diff --git a/doc/TS-MIGRATION.md b/doc/TS-MIGRATION.md
new file mode 100644
index 00000000..7cb5bb34
--- /dev/null
+++ b/doc/TS-MIGRATION.md
@@ -0,0 +1,7 @@
+# Typescript migration
+ 
+## Introduce `abstract` & `override`
+
+ - find all methods and getters that throw or are empty in base classes and turn into abstract method or if all methods are abstract, into an interface.
+ - change child impls to not call super.method and to add override
+ - don't allow implicit override in ts config
diff --git a/src/matrix/room/timeline/Direction.js b/src/matrix/room/timeline/Direction.ts
similarity index 70%
rename from src/matrix/room/timeline/Direction.js
rename to src/matrix/room/timeline/Direction.ts
index c83ec61c..24a8ff41 100644
--- a/src/matrix/room/timeline/Direction.js
+++ b/src/matrix/room/timeline/Direction.ts
@@ -15,34 +15,29 @@ limitations under the License.
 */
 
 export class Direction {
-    constructor(isForward) {
-        this._isForward = isForward;
+    constructor(public readonly isForward: boolean) {
     }
 
-    get isForward() {
-        return this._isForward;
-    }
-
-    get isBackward() {
+    get isBackward(): boolean {
         return !this.isForward;
     }
 
-    asApiString() {
+    asApiString(): string {
         return this.isForward ? "f" : "b";
     }
 
-    reverse() {
+    reverse(): Direction {
         return this.isForward ? Direction.Backward : Direction.Forward
     }
 
-    static get Forward() {
+    static get Forward(): Direction {
         return _forward;
     }
 
-    static get Backward() {
+    static get Backward(): Direction {
         return _backward;
     }
 }
 
-const _forward = Object.freeze(new Direction(true));
-const _backward = Object.freeze(new Direction(false));
+const _forward = new Direction(true);
+const _backward = new Direction(false);
diff --git a/src/matrix/room/timeline/EventKey.js b/src/matrix/room/timeline/EventKey.ts
similarity index 75%
rename from src/matrix/room/timeline/EventKey.js
rename to src/matrix/room/timeline/EventKey.ts
index af01a7e5..8775df26 100644
--- a/src/matrix/room/timeline/EventKey.js
+++ b/src/matrix/room/timeline/EventKey.ts
@@ -15,20 +15,22 @@ limitations under the License.
 */
 
 import {KeyLimits} from "../../storage/common";
+import {Direction} from "./Direction";
 
 // key for events in the timelineEvents store
 export class EventKey {
-    constructor(fragmentId, eventIndex) {
-        this.fragmentId = fragmentId;
-        this.eventIndex = eventIndex;
+    constructor(
+        public fragmentId: number,
+        public eventIndex: number
+    ) {
     }
 
-    nextFragmentKey() {
+    nextFragmentKey(): EventKey {
         // could take MIN_EVENT_INDEX here if it can't be paged back
         return new EventKey(this.fragmentId + 1, KeyLimits.middleStorageKey);
     }
 
-    nextKeyForDirection(direction) {
+    nextKeyForDirection(direction: Direction): EventKey {
         if (direction.isForward) {
             return this.nextKey();
         } else {
@@ -36,35 +38,35 @@ export class EventKey {
         }
     }
 
-    previousKey() {
+    previousKey(): EventKey {
         return new EventKey(this.fragmentId, this.eventIndex - 1);
     }
 
-    nextKey() {
+    nextKey(): EventKey {
         return new EventKey(this.fragmentId, this.eventIndex + 1);
     }
 
-    static get maxKey() {
+    static get maxKey(): EventKey {
         return new EventKey(KeyLimits.maxStorageKey, KeyLimits.maxStorageKey);
     }
 
-    static get minKey() {
+    static get minKey(): EventKey {
         return new EventKey(KeyLimits.minStorageKey, KeyLimits.minStorageKey);
     }
 
-    static get defaultLiveKey() {
+    static get defaultLiveKey(): EventKey {
         return EventKey.defaultFragmentKey(KeyLimits.minStorageKey);
     }
 
-    static defaultFragmentKey(fragmentId) {
+    static defaultFragmentKey(fragmentId: number): EventKey {
         return new EventKey(fragmentId, KeyLimits.middleStorageKey);
     }
 
-    toString() {
+    toString(): string {
         return `[${this.fragmentId}/${this.eventIndex}]`;
     }
 
-    equals(other) {
+    equals(other: EventKey): boolean {
         return this.fragmentId === other?.fragmentId && this.eventIndex === other?.eventIndex;
     }
 }
diff --git a/src/matrix/room/timeline/Timeline.js b/src/matrix/room/timeline/Timeline.js
index fdfaee48..022eda4e 100644
--- a/src/matrix/room/timeline/Timeline.js
+++ b/src/matrix/room/timeline/Timeline.js
@@ -17,7 +17,7 @@ limitations under the License.
 
