diff --git a/src/BasePlatform.js b/src/BasePlatform.ts
similarity index 79%
rename from src/BasePlatform.js
rename to src/BasePlatform.ts
index e3cbc4dcf0..3f11f25d62 100644
--- a/src/BasePlatform.js
+++ b/src/BasePlatform.ts
@@ -1,5 +1,3 @@
-// @flow
-
 /*
 Copyright 2016 Aviral Dasgupta
 Copyright 2016 OpenMarket Ltd
@@ -19,9 +17,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {MatrixClient} from "matrix-js-sdk";
+import {MatrixClient} from "matrix-js-sdk/src/client";
 import dis from './dispatcher/dispatcher';
 import BaseEventIndexManager from './indexing/BaseEventIndexManager';
+import {ActionPayload} from "./dispatcher/payloads";
 
 /**
  * Base class for classes that provide platform-specific functionality
@@ -29,27 +28,25 @@ import BaseEventIndexManager from './indexing/BaseEventIndexManager';
  *
  * Instances of this class are provided by the application.
  */
-export default class BasePlatform {
-    constructor() {
-        this.notificationCount = 0;
-        this.errorDidOccur = false;
+export default abstract class BasePlatform {
+    protected notificationCount: number = 0;
+    protected errorDidOccur: boolean = false;
 
-        dis.register(this._onAction.bind(this));
+    constructor() {
+        dis.register(this.onAction);
     }
 
-    _onAction(payload: Object) {
+    protected onAction = (payload: ActionPayload) => {
         switch (payload.action) {
             case 'on_client_not_viable':
             case 'on_logged_out':
                 this.setNotificationCount(0);
                 break;
         }
-    }
+    };
 
     // Used primarily for Analytics
-    getHumanReadableName(): string {
-        return 'Base Platform';
-    }
+    abstract getHumanReadableName(): string;
 
     setNotificationCount(count: number) {
         this.notificationCount = count;
@@ -84,22 +81,16 @@ export default class BasePlatform {
      * that is 'granted' if the user allowed the request or
      * 'denied' otherwise.
      */
-    requestNotificationPermission(): Promise<string> {
-    }
+    abstract requestNotificationPermission(): Promise<string>;
 
-    displayNotification(title: string, msg: string, avatarUrl: string, room: Object) {
-    }
+    abstract displayNotification(title: string, msg: string, avatarUrl: string, room: Object);
 
-    loudNotification(ev: Event, room: Object) {
-    }
+    abstract loudNotification(ev: Event, room: Object);
 
     /**
-     * Returns a promise that resolves to a string representing
-     * the current version of the application.
+     * Returns a promise that resolves to a string representing the current version of the application.
      */
-    getAppVersion(): Promise<string> {
-        throw new Error("getAppVersion not implemented!");
-    }
+    abstract getAppVersion(): Promise<string>;
 
     /*
      * If it's not expected that capturing the screen will work
@@ -114,20 +105,18 @@ export default class BasePlatform {
      * Restarts the application, without neccessarily reloading
      * any application code
      */
-    reload() {
-        throw new Error("reload not implemented!");
-    }
+    abstract reload();
 
     supportsAutoLaunch(): boolean {
         return false;
     }
 
     // XXX: Surely this should be a setting like any other?
-    async getAutoLaunchEnabled(): boolean {
+    async getAutoLaunchEnabled(): Promise<boolean> {
         return false;
     }
 
-    async setAutoLaunchEnabled(enabled: boolean): void {
+    async setAutoLaunchEnabled(enabled: boolean): Promise<void> {
         throw new Error("Unimplemented");
     }
 
@@ -135,11 +124,11 @@ export default class BasePlatform {
         return false;
     }
 
-    async getAutoHideMenuBarEnabled(): boolean {
+    async getAutoHideMenuBarEnabled(): Promise<boolean> {
         return false;
     }
 
-    async setAutoHideMenuBarEnabled(enabled: boolean): void {
+    async setAutoHideMenuBarEnabled(enabled: boolean): Promise<void> {
         throw new Error("Unimplemented");
     }
 
@@ -147,11 +136,11 @@ export default class BasePlatform {
         return false;
     }
 
-    async getMinimizeToTrayEnabled(): boolean {
+    async getMinimizeToTrayEnabled(): Promise<boolean> {
         return false;
     }
 
-    async setMinimizeToTrayEnabled(enabled: boolean): void {
+    async setMinimizeToTrayEnabled(enabled: boolean): Promise<void> {
         throw new Error("Unimplemented");
     }
 
diff --git a/src/indexing/BaseEventIndexManager.js b/src/indexing/BaseEventIndexManager.ts
similarity index 97%
rename from src/indexing/BaseEventIndexManager.js
rename to src/indexing/BaseEventIndexManager.ts
index f780c8e9ce..c40d1300ea 100644
--- a/src/indexing/BaseEventIndexManager.js
+++ b/src/indexing/BaseEventIndexManager.ts
@@ -1,5 +1,5 @@
 /*
-Copyright 2019 The Matrix.org Foundation C.I.C.
+Copyright 2019, 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.
@@ -20,7 +20,7 @@ export interface MatrixEvent {
     content: {};
     event_id: string;
     origin_server_ts: number;
-    unsigned: ?{};
+    unsigned?: {};
     room_id: string;
 }
 
@@ -59,7 +59,7 @@ export interface SearchArgs {
     before_limit: number;
     after_limit: number;
     order_by_recency: boolean;
-    room_id: ?string;
+    room_id?: string;
 }
 
 export interface EventAndProfile {
@@ -85,7 +85,7 @@ export interface IndexStats {
  *
  * Instances of this class are provided by the application.
  */
-export default class BaseEventIndexManager {
+export default abstract class BaseEventIndexManager {
     /**
      * Does our EventIndexManager support event indexing.
      *
@@ -119,7 +119,7 @@ export default class BaseEventIndexManager {
      * @return {Promise} A promise that will resolve when the was queued up for
      * addition.
      */
-    async addEventToIndex(ev: MatrixEvent, profile: MatrixProfile): Promise<> {
+    async addEventToIndex(ev: MatrixEvent, profile: MatrixProfile): Promise<void> {
         throw new Error("Unimplemented");
     }
 
@@ -188,7 +188,7 @@ export default class BaseEventIndexManager {
         events: [EventAndProfile],
         checkpoint: CrawlerCheckpoint | null,
         oldCheckpoint: CrawlerCheckpoint | null,
-    ): Promise<bool> {
+    ): Promise<boolean> {
         throw new Error("Unimplemented");
     }