diff --git a/package.json b/package.json
index 41ba3f47c1..3fd7703afb 100644
--- a/package.json
+++ b/package.json
@@ -120,6 +120,7 @@
     "@babel/register": "^7.7.4",
     "@peculiar/webcrypto": "^1.0.22",
     "@types/classnames": "^2.2.10",
+    "@types/counterpart": "^0.18.1",
     "@types/flux": "^3.1.9",
     "@types/lodash": "^4.14.152",
     "@types/modernizr": "^3.5.3",
diff --git a/src/@types/global.d.ts b/src/@types/global.d.ts
index ffd3277892..2c2fec759c 100644
--- a/src/@types/global.d.ts
+++ b/src/@types/global.d.ts
@@ -20,6 +20,7 @@ import { IMatrixClientPeg } from "../MatrixClientPeg";
 import ToastStore from "../stores/ToastStore";
 import DeviceListener from "../DeviceListener";
 import { RoomListStore2 } from "../stores/room-list/RoomListStore2";
+import { PlatformPeg } from "../PlatformPeg";
 
 declare global {
     interface Window {
@@ -33,6 +34,7 @@ declare global {
         mx_ToastStore: ToastStore;
         mx_DeviceListener: DeviceListener;
         mx_RoomListStore2: RoomListStore2;
+        mxPlatformPeg: PlatformPeg;
     }
 
     // workaround for https://github.com/microsoft/TypeScript/issues/30933
@@ -45,6 +47,10 @@ declare global {
         hasStorageAccess?: () => Promise<boolean>;
     }
 
+    interface Navigator {
+        userLanguage?: string;
+    }
+
     interface StorageEstimate {
         usageDetails?: {[key: string]: number};
     }
diff --git a/src/BasePlatform.ts b/src/BasePlatform.ts
index 1d11495e61..acf72a986c 100644
--- a/src/BasePlatform.ts
+++ b/src/BasePlatform.ts
@@ -53,6 +53,10 @@ export default abstract class BasePlatform {
         this.startUpdateCheck = this.startUpdateCheck.bind(this);
     }
 
+    abstract async getConfig(): Promise<{}>;
+
+    abstract getDefaultDeviceDisplayName(): string;
+
     protected onAction = (payload: ActionPayload) => {
         switch (payload.action) {
             case 'on_client_not_viable':
diff --git a/src/PlatformPeg.js b/src/PlatformPeg.ts
similarity index 80%
rename from src/PlatformPeg.js
rename to src/PlatformPeg.ts
index 34131fde7d..1d2b813ebc 100644
--- a/src/PlatformPeg.js
+++ b/src/PlatformPeg.ts
@@ -1,5 +1,6 @@
 /*
 Copyright 2016 OpenMarket Ltd
+Copyright 2020 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 +15,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import BasePlatform from "./BasePlatform";
+
 /*
  * Holds the current Platform object used by the code to do anything
  * specific to the platform we're running on (eg. web, electron)
@@ -21,10 +24,8 @@ limitations under the License.
  * This allows the app layer to set a Platform without necessarily
  * having to have a MatrixChat object
  */
-class PlatformPeg {
-    constructor() {
-        this.platform = null;
-    }
+export class PlatformPeg {
+    platform: BasePlatform = null;
 
     /**
      * Returns the current Platform object for the application.
@@ -39,12 +40,12 @@ class PlatformPeg {
      * application.
      * This should be an instance of a class extending BasePlatform.
      */
-    set(plaf) {
+    set(plaf: BasePlatform) {
         this.platform = plaf;
     }
 }
 
-if (!global.mxPlatformPeg) {
-    global.mxPlatformPeg = new PlatformPeg();
+if (!window.mxPlatformPeg) {
+    window.mxPlatformPeg = new PlatformPeg();
 }
-export default global.mxPlatformPeg;
+export default window.mxPlatformPeg;
diff --git a/src/components/views/elements/EditableItemList.js b/src/components/views/elements/EditableItemList.js
index 50d5a3d10f..34e53906a2 100644
--- a/src/components/views/elements/EditableItemList.js
+++ b/src/components/views/elements/EditableItemList.js
@@ -16,7 +16,7 @@ limitations under the License.
 
