diff --git a/.eslintrc.cjs b/.eslintrc.cjs
index 50ca8d622..7bad5c817 100644
--- a/.eslintrc.cjs
+++ b/.eslintrc.cjs
@@ -138,6 +138,7 @@ module.exports = {
                         tryExtensions: [".ts"],
                     },
                 ],
+                "no-extra-boolean-cast": "error",
             },
         },
         {
diff --git a/spec/integ/matrix-client-methods.spec.ts b/spec/integ/matrix-client-methods.spec.ts
index b5347e3b5..d5ec9677b 100644
--- a/spec/integ/matrix-client-methods.spec.ts
+++ b/spec/integ/matrix-client-methods.spec.ts
@@ -1915,6 +1915,28 @@ describe("MatrixClient", function () {
             return prom;
         });
     });
+
+    describe("getDomain", () => {
+        it("should return null if no userId is set", () => {
+            const client = new MatrixClient({ baseUrl: "http://localhost" });
+            expect(client.getDomain()).toBeNull();
+        });
+
+        it("should return the domain of the userId", () => {
+            expect(client.getDomain()).toBe("localhost");
+        });
+    });
+
+    describe("getUserIdLocalpart", () => {
+        it("should return null if no userId is set", () => {
+            const client = new MatrixClient({ baseUrl: "http://localhost" });
+            expect(client.getUserIdLocalpart()).toBeNull();
+        });
+
+        it("should return the localpart of the userId", () => {
+            expect(client.getUserIdLocalpart()).toBe("alice");
+        });
+    });
 });
 
 function withThreadId(event: MatrixEvent, newThreadId: string): MatrixEvent {
diff --git a/spec/test-utils/webrtc.ts b/spec/test-utils/webrtc.ts
index fbc6d93be..924d024ca 100644
--- a/spec/test-utils/webrtc.ts
+++ b/spec/test-utils/webrtc.ts
@@ -481,6 +481,9 @@ export class MockCallMatrixClient extends TypedEventEmitter<EmittedEvents, Emitt
     public getUserId(): string {
         return this.userId;
     }
+    public getSafeUserId(): string {
+        return this.userId;
+    }
 
     public getDeviceId(): string {
         return this.deviceId;
diff --git a/spec/unit/webrtc/groupCall.spec.ts b/spec/unit/webrtc/groupCall.spec.ts
index dcdd86008..669542e76 100644
--- a/spec/unit/webrtc/groupCall.spec.ts
+++ b/spec/unit/webrtc/groupCall.spec.ts
@@ -1430,7 +1430,7 @@ describe("Group Call", function () {
         let client: MatrixClient;
 
         beforeEach(() => {
-            client = new MatrixClient({ baseUrl: "base_url" });
+            client = new MatrixClient({ baseUrl: "base_url", userId: "my_user_id" });
 
             jest.spyOn(client, "sendStateEvent").mockResolvedValue({} as any);
         });
diff --git a/src/@types/global.d.ts b/src/@types/global.d.ts
index 0b51c2b22..5a280f0e2 100644
--- a/src/@types/global.d.ts
+++ b/src/@types/global.d.ts
@@ -65,6 +65,6 @@ declare global {
     interface Navigator {
         // We check for the webkit-prefixed getUserMedia to detect if we're
         // on webkit: we should check if we still need to do this
-        webkitGetUserMedia: DummyInterfaceWeShouldntBeUsingThis;
+        webkitGetUserMedia?: DummyInterfaceWeShouldntBeUsingThis;
     }
 }
diff --git a/src/autodiscovery.ts b/src/autodiscovery.ts
index b321dcddc..d9243e4da 100644
--- a/src/autodiscovery.ts
+++ b/src/autodiscovery.ts
@@ -130,7 +130,7 @@ export class AutoDiscovery {
      * configuration, which may include error states. Rejects on unexpected
      * failure, not when verification fails.
      */
-    public static async fromDiscoveryConfig(wellknown: IClientWellKnown): Promise<ClientConfig> {
+    public static async fromDiscoveryConfig(wellknown?: IClientWellKnown): Promise<ClientConfig> {
         // Step 1 is to get the config, which is provided to us here.
 
