-sdk/src/store/session/webstorage";
-import {IndexedDBStore} from "matrix-js-sdk/src/store/indexeddb";
+import { createClient, ICreateClientOpts } from "matrix-js-sdk/src/matrix";
+import { IndexedDBCryptoStore } from "matrix-js-sdk/src/crypto/store/indexeddb-crypto-store";
+import { WebStorageSessionStore } from "matrix-js-sdk/src/store/session/webstorage";
+import { IndexedDBStore } from "matrix-js-sdk/src/store/indexeddb";
 
 const localStorage = window.localStorage;
 
@@ -41,8 +41,8 @@ try {
  *
  * @returns {MatrixClient} the newly-created MatrixClient
  */
-export default function createMatrixClient(opts) {
-    const storeOpts = {
+export default function createMatrixClient(opts: ICreateClientOpts) {
+    const storeOpts: Partial<ICreateClientOpts> = {
         useAuthorizationHeader: true,
     };
 
@@ -65,9 +65,10 @@ export default function createMatrixClient(opts) {
         );
     }
 
-    opts = Object.assign(storeOpts, opts);
-
-    return createClient(opts);
+    return createClient({
+        ...storeOpts,
+        ...opts,
+    });
 }
 
 createMatrixClient.indexedDbWorkerScript = null;
diff --git a/yarn.lock b/yarn.lock
index 0d424cb93d..32ca30a996 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1486,6 +1486,11 @@
   resolved "https://registry.yarnpkg.com/@types/counterpart/-/counterpart-0.18.1.tgz#b1b784d9e54d9879f0a8cb12f2caedab65430fe8"
   integrity sha512-PRuFlBBkvdDOtxlIASzTmkEFar+S66Ek48NVVTWMUjtJAdn5vyMSN8y6IZIoIymGpR36q2nZbIYazBWyFxL+IQ==
 
+"@types/diff-match-patch@^1.0.32":
+  version "1.0.32"
+  resolved "https://registry.yarnpkg.com/@types/diff-match-patch/-/diff-match-patch-1.0.32.tgz#d9c3b8c914aa8229485351db4865328337a3d09f"
+  integrity sha512-bPYT5ECFiblzsVzyURaNhljBH2Gh1t9LowgUwciMrNAhFewLkHT2H0Mto07Y4/3KCOGZHRQll3CTtQZ0X11D/A==
+
 "@types/events@^3.0.0":
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/@types/events/-/events-3.0.0.tgz#2862f3f58a9a7f7c3e78d79f130dd4d71c25c2a7"