 import React from 'react';
 import PropTypes from 'prop-types';
-import {_t} from '../../../languageHandler.js';
+import {_t} from '../../../languageHandler';
 import Field from "./Field";
 import AccessibleButton from "./AccessibleButton";
 
diff --git a/src/components/views/toasts/GenericToast.tsx b/src/components/views/toasts/GenericToast.tsx
index 9f8885ba47..6cd881b9eb 100644
--- a/src/components/views/toasts/GenericToast.tsx
+++ b/src/components/views/toasts/GenericToast.tsx
@@ -14,13 +14,13 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import React, {ReactChild} from "react";
+import React, {ReactNode} from "react";
 
 import FormButton from "../elements/FormButton";
 import {XOR} from "../../../@types/common";
 
 export interface IProps {
-    description: ReactChild;
+    description: ReactNode;
     acceptLabel: string;
 
     onAccept();
diff --git a/src/contexts/MatrixClientContext.js b/src/contexts/MatrixClientContext.ts
similarity index 85%
rename from src/contexts/MatrixClientContext.js
rename to src/contexts/MatrixClientContext.ts
index 54a23ca132..7e8a92064d 100644
--- a/src/contexts/MatrixClientContext.js
+++ b/src/contexts/MatrixClientContext.ts
@@ -15,7 +15,8 @@ limitations under the License.
 */
 
 import { createContext } from "react";
+import { MatrixClient } from "matrix-js-sdk/src/client";
 
-const MatrixClientContext = createContext(undefined);
+const MatrixClientContext = createContext<MatrixClient>(undefined);
 MatrixClientContext.displayName = "MatrixClientContext";
 export default MatrixClientContext;
diff --git a/src/createRoom.js b/src/createRoom.ts
similarity index 81%
rename from src/createRoom.js
rename to src/createRoom.ts
index affdf196a7..c436196c27 100644
--- a/src/createRoom.js
+++ b/src/createRoom.ts
@@ -15,6 +15,9 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import {MatrixClient} from "matrix-js-sdk/src/client";
+import {Room} from "matrix-js-sdk/src/models/room";
+
 import {MatrixClientPeg} from './MatrixClientPeg';
 import Modal from './Modal';
 import * as sdk from './index';
@@ -26,6 +29,56 @@ import {getAddressType} from "./UserAddress";
 
 const E2EE_WK_KEY = "im.vector.riot.e2ee";
 
+// TODO move these interfaces over to js-sdk once it has been typescripted enough to accept them
+enum Visibility {
+    Public = "public",
+    Private = "private",
+}
+
+enum Preset {
+    PrivateChat = "private_chat",
+    TrustedPrivateChat = "trusted_private_chat",
+    PublicChat = "public_chat",
+}
+
+interface Invite3PID {
+    id_server: string;
+    id_access_token?: string; // this gets injected by the js-sdk
+    medium: string;
+    address: string;
+}
+
+interface IStateEvent {
+    type: string;
+    state_key?: string; // defaults to an empty string
+    content: object;
+}
+
+interface ICreateOpts {
+    visibility?: Visibility;
+    room_alias_name?: string;
+    name?: string;
+    topic?: string;
+    invite?: string[];
+    invite_3pid?: Invite3PID[];
+    room_version?: string;
+    creation_content?: object;
+    initial_state?: IStateEvent[];
+    preset?: Preset;
+    is_direct?: boolean;
+    power_level_content_override?: object;
+}
+
+interface IOpts {
+    dmUserId?: string;
+    createOpts?: ICreateOpts;
+    spinner?: boolean;
+    guestAccess?: boolean;
+    encryption?: boolean;
+    inlineErrors?: boolean;
+    andView?: boolean;
+}
+
 /**
  * Create a new room, and switch to it.
  *
@@ -40,11 +93,12 @@ const E2EE_WK_KEY = "im.vector.riot.e2ee";
  *     Default: False
  * @param {bool=} opts.inlineErrors True to raise errors off the promise instead of resolving to null.
  *     Default: False
+ * @param {bool=} opts.andView True to dispatch an action to view the room once it has been created.
  *
  * @returns {Promise} which resolves to the room id, or null if the
  * action was aborted or failed.
  */
-export default function createRoom(opts) {
+export default function createRoom(opts: IOpts): Promise<string | null> {
     opts = opts || {};
     if (opts.spinner === undefined) opts.spinner = true;
     if (opts.guestAccess === undefined) opts.guestAccess = true;
@@ -59,12 +113,12 @@ export default function createRoom(opts) {
         return Promise.resolve(null);
     }
 
