diff --git a/src/matrix/storage/common.ts b/src/matrix/storage/common.ts
index 4784862f..926ccbbf 100644
--- a/src/matrix/storage/common.ts
+++ b/src/matrix/storage/common.ts
@@ -41,8 +41,8 @@ export const STORE_MAP: Readonly<{ [name : string]: string }> = Object.freeze(ST
 }, {}));
 
 export class StorageError extends Error {
-    errcode?: string
-    cause?: Error
+    errcode?: string;
+    cause?: Error;
 
     constructor(message: string, cause?: Error) {
         super(message);
diff --git a/src/matrix/storage/idb/QueryTarget.ts b/src/matrix/storage/idb/QueryTarget.ts
index ac9e3d16..71da4d76 100644
--- a/src/matrix/storage/idb/QueryTarget.ts
+++ b/src/matrix/storage/idb/QueryTarget.ts
@@ -36,7 +36,7 @@ export class QueryTarget<T> {
         this._target = target;
     }
 
-    _openCursor(range?: IDBQuery, direction?: IDBCursorDirection) {
+    _openCursor(range?: IDBQuery, direction?: IDBCursorDirection): IDBRequest<IDBCursorWithValue | null> {
         if (range && direction) {
             return this._target.openCursor(range, direction);
         } else if (range) {
@@ -124,7 +124,7 @@ export class QueryTarget<T> {
 
     async findMaxKey(range: IDBQuery): Promise<IDBValidKey | undefined> {
         const cursor = this._target.openKeyCursor(range, "prev");
-        let maxKey;
+        let maxKey: IDBValidKey | undefined;
         await iterateCursor(cursor, (_, key) => {
             maxKey = key;
             return {done: true};
@@ -133,14 +133,14 @@ export class QueryTarget<T> {
     }
 
 
-    async iterateValues(range: IDBQuery, callback: (val: T, key: IDBValidKey, cur: IDBCursorWithValue) => boolean)  {
+    async iterateValues(range: IDBQuery, callback: (val: T, key: IDBValidKey, cur: IDBCursorWithValue) => boolean): Promise<void>  {
         const cursor = this._target.openCursor(range, "next");
         await iterateCursor<T>(cursor, (value, key, cur) => {
             return {done: callback(value, key, cur)};
         });
     }
 
-    async iterateKeys(range: IDBQuery, callback: (key: IDBValidKey, cur: IDBCursor) => boolean) {
+    async iterateKeys(range: IDBQuery, callback: (key: IDBValidKey, cur: IDBCursor) => boolean): Promise<void> {
         const cursor = this._target.openKeyCursor(range, "next");
         await iterateCursor(cursor, (_, key, cur) => {
             return {done: callback(key, cur)};
@@ -153,7 +153,7 @@ export class QueryTarget<T> {
      * If the callback returns true, the search is halted and callback won't be called again.
      * `callback` is called with the same instances of the key as given in `keys`, so direct comparison can be used.
      */
-    async findExistingKeys(keys: IDBValidKey[], backwards: boolean, callback: (key: IDBValidKey, found: boolean) => boolean) {
+    async findExistingKeys(keys: IDBValidKey[], backwards: boolean, callback: (key: IDBValidKey, found: boolean) => boolean): Promise<void> {
         const direction = backwards ? "prev" : "next";
         const compareKeys = (a, b) => backwards ? -indexedDB.cmp(a, b) : indexedDB.cmp(a, b);
         const sortedKeys = keys.slice().sort(compareKeys);
@@ -225,7 +225,7 @@ export class QueryTarget<T> {
         return results;
     }
 
-    async iterateWhile(range: IDBQuery, predicate: (v: T) => boolean) {
+    async iterateWhile(range: IDBQuery, predicate: (v: T) => boolean): Promise<void> {
         const cursor = this._openCursor(range, "next");
         await iterateCursor<T>(cursor, (value) => {
             const passesPredicate = predicate(value);
diff --git a/src/matrix/storage/idb/Storage.ts b/src/matrix/storage/idb/Storage.ts
index cc460b26..8dd33522 100644
--- a/src/matrix/storage/idb/Storage.ts
+++ b/src/matrix/storage/idb/Storage.ts
@@ -37,7 +37,7 @@ export class Storage {
         this.storeNames = Object.freeze(nameMap);
     }
 