         // We default to an error state to make the first few checks easier to
diff --git a/src/client.ts b/src/client.ts
index 5ef823a29..478e2784e 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -1832,10 +1832,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns MXID for the logged-in user, or null if not logged in
      */
     public getUserId(): string | null {
-        if (this.credentials && this.credentials.userId) {
-            return this.credentials.userId;
-        }
-        return null;
+        return this.credentials?.userId ?? null;
     }
 
     /**
@@ -1857,7 +1854,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns Domain of this MXID
      */
     public getDomain(): string | null {
-        if (this.credentials && this.credentials.userId) {
+        if (this.credentials?.userId) {
             return this.credentials.userId.replace(/^.*?:/, "");
         }
         return null;
@@ -1868,10 +1865,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      * @returns The user ID localpart or null.
      */
     public getUserIdLocalpart(): string | null {
-        if (this.credentials && this.credentials.userId) {
-            return this.credentials.userId.split(":")[0].substring(1);
-        }
-        return null;
+        return this.credentials?.userId?.split(":")[0].substring(1) ?? null;
     }
 
     /**
@@ -4313,7 +4307,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
      */
     public getIgnoredUsers(): string[] {
         const event = this.getAccountData("m.ignored_user_list");
-        if (!event || !event.getContent() || !event.getContent()["ignored_users"]) return [];
+        if (!event?.getContent()["ignored_users"]) return [];
         return Object.keys(event.getContent()["ignored_users"]);
     }
 
diff --git a/src/crypto/EncryptionSetup.ts b/src/crypto/EncryptionSetup.ts
index dab5d5606..bc51dd5ef 100644
--- a/src/crypto/EncryptionSetup.ts
+++ b/src/crypto/EncryptionSetup.ts
@@ -106,7 +106,7 @@ export class EncryptionSetupBuilder {
         if (!this.keySignatures) {
             this.keySignatures = {};
         }
-        const userSignatures = this.keySignatures[userId] || {};
+        const userSignatures = this.keySignatures[userId] ?? {};
         this.keySignatures[userId] = userSignatures;
         userSignatures[deviceId] = signature;
     }
diff --git a/src/crypto/dehydration.ts b/src/crypto/dehydration.ts
index cb72691f1..31f69ee0a 100644
--- a/src/crypto/dehydration.ts
+++ b/src/crypto/dehydration.ts
@@ -27,8 +27,8 @@ import decryptAESSecretStorageItem from "../utils/decryptAESSecretStorageItem.ts
 import encryptAESSecretStorageItem from "../utils/encryptAESSecretStorageItem.ts";
 
 export interface IDehydratedDevice {
-    device_id: string; // eslint-disable-line camelcase
-    device_data: SecretStorageKeyDescription & {
+    device_id?: string; // eslint-disable-line camelcase
+    device_data?: SecretStorageKeyDescription & {
         // eslint-disable-line camelcase
         algorithm: string;
         account: string; // pickle
@@ -90,7 +90,7 @@ export class DehydrationManager {
     }
 
     public async setKey(
-        key: Uint8Array,
+        key?: Uint8Array,
         keyInfo: { [props: string]: any } = {},
         deviceDisplayName?: string,
     ): Promise<boolean | undefined> {
diff --git a/src/crypto/store/localStorage-crypto-store.ts b/src/crypto/store/localStorage-crypto-store.ts
index d24e648d6..818a83140 100644
--- a/src/crypto/store/localStorage-crypto-store.ts
+++ b/src/crypto/store/localStorage-crypto-store.ts
@@ -162,7 +162,7 @@ export class LocalStorageCryptoStore extends MemoryCryptoStore implements Crypto
         func: (session: ISessionInfo) => void,
     ): void {
         const sessions = this._getEndToEndSessions(deviceKey);
-        func(sessions[sessionId] || {});
+        func(sessions[sessionId] ?? {});
     }
 