-    const defaultPreset = opts.dmUserId ? 'trusted_private_chat' : 'private_chat';
+    const defaultPreset = opts.dmUserId ? Preset.TrustedPrivateChat : Preset.PrivateChat;
 
     // set some defaults for the creation
     const createOpts = opts.createOpts || {};
     createOpts.preset = createOpts.preset || defaultPreset;
-    createOpts.visibility = createOpts.visibility || 'private';
+    createOpts.visibility = createOpts.visibility || Visibility.Private;
     if (opts.dmUserId && createOpts.invite === undefined) {
         switch (getAddressType(opts.dmUserId)) {
             case 'mx-user-id':
@@ -166,7 +220,7 @@ export default function createRoom(opts) {
     });
 }
 
-export function findDMForUser(client, userId) {
+export function findDMForUser(client: MatrixClient, userId: string): Room {
     const roomIds = DMRoomMap.shared().getDMRoomsForUserId(userId);
     const rooms = roomIds.map(id => client.getRoom(id));
     const suitableDMRooms = rooms.filter(r => {
@@ -189,7 +243,7 @@ export function findDMForUser(client, userId) {
  * NOTE: this assumes you've just created the room and there's not been an opportunity
  * for other code to run, so we shouldn't miss RoomState.newMember when it comes by.
  */
-export async function _waitForMember(client, roomId, userId, opts = { timeout: 1500 }) {
+export async function _waitForMember(client: MatrixClient, roomId: string, userId: string, opts = { timeout: 1500 }) {
     const { timeout } = opts;
     let handler;
     return new Promise((resolve) => {
@@ -212,7 +266,7 @@ export async function _waitForMember(client, roomId, userId, opts = { timeout: 1
  * Ensure that for every user in a room, there is at least one device that we
  * can encrypt to.
  */
-export async function canEncryptToAllUsers(client, userIds) {
+export async function canEncryptToAllUsers(client: MatrixClient, userIds: string[]) {
     const usersDeviceMap = await client.downloadKeys(userIds);
     // { "@user:host": { "DEVICE": {...}, ... }, ... }
     return Object.values(usersDeviceMap).every((userDevices) =>
@@ -221,7 +275,7 @@ export async function canEncryptToAllUsers(client, userIds) {
     );
 }
 
-export async function ensureDMExists(client, userId) {
+export async function ensureDMExists(client: MatrixClient, userId: string): Promise<string> {
     const existingDMRoom = findDMForUser(client, userId);
     let roomId;
     if (existingDMRoom) {
diff --git a/src/groups.js b/src/groups.js
index 860cf71fff..e73af15c79 100644
--- a/src/groups.js
+++ b/src/groups.js
@@ -15,7 +15,8 @@ limitations under the License.
 */
 
 import PropTypes from 'prop-types';
-import { _t } from './languageHandler.js';
+
+import { _t } from './languageHandler';
 
 export const GroupMemberType = PropTypes.shape({
     userId: PropTypes.string.isRequired,
diff --git a/src/languageHandler.js b/src/languageHandler.tsx
similarity index 87%
rename from src/languageHandler.js
rename to src/languageHandler.tsx
index 79a172015a..91d90d4e6c 100644
--- a/src/languageHandler.js
+++ b/src/languageHandler.tsx
@@ -1,7 +1,7 @@
 /*
 Copyright 2017 MTRNord and Cooperative EITA
 Copyright 2017 Vector Creations Ltd.
-Copyright 2019 The Matrix.org Foundation C.I.C.
+Copyright 2019, 2020 The Matrix.org Foundation C.I.C.
 Copyright 2019 Michael Telatynski <7t3chguy@gmail.com>
 
