diff --git a/package.json b/package.json
index 0105b5146..9944fbd28 100644
--- a/package.json
+++ b/package.json
@@ -15,7 +15,7 @@
     "build:minify-browser": "terser dist/browser-matrix.js --compress --mangle --source-map --output dist/browser-matrix.min.js",
     "gendoc": "jsdoc -c jsdoc.json -P package.json",
     "lint": "yarn lint:types && yarn lint:js",
-    "lint:js": "eslint --max-warnings 7 src spec",
+    "lint:js": "eslint --max-warnings 4 src spec",
     "lint:js-fix": "eslint --fix src spec",
     "lint:types": "tsc --noEmit",
     "test": "jest",
diff --git a/spec/integ/matrix-client-retrying.spec.ts b/spec/integ/matrix-client-retrying.spec.ts
index 99214ca62..d0335668f 100644
--- a/spec/integ/matrix-client-retrying.spec.ts
+++ b/spec/integ/matrix-client-retrying.spec.ts
@@ -4,13 +4,13 @@ import { Room } from "../../src/models/room";
 import { TestClient } from "../TestClient";
 
 describe("MatrixClient retrying", function() {
-    let client = null;
-    let httpBackend = null;
+    let client: TestClient = null;
+    let httpBackend: TestClient["httpBackend"] = null;
     let scheduler;
     const userId = "@alice:localhost";
     const accessToken = "aseukfgwef";
     const roomId = "!room:here";
-    let room;
+    let room: Room;
 
     beforeEach(function() {
         scheduler = new MatrixScheduler();
@@ -53,10 +53,10 @@ describe("MatrixClient retrying", function() {
         const p1 = client.sendMessage(roomId, {
             "msgtype": "m.text",
             "body": "m1",
-        }).then(function(ev) {
+        }).then(function() {
             // we expect the first message to fail
             throw new Error('Message 1 unexpectedly sent successfully');
-        }, (e) => {
+        }, () => {
             // this is expected
         });
 
@@ -78,7 +78,7 @@ describe("MatrixClient retrying", function() {
         expect(ev2.status).toEqual(EventStatus.SENDING);
 
         // the first message should get sent, and the second should get queued
-        httpBackend.when("PUT", "/send/m.room.message/").check(function(rq) {
+        httpBackend.when("PUT", "/send/m.room.message/").check(function() {
             // ev2 should now have been queued
             expect(ev2.status).toEqual(EventStatus.QUEUED);
 
diff --git a/spec/unit/models/MSC3089TreeSpace.spec.ts b/spec/unit/models/MSC3089TreeSpace.spec.ts
index 61d1daace..9c3a93e53 100644
--- a/spec/unit/models/MSC3089TreeSpace.spec.ts
+++ b/spec/unit/models/MSC3089TreeSpace.spec.ts
@@ -1018,7 +1018,7 @@ describe("MSC3089TreeSpace", () => {
     it('should return falsy for unknown files', () => {
         const fileEventId = "$file";
         room.currentState = {
-            getStateEvents: (eventType: string, stateKey?: string) => {
+            getStateEvents: (eventType: string, stateKey?: string): MatrixEvent[] | MatrixEvent | null => {
                 expect(eventType).toEqual(UNSTABLE_MSC3089_BRANCH.unstable); // test to ensure we're definitely using unstable
                 expect(stateKey).toEqual(fileEventId);
                 return null;
diff --git a/spec/unit/utils.spec.ts b/spec/unit/utils.spec.ts
index 12a83b235..61d11b1cf 100644
--- a/spec/unit/utils.spec.ts
+++ b/spec/unit/utils.spec.ts
@@ -111,10 +111,10 @@ describe("utils", function() {
 
     describe("deepCompare", function() {
         const assert = {
-            isTrue: function(x) {
+            isTrue: function(x: any) {
                 expect(x).toBe(true);
             },
-            isFalse: function(x) {
+            isFalse: function(x: any) {
                 expect(x).toBe(false);
             },
         };
@@ -176,10 +176,10 @@ describe("utils", function() {
             // no two different function is equal really, they capture their
             // context variables so even if they have same toString(), they
             // won't have same functionality
-            const func = function(x) {
+            const func = function() {
                 return true;
             };
-            const func2 = function(x) {
+            const func2 = function() {
                 return true;
             };
             assert.isTrue(utils.deepCompare(func, func));
@@ -189,66 +189,6 @@ describe("utils", function() {
         });
     });
 
-    describe("extend", function() {
-        const SOURCE = { "prop2": 1, "string2": "x", "newprop": "new" };
-
-        it("should extend", function() {
-            const target = {
-                "prop1": 5, "prop2": 7, "string1": "baz", "string2": "foo",
-            };
-            const merged = {
-                "prop1": 5, "prop2": 1, "string1": "baz", "string2": "x",
-                "newprop": "new",
-            };
-            const sourceOrig = JSON.stringify(SOURCE);
-
-            utils.extend(target, SOURCE);
-            expect(JSON.stringify(target)).toEqual(JSON.stringify(merged));
-
-            // check the originial wasn't modified
-            expect(JSON.stringify(SOURCE)).toEqual(sourceOrig);
-        });
-
-        it("should ignore null", function() {
-            const target = {
-                "prop1": 5, "prop2": 7, "string1": "baz", "string2": "foo",
-            };
-            const merged = {
-                "prop1": 5, "prop2": 1, "string1": "baz", "string2": "x",
-                "newprop": "new",
-            };
-            const sourceOrig = JSON.stringify(SOURCE);
-
-            utils.extend(target, null, SOURCE);
-            expect(JSON.stringify(target)).toEqual(JSON.stringify(merged));
-
-            // check the originial wasn't modified
-            expect(JSON.stringify(SOURCE)).toEqual(sourceOrig);
-        });
-
-        it("should handle properties created with defineProperties", function() {
-            const source = Object.defineProperties({}, {
-                "enumerableProp": {
-                    get: function() {
-                        return true;
-                    },
-                    enumerable: true,
-                },
-                "nonenumerableProp": {
-                    get: function() {
-                        return true;
-                    },
-                },
-            });
-
-            // TODO: Fix type
-            const target: any = {};
-            utils.extend(target, source);
-            expect(target.enumerableProp).toBe(true);
-            expect(target.nonenumerableProp).toBe(undefined);
-        });
-    });
-
     describe("chunkPromises", function() {
         it("should execute promises in chunks", async function() {
             let promiseCount = 0;
@@ -273,7 +213,7 @@ describe("utils", function() {
         it('should retry', async () => {
             let count = 0;
             const val = {};
-            const fn = (attempt) => {
+            const fn = (attempt: any) => {
                 count++;
 