-    _validateStoreNames(storeNames: string[]) {
+    _validateStoreNames(storeNames: string[]): void {
         const idx = storeNames.findIndex(name => !STORE_NAMES.includes(name));
         if (idx !== -1) {
             throw new StorageError(`Tried top, a transaction unknown store ${storeNames[idx]}`);
@@ -76,7 +76,7 @@ export class Storage {
         }
     }
 
-    close() {
+    close(): void {
         this._db.close();
     }
 }
diff --git a/src/matrix/storage/idb/StorageFactory.ts b/src/matrix/storage/idb/StorageFactory.ts
index 93f4c3ac..ce181a0c 100644
--- a/src/matrix/storage/idb/StorageFactory.ts
+++ b/src/matrix/storage/idb/StorageFactory.ts
@@ -57,7 +57,7 @@ export class StorageFactory {
         this._IDBKeyRange = IDBKeyRange;
     }
 
-    async create(sessionId: string) {
+    async create(sessionId: string): Promise<Storage> {
         await this._serviceWorkerHandler?.preventConcurrentSessionAccess(sessionId);
         requestPersistedStorage().then(persisted => {
             // Firefox lies here though, and returns true even if the user denied the request
diff --git a/src/matrix/storage/idb/Store.ts b/src/matrix/storage/idb/Store.ts
index 7dd0b588..585b3948 100644
--- a/src/matrix/storage/idb/Store.ts
+++ b/src/matrix/storage/idb/Store.ts
@@ -21,7 +21,7 @@ import {Transaction} from "./Transaction";
 
 const LOG_REQUESTS = false;
 
-function logRequest(method: string, params: any[], source: any) {
+function logRequest(method: string, params: any[], source: any): void {
     const storeName = source?.name;
     const databaseName = source?.transaction?.db?.name;
     console.info(`${databaseName}.${storeName}.${method}(${params.map(p => JSON.stringify(p)).join(", ")})`);
diff --git a/src/matrix/storage/idb/Transaction.ts b/src/matrix/storage/idb/Transaction.ts
index 6ad66f22..5aa5fc18 100644
--- a/src/matrix/storage/idb/Transaction.ts
+++ b/src/matrix/storage/idb/Transaction.ts
@@ -136,11 +136,11 @@ export class Transaction {
         return this._store("accountData", idbStore => new AccountDataStore(idbStore));
     }
 
-    complete() {
+    complete(): Promise<void> {
         return txnAsPromise(this._txn);
     }
 
-    abort() {
+    abort(): void {
         // TODO: should we wrap the exception in a StorageError?
         this._txn.abort();
     }
diff --git a/src/matrix/storage/idb/error.ts b/src/matrix/storage/idb/error.ts
index 3694b587..02953886 100644
--- a/src/matrix/storage/idb/error.ts
+++ b/src/matrix/storage/idb/error.ts
@@ -18,8 +18,8 @@ limitations under the License.
 import { StorageError } from "../common";
 
 export class IDBError extends StorageError {
-    storeName: string
-    databaseName: string
+    storeName: string;
+    databaseName: string;
 
     constructor(message: string, source, cause: DOMException | null) {
         const storeName = source?.name || "<unknown store>";
diff --git a/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts b/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
index bc49e146..c1ebfc63 100644
--- a/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
+++ b/src/matrix/storage/idb/stores/InboundGroupSessionStore.ts
@@ -27,7 +27,7 @@ interface InboundGroupSession {
     key: string;
 }
 