 Licensed under the Apache License, Version 2.0 (the "License");
@@ -20,10 +20,11 @@ limitations under the License.
 import request from 'browser-request';
 import counterpart from 'counterpart';
 import React from 'react';
+
 import SettingsStore, {SettingLevel} from "./settings/SettingsStore";
 import PlatformPeg from "./PlatformPeg";
 
-// $webapp is a webpack resolve alias pointing to the output directory, see webpack config
+// @ts-ignore - $webapp is a webpack resolve alias pointing to the output directory, see webpack config
 import webpackLangJsonUrl from "$webapp/i18n/languages.json";
 
 const i18nFolder = 'i18n/';
@@ -37,27 +38,31 @@ counterpart.setSeparator('|');
 // Fall back to English
 counterpart.setFallbackLocale('en');
 
+interface ITranslatableError extends Error {
+    translatedMessage: string;
+}
+
 /**
  * Helper function to create an error which has an English message
  * with a translatedMessage property for use by the consumer.
  * @param {string} message Message to translate.
  * @returns {Error} The constructed error.
  */
-export function newTranslatableError(message) {
-    const error = new Error(message);
+export function newTranslatableError(message: string) {
+    const error = new Error(message) as ITranslatableError;
     error.translatedMessage = _t(message);
     return error;
 }
 
 // Function which only purpose is to mark that a string is translatable
 // Does not actually do anything. It's helpful for automatic extraction of translatable strings
-export function _td(s) {
+export function _td(s: string): string {
     return s;
 }
 
 // Wrapper for counterpart's translation function so that it handles nulls and undefineds properly
 // Takes the same arguments as counterpart.translate()
-function safeCounterpartTranslate(text, options) {
+function safeCounterpartTranslate(text: string, options?: object) {
     // Horrible hack to avoid https://github.com/vector-im/riot-web/issues/4191
     // The interpolation library that counterpart uses does not support undefined/null
     // values and instead will throw an error. This is a problem since everywhere else
@@ -89,6 +94,13 @@ function safeCounterpartTranslate(text, options) {
     return translated;
 }
 
+interface IVariables {
+    count?: number;
+    [key: string]: number | string;
+}
+
+type Tags = Record<string, (sub: string) => React.ReactNode>;
+
 /*
  * Translates text and optionally also replaces XML-ish elements in the text with e.g. React components
  * @param {string} text The untranslated text, e.g "click <a>here</a> now to %(foo)s".
@@ -105,7 +117,9 @@ function safeCounterpartTranslate(text, options) {
  *
  * @return a React <span> component if any non-strings were used in substitutions, otherwise a string
  */
-export function _t(text, variables, tags) {
+export function _t(text: string, variables?: IVariables): string;
+export function _t(text: string, variables: IVariables, tags: Tags): React.ReactNode;
+export function _t(text: string, variables?: IVariables, tags?: Tags): string | React.ReactNode {
     // Don't do substitutions in counterpart. We handle it ourselves so we can replace with React components
     // However, still pass the variables to counterpart so that it can choose the correct plural if count is given
     // It is enough to pass the count variable, but in the future counterpart might make use of other information too
@@ -141,23 +155,25 @@ export function _t(text, variables, tags) {
  *
  * @return a React <span> component if any non-strings were used in substitutions, otherwise a string
  */
-export function substitute(text, variables, tags) {
-    let result = text;
+export function substitute(text: string, variables?: IVariables): string;
+export function substitute(text: string, variables: IVariables, tags: Tags): string;
+export function substitute(text: string, variables?: IVariables, tags?: Tags): string | React.ReactNode {
+    let result: React.ReactNode | string = text;
 