                 // If this expectation fails then it can appear as a Jest Timeout due to
@@ -480,7 +420,7 @@ describe("utils", function() {
                 },
                 [72]: "test",
             };
-            const output = [
+            const output: any = [
                 ["72", "test"],
                 ["a", 42],
                 ["b", [
diff --git a/spec/unit/webrtc/call.spec.ts b/spec/unit/webrtc/call.spec.ts
index 9e12db20f..bcded3fb7 100644
--- a/spec/unit/webrtc/call.spec.ts
+++ b/spec/unit/webrtc/call.spec.ts
@@ -17,6 +17,7 @@ limitations under the License.
 import { TestClient } from '../../TestClient';
 import { MatrixCall, CallErrorCode, CallEvent } from '../../../src/webrtc/call';
 import { SDPStreamMetadataKey, SDPStreamMetadataPurpose } from '../../../src/webrtc/callEventTypes';
+import { RoomMember } from "../../../src";
 
 const DUMMY_SDP = (
     "v=0\r\n" +
@@ -85,7 +86,7 @@ class MockRTCPeerConnection {
 
 class MockMediaStream {
     constructor(
-        public id,
+        public id: string,
     ) {}
 
     getTracks() { return []; }
@@ -362,7 +363,7 @@ describe('Call', function() {
         await callPromise;
 
         call.getOpponentMember = () => {
-            return { userId: "@bob:bar.uk" };
+            return { userId: "@bob:bar.uk" } as RoomMember;
         };
 
         await call.onAnswerReceived({
diff --git a/src/ReEmitter.ts b/src/ReEmitter.ts
index b1060cbb8..03c13dd60 100644
--- a/src/ReEmitter.ts
+++ b/src/ReEmitter.ts
@@ -30,7 +30,7 @@ export class ReEmitter {
             // We include the source as the last argument for event handlers which may need it,
             // such as read receipt listeners on the client class which won't have the context
             // of the room.
-            const forSource = (...args) => {
+            const forSource = (...args: any[]) => {
                 // EventEmitter special cases 'error' to make the emit function throw if no
                 // handler is attached, which sort of makes sense for making sure that something
                 // handles an error, but for re-emitting, there could be a listener on the original
diff --git a/src/client.ts b/src/client.ts
index ecb684cfc..2244632f6 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -1423,7 +1423,7 @@ export class MatrixClient extends EventEmitter {
         // We swallow errors because we need a default object anyhow
         return this.http.authedRequest(
             undefined, "GET", "/capabilities",
-        ).catch((e) => {
+        ).catch((e: Error) => {
             logger.error(e);
             return null; // otherwise consume the error
         }).then((r) => {
@@ -3106,7 +3106,7 @@ export class MatrixClient extends EventEmitter {
      * data event.
      * @return {module:http-api.MatrixError} Rejects: with an error response.
      */
-    public async getAccountDataFromServer(eventType: string): Promise<Record<string, any>> {
+    public async getAccountDataFromServer<T>(eventType: string): Promise<T> {
         if (this.isInitialSyncComplete()) {
             const event = this.store.getAccountData(eventType);
             if (!event) {
@@ -3201,7 +3201,7 @@ export class MatrixClient extends EventEmitter {
             );
         }
 
-        const queryString = {};
+        const queryString: Record<string, string | string[]> = {};
         if (opts.viaServers) {
             queryString["server_name"] = opts.viaServers;
         }
@@ -3411,7 +3411,7 @@ export class MatrixClient extends EventEmitter {
         content: IContent,
         txnId?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendEvent(
         roomId: string,
         threadId: string | null,
@@ -3419,7 +3419,7 @@ export class MatrixClient extends EventEmitter {
         content: IContent,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendEvent(
         roomId: string,
         threadId: string | null,
@@ -3699,14 +3699,14 @@ export class MatrixClient extends EventEmitter {
         eventId: string,
         txnId?: string | undefined,
         cbOrOpts?: Callback | IRedactOpts,
-    );
+    ): Promise<ISendEventResponse>;
     public redactEvent(
         roomId: string,
         threadId: string | null,
         eventId: string,
         txnId?: string | undefined,
         cbOrOpts?: Callback | IRedactOpts,
-    );
+    ): Promise<ISendEventResponse>;
     public redactEvent(
         roomId: string,
         threadId: string | null,
@@ -3744,14 +3744,14 @@ export class MatrixClient extends EventEmitter {
         content: IContent,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendMessage(
         roomId: string,
         threadId: string | null,
         content: IContent,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendMessage(
         roomId: string,
         threadId: string | null | IContent,
@@ -3793,14 +3793,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendTextMessage(
         roomId: string,
         threadId: string | null,
         body: string,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendTextMessage(
         roomId: string,
         threadId: string | null,
@@ -3832,14 +3832,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendNotice(
         roomId: string,
         threadId: string | null,
         body: string,
         txnId?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendNotice(
         roomId: string,
         threadId: string | null,
@@ -3871,14 +3871,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         txnId?: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendEmoteMessage(
         roomId: string,
         threadId: string | null,
         body: string,
         txnId?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendEmoteMessage(
         roomId: string,
         threadId: string | null,
@@ -3912,7 +3912,7 @@ export class MatrixClient extends EventEmitter {
         info?: IImageInfo,
         text?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendImageMessage(
         roomId: string,
         threadId: string | null,
@@ -3920,7 +3920,7 @@ export class MatrixClient extends EventEmitter {
         info?: IImageInfo,
         text?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendImageMessage(
         roomId: string,
         threadId: string | null,
@@ -3965,7 +3965,7 @@ export class MatrixClient extends EventEmitter {
         info?: IImageInfo,
         text?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendStickerMessage(
         roomId: string,
         threadId: string | null,
@@ -3973,7 +3973,7 @@ export class MatrixClient extends EventEmitter {
         info?: IImageInfo,
         text?: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendStickerMessage(
         roomId: string,
         threadId: string | null,
@@ -4015,14 +4015,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         htmlBody: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendHtmlMessage(
         roomId: string,
         threadId: string | null,
         body: string,
         htmlBody: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendHtmlMessage(
         roomId: string,
         threadId: string | null,
@@ -4053,14 +4053,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         htmlBody: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendHtmlNotice(
         roomId: string,
         threadId: string | null,
         body: string,
         htmlBody: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendHtmlNotice(
         roomId: string,
         threadId: string | null,
@@ -4092,14 +4092,14 @@ export class MatrixClient extends EventEmitter {
         body: string,
         htmlBody: string,
         callback?: Callback,
-    );
+    ): Promise<ISendEventResponse>;
     public sendHtmlEmote(
         roomId: string,
         threadId: string | null,
         body: string,
         htmlBody: string,
         callback?: Callback,
-    )
+    ): Promise<ISendEventResponse>;
     public sendHtmlEmote(
         roomId: string,
         threadId: string | null,
@@ -4420,7 +4420,7 @@ export class MatrixClient extends EventEmitter {
                 errcode: "ORG.MATRIX.JSSDK_MISSING_PARAM",
             }));
         }
-        const params = {
+        const params: Record<string, string> = {
             id_server: identityServerUrl,
             medium: medium,
             address: address,
@@ -4478,10 +4478,10 @@ export class MatrixClient extends EventEmitter {
             }
         }
 