     public getEndToEndSessions(
@@ -170,7 +170,7 @@ export class LocalStorageCryptoStore extends MemoryCryptoStore implements Crypto
         txn: unknown,
         func: (sessions: { [sessionId: string]: ISessionInfo }) => void,
     ): void {
-        func(this._getEndToEndSessions(deviceKey) || {});
+        func(this._getEndToEndSessions(deviceKey) ?? {});
     }
 
     public getAllEndToEndSessions(txn: unknown, func: (session: ISessionInfo) => void): void {
diff --git a/src/models/event.ts b/src/models/event.ts
index 5ce7e9fb9..bdb75f5ea 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -559,9 +559,9 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
             return {} as T;
         }
         if (this.clearEvent) {
-            return (this.clearEvent.content || {}) as T;
+            return (this.clearEvent.content ?? {}) as T;
         }
-        return (this.event.content || {}) as T;
+        return (this.event.content ?? {}) as T;
     }
 
     /**
@@ -575,7 +575,7 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
         if (this._localRedactionEvent) {
             return {} as T;
         } else if (this._replacingEvent) {
-            return this._replacingEvent.getContent()["m.new_content"] || {};
+            return this._replacingEvent.getContent()["m.new_content"] ?? {};
         } else {
             return this.getOriginalContent();
         }
@@ -1633,7 +1633,7 @@ export class MatrixEvent extends TypedEventEmitter<MatrixEventEmittedEvents, Mat
      * @param otherEvent - The other event to check against.
      * @returns True if the events are the same, false otherwise.
      */
-    public isEquivalentTo(otherEvent: MatrixEvent): boolean {
+    public isEquivalentTo(otherEvent?: MatrixEvent): boolean {
         if (!otherEvent) return false;
         if (otherEvent === this) return true;
         const myProps = deepSortedObjectEntries(this.event);
diff --git a/src/sliding-sync.ts b/src/sliding-sync.ts
index 487c447a2..4e93d8f21 100644
--- a/src/sliding-sync.ts
+++ b/src/sliding-sync.ts
@@ -196,8 +196,8 @@ class SlidingList {
      * @param list - The new list parameters
      */
     public replaceList(list: MSC3575List): void {
-        list.filters = list.filters || {};
-        list.ranges = list.ranges || [];
+        list.filters = list.filters ?? {};
+        list.ranges = list.ranges ?? [];
         this.list = JSON.parse(JSON.stringify(list));
         this.isModified = true;
 
@@ -894,9 +894,9 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
                     l.setModified(false);
                 });
                 // set default empty values so we don't need to null check
-                resp.lists = resp.lists || {};
-                resp.rooms = resp.rooms || {};
-                resp.extensions = resp.extensions || {};
+                resp.lists = resp.lists ?? {};
+                resp.rooms = resp.rooms ?? {};
+                resp.extensions = resp.extensions ?? {};
                 Object.keys(resp.lists).forEach((key: string) => {
                     const list = this.lists.get(key);
                     if (!list || !resp) {
@@ -934,7 +934,7 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
             const listKeysWithUpdates: Set<string> = new Set();
             if (!doNotUpdateList) {
                 for (const [key, list] of Object.entries(resp.lists)) {
-                    list.ops = list.ops || [];
+                    list.ops = list.ops ?? [];
                     if (list.ops.length > 0) {
                         listKeysWithUpdates.add(key);
                     }
diff --git a/src/store/indexeddb.ts b/src/store/indexeddb.ts
index dfed00df2..8705f4a5c 100644
--- a/src/store/indexeddb.ts
+++ b/src/store/indexeddb.ts
@@ -42,7 +42,7 @@ const WRITE_DELAY_MS = 1000 * 60 * 5; // once every 5 minutes
 