     if (variables !== undefined) {
-        const regexpMapping = {};
+        const regexpMapping: IVariables = {};
         for (const variable in variables) {
             regexpMapping[`%\\(${variable}\\)s`] = variables[variable];
         }
-        result = replaceByRegexes(result, regexpMapping);
+        result = replaceByRegexes(result as string, regexpMapping);
     }
 
     if (tags !== undefined) {
-        const regexpMapping = {};
+        const regexpMapping: Tags = {};
         for (const tag in tags) {
             regexpMapping[`(<${tag}>(.*?)<\\/${tag}>|<${tag}>|<${tag}\\s*\\/>)`] = tags[tag];
         }
-        result = replaceByRegexes(result, regexpMapping);
+        result = replaceByRegexes(result as string, regexpMapping);
     }
 
     return result;
@@ -172,7 +188,9 @@ export function substitute(text, variables, tags) {
  *
  * @return a React <span> component if any non-strings were used in substitutions, otherwise a string
  */
-export function replaceByRegexes(text, mapping) {
+export function replaceByRegexes(text: string, mapping: IVariables): string;
+export function replaceByRegexes(text: string, mapping: Tags): React.ReactNode;
+export function replaceByRegexes(text: string, mapping: IVariables | Tags): string | React.ReactNode {
     // We initially store our output as an array of strings and objects (e.g. React components).
     // This will then be converted to a string or a <span> at the end
     const output = [text];
@@ -189,7 +207,7 @@ export function replaceByRegexes(text, mapping) {
         // and everything after the match. Insert all three into the output. We need to do this because we can insert objects.
         // Otherwise there would be no need for the splitting and we could do simple replacement.
         let matchFoundSomewhere = false; // If we don't find a match anywhere we want to log it
-        for (const outputIndex in output) {
+        for (let outputIndex = 0; outputIndex < output.length; outputIndex++) {
             const inputText = output[outputIndex];
             if (typeof inputText !== 'string') { // We might have inserted objects earlier, don't try to replace them
                 continue;
@@ -216,7 +234,7 @@ export function replaceByRegexes(text, mapping) {
                 let replaced;
                 // If substitution is a function, call it
                 if (mapping[regexpString] instanceof Function) {
-                    replaced = mapping[regexpString].apply(null, capturedGroups);
+                    replaced = (mapping as Tags)[regexpString].apply(null, capturedGroups);
                 } else {
                     replaced = mapping[regexpString];
                 }
@@ -277,11 +295,11 @@ export function replaceByRegexes(text, mapping) {
 // Allow overriding the text displayed when no translation exists
 // Currently only used in unit tests to avoid having to load
 // the translations in riot-web
-export function setMissingEntryGenerator(f) {
+export function setMissingEntryGenerator(f: (value: string) => void) {
     counterpart.setMissingEntryGenerator(f);
 }
 
-export function setLanguage(preferredLangs) {
+export function setLanguage(preferredLangs: string | string[]) {
     if (!Array.isArray(preferredLangs)) {
         preferredLangs = [preferredLangs];
     }
@@ -358,8 +376,8 @@ export function getLanguageFromBrowser() {
  * @param {string} language The input language string
  * @return {string[]} List of normalised languages
  */
-export function getNormalizedLanguageKeys(language) {
-    const languageKeys = [];
+export function getNormalizedLanguageKeys(language: string) {
+    const languageKeys: string[] = [];
     const normalizedLanguage = normalizeLanguageKey(language);
     const languageParts = normalizedLanguage.split('-');
     if (languageParts.length === 2 && languageParts[0] === languageParts[1]) {
@@ -380,7 +398,7 @@ export function getNormalizedLanguageKeys(language) {
  * @param {string} language The language string to be normalized
  * @returns {string} The normalized language string
  */
-export function normalizeLanguageKey(language) {
+export function normalizeLanguageKey(language: string) {
     return language.toLowerCase().replace("_", "-");
 }
 