-        const populationResults = {}; // {roomId: Error}
+        const populationResults: Record<string, Error> = {}; // {roomId: Error}
         const promises = [];
 
-        const doLeave = (roomId) => {
+        const doLeave = (roomId: string) => {
             return this.leave(roomId).then(() => {
                 populationResults[roomId] = null;
             }).catch((err) => {
@@ -5065,14 +5065,14 @@ export class MatrixClient extends EventEmitter {
             return pendingRequest;
         }
 
-        let path;
-        let params;
+        let path: string;
+        let params: Record<string, string>;
         let promise;
 
         if (isNotifTimeline) {
             path = "/notifications";
             params = {
-                limit: ('limit' in opts) ? opts.limit : 30,
+                limit: (opts.limit ?? 30).toString(),
                 only: 'highlight',
             };
 
@@ -5509,7 +5509,7 @@ export class MatrixClient extends EventEmitter {
      * @return {module:http-api.MatrixError} Rejects: with an error response.
      */
     public setRoomMutePushRule(scope: string, roomId: string, mute: boolean): Promise<void> | void {
-        let deferred;
+        let promise: Promise<void>;
         let hasDontNotifyRule;
 
         // Get the existing room-kind push rule if any
@@ -5523,17 +5523,17 @@ export class MatrixClient extends EventEmitter {
         if (!mute) {
             // Remove the rule only if it is a muting rule
             if (hasDontNotifyRule) {
-                deferred = this.deletePushRule(scope, PushRuleKind.RoomSpecific, roomPushRule.rule_id);
+                promise = this.deletePushRule(scope, PushRuleKind.RoomSpecific, roomPushRule.rule_id);
             }
         } else {
             if (!roomPushRule) {
-                deferred = this.addPushRule(scope, PushRuleKind.RoomSpecific, roomId, {
+                promise = this.addPushRule(scope, PushRuleKind.RoomSpecific, roomId, {
                     actions: ["dont_notify"],
                 });
             } else if (!hasDontNotifyRule) {
                 // Remove the existing one before setting the mute push rule
                 // This is a workaround to SYN-590 (Push rule update fails)
-                deferred = utils.defer();
+                const deferred = utils.defer();
                 this.deletePushRule(scope, PushRuleKind.RoomSpecific, roomPushRule.rule_id)
                     .then(() => {
                         this.addPushRule(scope, PushRuleKind.RoomSpecific, roomId, {
@@ -5547,21 +5547,21 @@ export class MatrixClient extends EventEmitter {
                         deferred.reject(err);
                     });
 
-                deferred = deferred.promise;
+                promise = deferred.promise;
             }
         }
 
-        if (deferred) {
+        if (promise) {
             return new Promise<void>((resolve, reject) => {
                 // Update this.pushRules when the operation completes
-                deferred.then(() => {
+                promise.then(() => {
                     this.getPushRules().then((result) => {
                         this.pushRules = result;
                         resolve();
                     }).catch((err) => {
                         reject(err);
                     });
-                }).catch((err) => {
+                }).catch((err: Error) => {
                     // Update it even if the previous operation fails. This can help the
                     // app to recover when push settings has been modifed from another client
                     this.getPushRules().then((result) => {
@@ -6281,7 +6281,7 @@ export class MatrixClient extends EventEmitter {
             fetchedEventType,
             opts);
         const mapper = this.getEventMapper();
-        let originalEvent;
+        let originalEvent: MatrixEvent;
         if (result.original_event) {
             originalEvent = mapper(result.original_event);
         }
@@ -6564,7 +6564,7 @@ export class MatrixClient extends EventEmitter {
         };
 
         // merge data into loginData
-        utils.extend(loginData, data);
+        Object.assign(loginData, data);
 
         return this.http.authedRequest(
             (error, response) => {
@@ -7544,7 +7544,7 @@ export class MatrixClient extends EventEmitter {
      * @return {module:http-api.MatrixError} Rejects: with an error response.
      */
     public getPushRules(callback?: Callback): Promise<IPushRules> {
-        return this.http.authedRequest(callback, "GET", "/pushrules/").then(rules => {
+        return this.http.authedRequest(callback, "GET", "/pushrules/").then((rules: IPushRules) => {
             return PushProcessor.rewriteDefaultRules(rules);
         });
     }
@@ -8018,7 +8018,7 @@ export class MatrixClient extends EventEmitter {
         addressPairs: [string, string][],
         identityAccessToken: string,
     ): Promise<{ address: string, mxid: string }[]> {
-        const params = {
+        const params: Record<string, string | string[]> = {
             // addresses: ["email@example.org", "10005550000"],
             // algorithm: "sha256",
             // pepper: "abc123"
@@ -8032,7 +8032,7 @@ export class MatrixClient extends EventEmitter {
 
         params['pepper'] = hashes['lookup_pepper'];
 