-function encodeKey(roomId: string, senderKey: string, sessionId: string) {
+function encodeKey(roomId: string, senderKey: string, sessionId: string): string {
     return `${roomId}|${senderKey}|${sessionId}`;
 }
 
diff --git a/src/matrix/storage/idb/stores/RoomMemberStore.ts b/src/matrix/storage/idb/stores/RoomMemberStore.ts
index 1b3df104..847e8dae 100644
--- a/src/matrix/storage/idb/stores/RoomMemberStore.ts
+++ b/src/matrix/storage/idb/stores/RoomMemberStore.ts
@@ -50,7 +50,7 @@ export class RoomMemberStore {
         return this._roomMembersStore.get(encodeKey(roomId, userId));
     }
 
-    async set(member: MemberData) {
+    async set(member: MemberData): Promise<IDBValidKey> {
         // Object.assign would be more typesafe, but small objects 
         (member as any).key = encodeKey(member.roomId, member.userId);
         return this._roomMembersStore.put(member as MemberStorageEntry);
diff --git a/src/matrix/storage/idb/stores/SessionStore.ts b/src/matrix/storage/idb/stores/SessionStore.ts
index 1f79839f..bf3d6d11 100644
--- a/src/matrix/storage/idb/stores/SessionStore.ts
+++ b/src/matrix/storage/idb/stores/SessionStore.ts
@@ -34,15 +34,15 @@ export class SessionStore {
         }
     }
 
-    set(key: string, value: any) {
-        this._sessionStore.put({key, value});
+    set(key: string, value: any): Promise<IDBValidKey> {
+        return this._sessionStore.put({key, value});
     }
 
-    add(key: string, value: any) {
-        this._sessionStore.add({key, value});
+    add(key: string, value: any): Promise<IDBValidKey> {
+        return this._sessionStore.add({key, value});
     }
 
-    remove(key: IDBValidKey) {
-        this._sessionStore.delete(key);
+    remove(key: IDBValidKey): Promise<undefined> {
+        return this._sessionStore.delete(key);
     }
 }
diff --git a/src/matrix/storage/idb/stores/TimelineEventStore.ts b/src/matrix/storage/idb/stores/TimelineEventStore.ts
index e9bdce3a..764b886b 100644
--- a/src/matrix/storage/idb/stores/TimelineEventStore.ts
+++ b/src/matrix/storage/idb/stores/TimelineEventStore.ts
@@ -69,7 +69,7 @@ class Range {
         this._upperOpen = upperOpen;
     }
 
-    asIDBKeyRange(roomId: string) {
+    asIDBKeyRange(roomId: string): IDBKeyRange | undefined {
         try {
             // only
             if (this._only) {
@@ -240,7 +240,7 @@ export class TimelineEventStore {
         let firstFoundKey: string | undefined;
 
         // find first result that is found and has no undefined results before it
-        function firstFoundAndPrecedingResolved() {
+        function firstFoundAndPrecedingResolved(): string | undefined {
             for(let i = 0; i < results.length; ++i) {
                 if (results[i] === undefined) {
                     return;
diff --git a/src/matrix/storage/idb/stores/TimelineFragmentStore.ts b/src/matrix/storage/idb/stores/TimelineFragmentStore.ts
index 7bbae73d..cb89211a 100644
--- a/src/matrix/storage/idb/stores/TimelineFragmentStore.ts
+++ b/src/matrix/storage/idb/stores/TimelineFragmentStore.ts
@@ -74,13 +74,13 @@ export class TimelineFragmentStore {
     // should generate an id an return it?
     // depends if we want to do anything smart with fragment ids,
     // like give them meaning depending on range. not for now probably ...
-    add(fragment: Fragment) {
+    add(fragment: Fragment): Promise<IDBValidKey> {
         (fragment as any).key = encodeKey(fragment.roomId, fragment.id);
-        this._store.add(fragment as FragmentEntry);
+        return this._store.add(fragment as FragmentEntry);
     }
 
-    update(fragment: FragmentEntry) {
-        this._store.put(fragment);
+    update(fragment: FragmentEntry): Promise<IDBValidKey> {
+        return this._store.put(fragment);
     }
 
     get(roomId: string, fragmentId: number): Promise<FragmentEntry | null> {