@@ -396,7 +414,7 @@ export function getCurrentLanguage() {
  * @param {string[]} langs List of language codes to pick from
  * @returns {string} The most appropriate language code from langs
  */
-export function pickBestLanguage(langs) {
+export function pickBestLanguage(langs: string[]): string {
     const currentLang = getCurrentLanguage();
     const normalisedLangs = langs.map(normalizeLanguageKey);
 
@@ -408,13 +426,13 @@ export function pickBestLanguage(langs) {
 
     {
         // Failing that, a different dialect of the same language
-        const closeLangIndex = normalisedLangs.find((l) => l.substr(0, 2) === currentLang.substr(0, 2));
+        const closeLangIndex = normalisedLangs.findIndex((l) => l.substr(0, 2) === currentLang.substr(0, 2));
         if (closeLangIndex > -1) return langs[closeLangIndex];
     }
 
     {
         // Neither of those? Try an english variant.
-        const enIndex = normalisedLangs.find((l) => l.startsWith('en'));
+        const enIndex = normalisedLangs.findIndex((l) => l.startsWith('en'));
         if (enIndex > -1) return langs[enIndex];
     }
 
@@ -422,7 +440,7 @@ export function pickBestLanguage(langs) {
     return langs[0];
 }
 
-function getLangsJson() {
+function getLangsJson(): Promise<object> {
     return new Promise(async (resolve, reject) => {
         let url;
         if (typeof(webpackLangJsonUrl) === 'string') { // in Jest this 'url' isn't a URL, so just fall through
@@ -443,7 +461,7 @@ function getLangsJson() {
     });
 }
 
-function weblateToCounterpart(inTrs) {
+function weblateToCounterpart(inTrs: object): object {
     const outTrs = {};
 
     for (const key of Object.keys(inTrs)) {
@@ -463,7 +481,7 @@ function weblateToCounterpart(inTrs) {
     return outTrs;
 }
 
-function getLanguage(langPath) {
+function getLanguage(langPath: string): object {
     return new Promise((resolve, reject) => {
         request(
             { method: "GET", url: langPath },
diff --git a/src/utils/promise.ts b/src/utils/promise.ts
index c5c1cb9a56..d3ae2c3d1b 100644
--- a/src/utils/promise.ts
+++ b/src/utils/promise.ts
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 // Returns a promise which resolves with a given value after the given number of ms
-export function sleep<T>(ms: number, value: T): Promise<T> {
+export function sleep<T>(ms: number, value?: T): Promise<T> {
     return new Promise((resolve => { setTimeout(resolve, ms, value); }));
 }
 
diff --git a/yarn.lock b/yarn.lock
index 98b42a0b29..d8106febab 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1257,6 +1257,11 @@
   resolved "https://registry.yarnpkg.com/@types/classnames/-/classnames-2.2.10.tgz#cc658ca319b6355399efc1f5b9e818f1a24bf999"
   integrity sha512-1UzDldn9GfYYEsWWnn/P4wkTlkZDH7lDb0wBMGbtIQc9zXEQq7FlKBdZUn6OBqD8sKZZ2RQO2mAjGpXiDGoRmQ==
 
+"@types/counterpart@^0.18.1":
+  version "0.18.1"
+  resolved "https://registry.yarnpkg.com/@types/counterpart/-/counterpart-0.18.1.tgz#b1b784d9e54d9879f0a8cb12f2caedab65430fe8"
+  integrity sha512-PRuFlBBkvdDOtxlIASzTmkEFar+S66Ek48NVVTWMUjtJAdn5vyMSN8y6IZIoIymGpR36q2nZbIYazBWyFxL+IQ==
+
 "@types/fbemitter@*":
   version "2.0.32"
   resolved "https://registry.yarnpkg.com/@types/fbemitter/-/fbemitter-2.0.32.tgz#8ed204da0f54e9c8eaec31b1eec91e25132d082c"