-        const localMapping = {
+        const localMapping: Record<string, string> = {
             // hashed identifier => plain text address
             // For use in this function's return format
         };
diff --git a/src/content-repo.ts b/src/content-repo.ts
index 287259651..febd0d1c9 100644
--- a/src/content-repo.ts
+++ b/src/content-repo.ts
@@ -53,13 +53,13 @@ export function getHttpUriForMxc(
     }
     let serverAndMediaId = mxc.slice(6); // strips mxc://
     let prefix = "/_matrix/media/r0/download/";
-    const params = {};
+    const params: Record<string, string> = {};
 
     if (width) {
-        params["width"] = Math.round(width);
+        params["width"] = Math.round(width).toString();
     }
     if (height) {
-        params["height"] = Math.round(height);
+        params["height"] = Math.round(height).toString();
     }
     if (resizeMethod) {
         params["method"] = resizeMethod;
diff --git a/src/crypto/DeviceList.ts b/src/crypto/DeviceList.ts
index d27af3c54..c8a87cc56 100644
--- a/src/crypto/DeviceList.ts
+++ b/src/crypto/DeviceList.ts
@@ -265,8 +265,8 @@ export class DeviceList extends EventEmitter {
      * module:crypto/deviceinfo|DeviceInfo}.
      */
     public downloadKeys(userIds: string[], forceDownload: boolean): Promise<DeviceInfoMap> {
-        const usersToDownload = [];
-        const promises = [];
+        const usersToDownload: string[] = [];
+        const promises: Promise<unknown>[] = [];
 
         userIds.forEach((u) => {
             const trackingStatus = this.deviceTrackingStatus[u];
@@ -633,7 +633,7 @@ export class DeviceList extends EventEmitter {
             }
         });
 
-        const finished = (success) => {
+        const finished = (success: boolean): void => {
             this.emit("crypto.willUpdateDevices", users, !this.hasFetched);
             users.forEach((u) => {
                 this.dirty = true;
diff --git a/src/crypto/SecretStorage.ts b/src/crypto/SecretStorage.ts
index fb5665fa6..877d7507c 100644
--- a/src/crypto/SecretStorage.ts
+++ b/src/crypto/SecretStorage.ts
@@ -37,7 +37,7 @@ export interface ISecretRequest {
 
 export interface IAccountDataClient extends EventEmitter {
     // Subset of MatrixClient (which also uses any for the event content)
-    getAccountDataFromServer: (eventType: string) => Promise<Record<string, any>>;
+    getAccountDataFromServer: <T>(eventType: string) => Promise<T>;
     getAccountData: (eventType: string) => MatrixEvent;
     setAccountData: (eventType: string, content: any) => Promise<{}>;
 }
@@ -76,7 +76,7 @@ export class SecretStorage {
     ) {}
 
     public async getDefaultKeyId(): Promise<string> {
-        const defaultKey = await this.accountDataAdapter.getAccountDataFromServer(
+        const defaultKey = await this.accountDataAdapter.getAccountDataFromServer<{ key: string }>(
             'm.secret_storage.default_key',
         );
         if (!defaultKey) return null;
@@ -230,7 +230,7 @@ export class SecretStorage {
      *     or null/undefined to use the default key.
      */
     public async store(name: string, secret: string, keys?: string[]): Promise<void> {
-        const encrypted = {};
+        const encrypted: Record<string, IEncryptedPayload> = {};
 
         if (!keys) {
             const defaultKeyId = await this.getDefaultKeyId();
@@ -246,9 +246,9 @@ export class SecretStorage {
 
         for (const keyId of keys) {
             // get key information from key storage
-            const keyInfo = await this.accountDataAdapter.getAccountDataFromServer(
+            const keyInfo = await this.accountDataAdapter.getAccountDataFromServer<ISecretStorageKeyInfo>(
                 "m.secret_storage.key." + keyId,
-            ) as ISecretStorageKeyInfo;
+            );
             if (!keyInfo) {
                 throw new Error("Unknown key: " + keyId);
             }
@@ -277,7 +277,7 @@ export class SecretStorage {
      * @return {string} the contents of the secret
      */
     public async get(name: string): Promise<string> {
-        const secretInfo = await this.accountDataAdapter.getAccountDataFromServer(name);
+        const secretInfo = await this.accountDataAdapter.getAccountDataFromServer<any>(name); // TODO types
         if (!secretInfo) {
             return;
         }
@@ -286,11 +286,13 @@ export class SecretStorage {
         }
 
         // get possible keys to decrypt
-        const keys = {};
+        const keys: Record<string, ISecretStorageKeyInfo> = {};
         for (const keyId of Object.keys(secretInfo.encrypted)) {
             // get key information from key storage
-            const keyInfo = await this.accountDataAdapter.getAccountDataFromServer(
-                "m.secret_storage.key." + keyId,
+            const keyInfo = (
+                await this.accountDataAdapter.getAccountDataFromServer<ISecretStorageKeyInfo>(
+                    "m.secret_storage.key." + keyId,
+                )
             );
             const encInfo = secretInfo.encrypted[keyId];
             // only use keys we understand the encryption algorithm of
@@ -306,7 +308,7 @@ export class SecretStorage {
                 `the keys it is encrypted with are for a supported algorithm`);
         }
 