 import {SortedArray, AsyncMappedList, ConcatList, ObservableArray} from "../../../observable/index.js";
 import {Disposables} from "../../../utils/Disposables.js";
-import {Direction} from "./Direction.js";
+import {Direction} from "./Direction";
 import {TimelineReader} from "./persistence/TimelineReader.js";
 import {PendingEventEntry} from "./entries/PendingEventEntry.js";
 import {RoomMember} from "../members/RoomMember.js";
diff --git a/src/matrix/room/timeline/entries/BaseEntry.ts b/src/matrix/room/timeline/entries/BaseEntry.ts
index 1ef6a863..23ec268c 100644
--- a/src/matrix/room/timeline/entries/BaseEntry.ts
+++ b/src/matrix/room/timeline/entries/BaseEntry.ts
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 //entries can be sorted, first by fragment, then by entry index.
-import {EventKey} from "../EventKey.js";
+import {EventKey} from "../EventKey";
 export const PENDING_FRAGMENT_ID = Number.MAX_SAFE_INTEGER;
 
 interface FragmentIdComparer {
diff --git a/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js b/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
index 0697fbdd..7a8ed5d0 100644
--- a/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
+++ b/src/matrix/room/timeline/entries/FragmentBoundaryEntry.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {BaseEntry} from "./BaseEntry";
-import {Direction} from "../Direction.js";
+import {Direction} from "../Direction";
 import {isValidFragmentId} from "../common.js";
 import {KeyLimits} from "../../../storage/common";
 
diff --git a/src/matrix/room/timeline/persistence/GapWriter.js b/src/matrix/room/timeline/persistence/GapWriter.js
index 2fc56a21..e133d713 100644
--- a/src/matrix/room/timeline/persistence/GapWriter.js
+++ b/src/matrix/room/timeline/persistence/GapWriter.js
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {EventKey} from "../EventKey.js";
+import {EventKey} from "../EventKey";
 import {EventEntry} from "../entries/EventEntry.js";
 import {createEventEntry, directionalAppend} from "./common.js";
 import {RoomMember, EVENT_TYPE as MEMBER_EVENT_TYPE} from "../../members/RoomMember.js";
diff --git a/src/matrix/room/timeline/persistence/SyncWriter.js b/src/matrix/room/timeline/persistence/SyncWriter.js
index 54fd5a0b..1ee2ce7d 100644
--- a/src/matrix/room/timeline/persistence/SyncWriter.js
+++ b/src/matrix/room/timeline/persistence/SyncWriter.js
@@ -15,7 +15,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {EventKey} from "../EventKey.js";
+import {EventKey} from "../EventKey";
 import {EventEntry} from "../entries/EventEntry.js";
 import {FragmentBoundaryEntry} from "../entries/FragmentBoundaryEntry.js";
 import {createEventEntry} from "./common.js";
diff --git a/src/matrix/room/timeline/persistence/TimelineReader.js b/src/matrix/room/timeline/persistence/TimelineReader.js
index 27b88ea9..ffad80ec 100644
--- a/src/matrix/room/timeline/persistence/TimelineReader.js
+++ b/src/matrix/room/timeline/persistence/TimelineReader.js
@@ -15,7 +15,7 @@ limitations under the License.
 */
 
 import {directionalConcat, directionalAppend} from "./common.js";
-import {Direction} from "../Direction.js";
+import {Direction} from "../Direction";
 import {EventEntry} from "../entries/EventEntry.js";
 import {FragmentBoundaryEntry} from "../entries/FragmentBoundaryEntry.js";
 
diff --git a/src/matrix/storage/idb/stores/TimelineEventStore.ts b/src/matrix/storage/idb/stores/TimelineEventStore.ts
index f659bdbd..49892771 100644
--- a/src/matrix/storage/idb/stores/TimelineEventStore.ts
+++ b/src/matrix/storage/idb/stores/TimelineEventStore.ts
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import {EventKey} from "../../../room/timeline/EventKey.js";
+import {EventKey} from "../../../room/timeline/EventKey";
 import { StorageError } from "../../common";
 import { encodeUint32 } from "../utils";
 import {KeyLimits} from "../../common";
diff --git a/tsconfig.json b/tsconfig.json
index b7ae64e8..0179e267 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -2,7 +2,7 @@
     "compilerOptions": {
         "strictNullChecks": true,
         "noEmit": true,
-        "target": "es6"
+        "target": "ES2020"
     },
     "include": ["src/**/*"],
 }