 interface IOpts extends IBaseOpts {
     /** The Indexed DB interface e.g. `window.indexedDB` */
-    indexedDB: IDBFactory;
+    indexedDB?: IDBFactory;
     /** Optional database name. The same name must be used to open the same database. */
     dbName?: string;
     /** Optional factory to spin up a Worker to execute the IDB transactions within. */
diff --git a/src/webrtc/call.ts b/src/webrtc/call.ts
index c2b24cadb..c1203e247 100644
--- a/src/webrtc/call.ts
+++ b/src/webrtc/call.ts
@@ -3016,9 +3016,9 @@ export function supportsMatrixCall(): boolean {
     // is that the browser throwing a SecurityError will brick the client creation process.
     try {
         const supported = Boolean(
-            window.RTCPeerConnection ||
-                window.RTCSessionDescription ||
-                window.RTCIceCandidate ||
+            window.RTCPeerConnection ??
+                window.RTCSessionDescription ??
+                window.RTCIceCandidate ??
                 navigator.mediaDevices,
         );
         if (!supported) {
diff --git a/src/webrtc/groupCall.ts b/src/webrtc/groupCall.ts
index 6f0266d4a..0d2538538 100644
--- a/src/webrtc/groupCall.ts
+++ b/src/webrtc/groupCall.ts
@@ -1445,7 +1445,7 @@ export class GroupCall extends TypedEventEmitter<
      * Recalculates and updates the participant map to match the room state.
      */
     private updateParticipants(): void {
-        const localMember = this.room.getMember(this.client.getUserId()!)!;
+        const localMember = this.room.getMember(this.client.getSafeUserId());
         if (!localMember) {
             // The client hasn't fetched enough of the room state to get our own member
             // event. This probably shouldn't happen, but sanity check & exit for now.
diff --git a/src/webrtc/stats/callFeedStatsReporter.ts b/src/webrtc/stats/callFeedStatsReporter.ts
index d219f7172..e78438acb 100644
--- a/src/webrtc/stats/callFeedStatsReporter.ts
+++ b/src/webrtc/stats/callFeedStatsReporter.ts
@@ -49,8 +49,8 @@ export class CallFeedStatsReporter {
         return {
             id: track.id,
             kind: track.kind,
-            settingDeviceId: settingDeviceId ? settingDeviceId : "unknown",
-            constrainDeviceId: constrainDeviceId ? constrainDeviceId : "unknown",
+            settingDeviceId: settingDeviceId ?? "unknown",
+            constrainDeviceId: constrainDeviceId ?? "unknown",
             muted: track.muted,
             enabled: track.enabled,
             readyState: track.readyState,
@@ -63,9 +63,6 @@ export class CallFeedStatsReporter {
         callFeeds: CallFeed[],
         prefix = "unknown",
     ): CallFeedReport {
-        if (!report.callFeeds) {
-            report.callFeeds = [];
-        }
         callFeeds.forEach((feed) => {
             const audioTracks = feed.stream.getAudioTracks();
             const videoTracks = feed.stream.getVideoTracks();
diff --git a/src/webrtc/stats/media/mediaTrackHandler.ts b/src/webrtc/stats/media/mediaTrackHandler.ts
index 31f1264b3..8d2108c0b 100644
--- a/src/webrtc/stats/media/mediaTrackHandler.ts
+++ b/src/webrtc/stats/media/mediaTrackHandler.ts
@@ -49,18 +49,12 @@ export class MediaTrackHandler {
 
     public getLocalTrackIdByMid(mid: string): string | undefined {
         const transceiver = this.pc.getTransceivers().find((t) => t.mid === mid);
-        if (transceiver !== undefined && !!transceiver.sender && !!transceiver.sender.track) {
-            return transceiver.sender.track.id;
-        }
-        return undefined;
+        return transceiver?.sender?.track?.id;
     }
 
     public getRemoteTrackIdByMid(mid: string): string | undefined {
         const transceiver = this.pc.getTransceivers().find((t) => t.mid === mid);
-        if (transceiver !== undefined && !!transceiver.receiver && !!transceiver.receiver.track) {
-            return transceiver.receiver.track.id;
-        }
-        return undefined;
+        return transceiver?.receiver?.track?.id;
     }
 
     public getActiveSimulcastStreams(): number {