-        let keyId;
+        let keyId: string;
         let decryption;
         try {
             // fetch private key from app
@@ -337,7 +339,7 @@ export class SecretStorage {
      */
     public async isStored(name: string, checkKey: boolean): Promise<Record<string, ISecretStorageKeyInfo>> {
         // check if secret exists
-        const secretInfo = await this.accountDataAdapter.getAccountDataFromServer(name);
+        const secretInfo = await this.accountDataAdapter.getAccountDataFromServer<any>(name); // TODO types
         if (!secretInfo) return null;
         if (!secretInfo.encrypted) {
             return null;
@@ -350,7 +352,7 @@ export class SecretStorage {
         // filter secret encryption keys with supported algorithm
         for (const keyId of Object.keys(secretInfo.encrypted)) {
             // get key information from key storage
-            const keyInfo = await this.accountDataAdapter.getAccountDataFromServer(
+            const keyInfo = await this.accountDataAdapter.getAccountDataFromServer<any>( // TODO types
                 "m.secret_storage.key." + keyId,
             );
             if (!keyInfo) continue;
@@ -375,8 +377,8 @@ export class SecretStorage {
     public request(name: string, devices: string[]): ISecretRequest {
         const requestId = this.baseApis.makeTxnId();
 
-        let resolve: (string) => void;
-        let reject: (Error) => void;
+        let resolve: (s: string) => void;
+        let reject: (e: Error) => void;
         const promise = new Promise<string>((res, rej) => {
             resolve = res;
             reject = rej;
diff --git a/src/crypto/algorithms/base.ts b/src/crypto/algorithms/base.ts
index 89fa7034d..add9111ef 100644
--- a/src/crypto/algorithms/base.ts
+++ b/src/crypto/algorithms/base.ts
@@ -46,7 +46,7 @@ type DecryptionClassParams = Omit<IParams, "deviceId" | "config">;
  */
 export const DECRYPTION_CLASSES: Record<string, new (params: DecryptionClassParams) => DecryptionAlgorithm> = {};
 
-interface IParams {
+export interface IParams {
     userId: string;
     deviceId: string;
     crypto: Crypto;
diff --git a/src/crypto/algorithms/megolm.ts b/src/crypto/algorithms/megolm.ts
index 167c81130..d5a591997 100644
--- a/src/crypto/algorithms/megolm.ts
+++ b/src/crypto/algorithms/megolm.ts
@@ -26,6 +26,7 @@ import {
     DecryptionAlgorithm,
     DecryptionError,
     EncryptionAlgorithm,
+    IParams,
     registerAlgorithm,
     UnknownDeviceError,
 } from "./base";
@@ -99,6 +100,12 @@ interface IPayload extends Partial<IMessage> {
     algorithm?: string;
     sender_key?: string;
 }
+
+interface IEncryptedContent {
+    algorithm: string;
+    sender_key: string;
+    ciphertext: Record<string, string>;
+}
 /* eslint-enable camelcase */
 
 interface SharedWithData {
@@ -238,7 +245,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
         startTime: number;
     };
 
-    constructor(params) {
+    constructor(params: IParams) {
         super(params);
 
         this.sessionRotationPeriodMsgs = params.config?.rotation_period_msgs ?? 100;
@@ -263,7 +270,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
         blocked: IBlockedMap,
         singleOlmCreationPhase = false,
     ): Promise<OutboundSessionInfo> {
-        let session;
+        let session: OutboundSessionInfo;
 
         // takes the previous OutboundSessionInfo, and considers whether to create
         // a new one. Also shares the key with any (new) devices in the room.
@@ -302,7 +309,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
             }
 
             // now check if we need to share with any devices
-            const shareMap = {};
+            const shareMap: Record<string, DeviceInfo[]> = {};
 
             for (const [userId, userDevices] of Object.entries(devicesInRoom)) {
                 for (const [deviceId, deviceInfo] of Object.entries(userDevices)) {
@@ -350,7 +357,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
                         `Sharing keys (start phase 1) with new Olm sessions in ${this.roomId}`,
                         devicesWithoutSession,
                     );
-                    const errorDevices = [];
+                    const errorDevices: IOlmDevice[] = [];
 
                     // meanwhile, establish olm sessions for devices that we don't
                     // already have a session for, and share keys with them.  If
@@ -358,7 +365,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
                     // shorter timeout when fetching one-time keys for the first
                     // phase.
                     const start = Date.now();
-                    const failedServers = [];
+                    const failedServers: string[] = [];
                     await this.shareKeyWithDevices(
                         session, key, payload, devicesWithoutSession, errorDevices,
                         singleOlmCreationPhase ? 10000 : 2000, failedServers,
@@ -374,7 +381,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
                             // do this in the background and don't block anything else while we
                             // do this.  We only need to retry users from servers that didn't
                             // respond the first time.
-                            const retryDevices = {};
+                            const retryDevices: Record<string, DeviceInfo[]> = {};
                             const failedServerMap = new Set;
                             for (const server of failedServers) {
                                 failedServerMap.add(server);
@@ -584,12 +591,12 @@ class MegolmEncryption extends EncryptionAlgorithm {
         userDeviceMap: IOlmDevice[],
         payload: IPayload,
     ): Promise<void> {
-        const contentMap = {};
+        const contentMap: Record<string, Record<string, IEncryptedContent>> = {};
         const deviceInfoByDeviceId = new Map<string, DeviceInfo>();
 
-        const promises = [];
+        const promises: Promise<unknown>[] = [];
         for (let i = 0; i < userDeviceMap.length; i++) {
-            const encryptedContent = {
+            const encryptedContent: IEncryptedContent = {
                 algorithm: olmlib.OLM_ALGORITHM,
                 sender_key: this.olmDevice.deviceCurve25519Key,
                 ciphertext: {},
@@ -679,7 +686,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
         userDeviceMap: IOlmDevice<IBlockedDevice>[],
         payload: IPayload,
     ): Promise<void> {
-        const contentMap = {};
+        const contentMap: Record<string, Record<string, IPayload>> = {};
 
         for (const val of userDeviceMap) {
             const userId = val.userId;
@@ -1105,7 +1112,7 @@ class MegolmEncryption extends EncryptionAlgorithm {
      *   devices we should shared the session with.
      */
     private checkForUnknownDevices(devicesInRoom: DeviceInfoMap): void {
-        const unknownDevices = {};
+        const unknownDevices: Record<string, Record<string, DeviceInfo>> = {};
 
         Object.keys(devicesInRoom).forEach((userId)=>{
             Object.keys(devicesInRoom[userId]).forEach((deviceId)=>{
@@ -1304,8 +1311,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
                 content.sender_key, event.getTs() - 120000,
             );
             if (problem) {
-                let problemDescription = PROBLEM_DESCRIPTIONS[problem.type]
-                    || PROBLEM_DESCRIPTIONS.unknown;
+                let problemDescription = PROBLEM_DESCRIPTIONS[problem.type as "no_olm"] || PROBLEM_DESCRIPTIONS.unknown;
                 if (problem.fixed) {
                     problemDescription +=
                         " Trying to create a new secure channel and re-requesting the keys.";
@@ -1399,14 +1405,14 @@ class MegolmDecryption extends DecryptionAlgorithm {
         const senderKey = content.sender_key;
         const sessionId = content.session_id;
         const senderPendingEvents = this.pendingEvents[senderKey];
-        const pendingEvents = senderPendingEvents && senderPendingEvents.get(sessionId);
+        const pendingEvents = senderPendingEvents?.get(sessionId);
         if (!pendingEvents) {
             return;
         }
 
         pendingEvents.delete(event);
         if (pendingEvents.size === 0) {
-            senderPendingEvents.delete(senderKey);
+            senderPendingEvents.delete(sessionId);
         }
         if (senderPendingEvents.size === 0) {
             delete this.pendingEvents[senderKey];
@@ -1760,7 +1766,7 @@ class MegolmDecryption extends DecryptionAlgorithm {
         }));
 
         // If decrypted successfully, they'll have been removed from pendingEvents
-        return !((this.pendingEvents[senderKey] || {})[sessionId]);
+        return !this.pendingEvents[senderKey]?.has(sessionId);
     }
 
     public async retryDecryptionFromSender(senderKey: string): Promise<boolean> {
@@ -1794,12 +1800,12 @@ class MegolmDecryption extends DecryptionAlgorithm {
         for (const [senderKey, sessionId] of sharedHistorySessions) {
             const payload = await this.buildKeyForwardingMessage(this.roomId, senderKey, sessionId);
 
-            const promises = [];
-            const contentMap = {};
+            const promises: Promise<unknown>[] = [];
+            const contentMap: Record<string, Record<string, IEncryptedContent>> = {};
             for (const [userId, devices] of Object.entries(devicesByUser)) {
                 contentMap[userId] = {};
                 for (const deviceInfo of devices) {
-                    const encryptedContent = {
+                    const encryptedContent: IEncryptedContent = {
                         algorithm: olmlib.OLM_ALGORITHM,
                         sender_key: this.olmDevice.deviceCurve25519Key,
                         ciphertext: {},
diff --git a/src/crypto/algorithms/olm.ts b/src/crypto/algorithms/olm.ts
index 932871783..369178737 100644
--- a/src/crypto/algorithms/olm.ts
+++ b/src/crypto/algorithms/olm.ts
@@ -282,7 +282,7 @@ class OlmDecryption extends DecryptionAlgorithm {
         const sessionIds = await this.olmDevice.getSessionIdsForDevice(theirDeviceIdentityKey);
 
         // try each session in turn.
-        const decryptionErrors = {};
+        const decryptionErrors: Record<string, string> = {};
         for (let i = 0; i < sessionIds.length; i++) {
             const sessionId = sessionIds[i];
             try {
diff --git a/src/crypto/index.ts b/src/crypto/index.ts
index be30eb8bb..828067461 100644
--- a/src/crypto/index.ts
+++ b/src/crypto/index.ts
@@ -58,7 +58,7 @@ import { BackupManager } from "./backup";
 import { IStore } from "../store";
 import { Room } from "../models/room";
 import { RoomMember } from "../models/room-member";
-import { MatrixEvent, EventStatus } from "../models/event";
+import { MatrixEvent, EventStatus, IClearEvent } from "../models/event";
 import { MatrixClient, IKeysUploadResponse, SessionStore, ISignedKey, ICrossSigningKey } from "../client";
 import type { EncryptionAlgorithm, DecryptionAlgorithm } from "./algorithms/base";
 import type { IRoomEncryption, RoomList } from "./RoomList";
@@ -172,10 +172,10 @@ interface ISignableObject {
 }
 
 export interface IEventDecryptionResult {
-    clearEvent: object;
+    clearEvent: IClearEvent;
+    forwardingCurve25519KeyChain?: string[];
     senderCurve25519Key?: string;
     claimedEd25519Key?: string;
-    forwardingCurve25519KeyChain?: string[];
     untrusted?: boolean;
 }
 
diff --git a/src/crypto/keybackup.ts b/src/crypto/keybackup.ts
index 4d8981ff8..1b7ae5240 100644
--- a/src/crypto/keybackup.ts
+++ b/src/crypto/keybackup.ts
@@ -67,5 +67,5 @@ export interface IKeyBackupRestoreResult {
 
 export interface IKeyBackupRestoreOpts {
     cacheCompleteCallback?: () => void;
-    progressCallback?: ({ stage: string }) => void;
+    progressCallback?: (progress: { stage: string }) => void;
 }
diff --git a/src/crypto/olmlib.ts b/src/crypto/olmlib.ts
index 72268c3a2..f0424f672 100644
--- a/src/crypto/olmlib.ts
+++ b/src/crypto/olmlib.ts
@@ -27,7 +27,6 @@ import { Logger } from "loglevel";
 import { OlmDevice } from "./OlmDevice";
 import { DeviceInfo } from "./deviceinfo";
 import { logger } from '../logger';
-import * as utils from "../utils";
 import { IOneTimeKey } from "./dehydration";
 import { MatrixClient } from "../client";
 
@@ -126,7 +125,7 @@ export async function encryptMessageForDevice(
     // involved in the session. If we're looking to reduce data transfer in the
     // future, we could elide them for subsequent messages.
 
-    utils.extend(payload, payloadFields);
+    Object.assign(payload, payloadFields);
 
     resultsObject[deviceKey] = await olmDevice.encryptMessage(
         deviceKey, sessionId, JSON.stringify(payload),
diff --git a/src/crypto/verification/request/InRoomChannel.ts b/src/crypto/verification/request/InRoomChannel.ts
index 98dd58e5b..f07c39ffb 100644
--- a/src/crypto/verification/request/InRoomChannel.ts
+++ b/src/crypto/verification/request/InRoomChannel.ts
@@ -36,7 +36,7 @@ const M_RELATES_TO = "m.relates_to";
  * Uses the event id of the initial m.key.verification.request event as a transaction id.
  */
 export class InRoomChannel implements IVerificationChannel {
-    private requestEventId = null;
+    private requestEventId: string = null;
 
     /**
      * @param {MatrixClient} client the matrix client, to send messages with and get current user & device from.
diff --git a/src/http-api.js b/src/http-api.js
index 92c6e420f..a3d3c155b 100644
--- a/src/http-api.js
+++ b/src/http-api.js
@@ -657,7 +657,7 @@ MatrixHttpApi.prototype = {
             };
         }
 
-        const headers = utils.extend({}, opts.headers || {});
+        const headers = Object.assign({}, opts.headers || {});
         const json = opts.json === undefined ? true : opts.json;
         let bodyParser = opts.bodyParser;
 
diff --git a/src/interactive-auth.ts b/src/interactive-auth.ts
index 74d5d30ca..eb00c75db 100644
--- a/src/interactive-auth.ts
+++ b/src/interactive-auth.ts
@@ -18,7 +18,6 @@ limitations under the License.
 
 /** @module interactive-auth */
 
-import * as utils from "./utils";
 import { logger } from './logger';
 import { MatrixClient } from "./client";
 import { defer, IDeferred } from "./utils";
@@ -68,7 +67,7 @@ export enum AuthType {
 export interface IAuthDict {
     // [key: string]: any;
     type?: string;
-    // session?: string; // TODO
+    session?: string;
     // TODO: Remove `user` once servers support proper UIA
     // See https://github.com/vector-im/element-web/issues/10312
     user?: string;
@@ -360,12 +359,12 @@ export class InteractiveAuth {
         }
 
         // use the sessionid from the last request, if one is present.
-        let auth;
+        let auth: IAuthDict;
         if (this.data.session) {
             auth = {
                 session: this.data.session,
             };
-            utils.extend(auth, authData);
+            Object.assign(auth, authData);
         } else {
             auth = authData;
         }
diff --git a/src/logger.ts b/src/logger.ts
index 04763cd33..6084314a3 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -76,7 +76,7 @@ function extendLogger(logger: PrefixedLogger) {
 
 extendLogger(logger);
 
-function getPrefixedLogger(prefix): PrefixedLogger {
+function getPrefixedLogger(prefix: string): PrefixedLogger {
     const prefixLogger: PrefixedLogger = log.getLogger(`${DEFAULT_NAMESPACE}-${prefix}`);
     if (prefixLogger.prefix !== prefix) {
         // Only do this setup work the first time through, as loggers are saved by name.
diff --git a/src/matrix.ts b/src/matrix.ts
index e279997e9..f43d35d72 100644
--- a/src/matrix.ts
+++ b/src/matrix.ts
@@ -121,7 +121,7 @@ export interface ICryptoCallbacks {
     ) => Promise<string>;
     getDehydrationKey?: (
         keyInfo: ISecretStorageKeyInfo,
-        checkFunc: (Uint8Array) => void,
+        checkFunc: (key: Uint8Array) => void,
     ) => Promise<Uint8Array>;
     getBackupKey?: () => Promise<Uint8Array>;
 }
diff --git a/src/models/event-timeline-set.ts b/src/models/event-timeline-set.ts
index 1897d6741..b5c6e4800 100644
--- a/src/models/event-timeline-set.ts
+++ b/src/models/event-timeline-set.ts
@@ -33,7 +33,7 @@ import { Thread } from "./thread";
 // var DEBUG = false;
 const DEBUG = true;
 
-let debuglog;
+let debuglog: (...args: any[]) => void;
 if (DEBUG) {
     // using bind means that we get to keep useful line numbers in the console
     debuglog = logger.log.bind(logger);
diff --git a/src/models/event.ts b/src/models/event.ts
index 7b24ffe1a..29f2df904 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -29,7 +29,7 @@ import {
     MsgType,
     RelationType,
 } from "../@types/event";
-import { Crypto } from "../crypto";
+import { Crypto, IEventDecryptionResult } from "../crypto";
 import { deepSortedObjectEntries } from "../utils";
 import { RoomMember } from "./room-member";
 import { Thread, ThreadEvent } from "./thread";
@@ -85,7 +85,7 @@ export interface IUnsigned {
     age?: number;
     prev_sender?: string;
     prev_content?: IContent;
-    redacted_because?: IEvent;
+    redacted_because?: IClearEvent;
     transaction_id?: string;
     invite_room_state?: StrippedState[];
 }
@@ -124,25 +124,13 @@ export interface IEventRelation {
     key?: string;
 }
 
-interface IDecryptionResult {
-    clearEvent: {
-        room_id?: string;
-        type: string;
-        content: IContent;
-        unsigned?: IUnsigned;
-    };
-    forwardingCurve25519KeyChain?: string[];
-    senderCurve25519Key?: string;
-    claimedEd25519Key?: string;
-    untrusted?: boolean;
-}
-/* eslint-enable camelcase */
-
 export interface IClearEvent {
+    room_id?: string;
     type: string;
     content: Omit<IContent, "membership" | "avatar_url" | "displayname" | "m.relates_to">;
     unsigned?: IUnsigned;
 }
+/* eslint-enable camelcase */
 
 interface IKeyRequestRecipient {
     userId: string;
@@ -215,14 +203,14 @@ export class MatrixEvent extends EventEmitter {
     public sender: RoomMember = null;
     public target: RoomMember = null;
     public status: EventStatus = null;
-    public error = null;
+    public error: Error = null;
     public forwardLooking = true;
 
     /* If the event is a `m.key.verification.request` (or to_device `m.key.verification.start`) event,
      * `Crypto` will set this the `VerificationRequest` for the event
      * so it can be easily accessed from the timeline.
      */
-    public verificationRequest = null;
+    public verificationRequest: VerificationRequest = null;
 
     private readonly reEmitter: ReEmitter;
 
@@ -690,8 +678,8 @@ export class MatrixEvent extends EventEmitter {
         while (true) {
             this.retryDecryption = false;
 
-            let res;
-            let err;
+            let res: IEventDecryptionResult;
+            let err: Error;
             try {
                 if (!crypto) {
                     res = this.badEncryptedMessage("Encryption not enabled");
@@ -779,7 +767,7 @@ export class MatrixEvent extends EventEmitter {
         }
     }
 
-    private badEncryptedMessage(reason: string): IDecryptionResult {
+    private badEncryptedMessage(reason: string): IEventDecryptionResult {
         return {
             clearEvent: {
                 type: "m.room.message",
@@ -803,7 +791,7 @@ export class MatrixEvent extends EventEmitter {
      * @param {module:crypto~EventDecryptionResult} decryptionResult
      *     the decryption result, including the plaintext and some key info
      */
-    private setClearData(decryptionResult: IDecryptionResult): void {
+    private setClearData(decryptionResult: IEventDecryptionResult): void {
         this.clearEvent = decryptionResult.clearEvent;
         this.senderCurve25519Key =
             decryptionResult.senderCurve25519Key || null;
diff --git a/src/models/room.ts b/src/models/room.ts
index e66679ee8..d22082b53 100644
--- a/src/models/room.ts
+++ b/src/models/room.ts
@@ -48,19 +48,19 @@ const SAFE_ROOM_VERSIONS = ['1', '2', '3', '4', '5', '6'];
 
 function synthesizeReceipt(userId: string, event: MatrixEvent, receiptType: string): MatrixEvent {
     // console.log("synthesizing receipt for "+event.getId());
-    // This is really ugly because JS has no way to express an object literal
-    // where the name of a key comes from an expression
-    const fakeReceipt = {
-        content: {},
+    return new MatrixEvent({
+        content: {
+            [event.getId()]: {
+                [receiptType]: {
+                    [userId]: {
+                        ts: event.getTs(),
+                    },
+                },
+            },
+        },
         type: "m.receipt",
         room_id: event.getRoomId(),
-    };
-    fakeReceipt.content[event.getId()] = {};
-    fakeReceipt.content[event.getId()][receiptType] = {};
-    fakeReceipt.content[event.getId()][receiptType][userId] = {
-        ts: event.getTs(),
-    };
-    return new MatrixEvent(fakeReceipt);
+    });
 }
 
 interface IOpts {
@@ -240,7 +240,7 @@ export class Room extends EventEmitter {
             const serializedPendingEventList = client.sessionStore.store.getItem(pendingEventsKey(this.roomId));
             if (serializedPendingEventList) {
                 JSON.parse(serializedPendingEventList)
-                    .forEach(async serializedEvent => {
+                    .forEach(async (serializedEvent: Partial<IEvent>) => {
                         const event = new MatrixEvent(serializedEvent);
                         if (event.getType() === EventType.RoomMessageEncrypted) {
                             await event.attemptDecryption(this.client.crypto);
@@ -997,14 +997,14 @@ export class Room extends EventEmitter {
      * @return {array} The room's alias as an array of strings
      */
     public getAliases(): string[] {
-        const aliasStrings = [];
+        const aliasStrings: string[] = [];
 
         const aliasEvents = this.currentState.getStateEvents(EventType.RoomAliases);
         if (aliasEvents) {
             for (let i = 0; i < aliasEvents.length; ++i) {
                 const aliasEvent = aliasEvents[i];
                 if (Array.isArray(aliasEvent.getContent().aliases)) {
-                    const filteredAliases = aliasEvent.getContent().aliases.filter(a => {
+                    const filteredAliases = aliasEvent.getContent<{ aliases: string[] }>().aliases.filter(a => {
                         if (typeof(a) !== "string") return false;
                         if (a[0] !== '#') return false;
                         if (!a.endsWith(`:${aliasEvent.getStateKey()}`)) return false;
@@ -1992,7 +1992,7 @@ export class Room extends EventEmitter {
      * @return {Object} Map of receipts by event ID
      */
     private buildReceiptCache(receipts: Receipts): ReceiptCache {
-        const receiptCacheByEventId = {};
+        const receiptCacheByEventId: ReceiptCache = {};
         Object.keys(receipts).forEach(function(receiptType) {
             Object.keys(receipts[receiptType]).forEach(function(userId) {
                 const receipt = receipts[receiptType][userId];
@@ -2185,7 +2185,7 @@ export class Room extends EventEmitter {
         }
 
         // get members that are NOT ourselves and are actually in the room.
-        let otherNames = null;
+        let otherNames: string[] = null;
         if (this.summaryHeroes) {
             // if we have a summary, the member state events
             // should be in the room state
@@ -2266,31 +2266,29 @@ function pendingEventsKey(roomId: string): string {
 
 /* a map from current event status to a list of allowed next statuses
      */
-const ALLOWED_TRANSITIONS = {};
-
-ALLOWED_TRANSITIONS[EventStatus.ENCRYPTING] = [
-    EventStatus.SENDING,
-    EventStatus.NOT_SENT,
-];
-
-ALLOWED_TRANSITIONS[EventStatus.SENDING] = [
-    EventStatus.ENCRYPTING,
-    EventStatus.QUEUED,
-    EventStatus.NOT_SENT,
-    EventStatus.SENT,
-];
-
-ALLOWED_TRANSITIONS[EventStatus.QUEUED] =
-    [EventStatus.SENDING, EventStatus.CANCELLED];
-
-ALLOWED_TRANSITIONS[EventStatus.SENT] =
-    [];
-
-ALLOWED_TRANSITIONS[EventStatus.NOT_SENT] =
-    [EventStatus.SENDING, EventStatus.QUEUED, EventStatus.CANCELLED];
-
-ALLOWED_TRANSITIONS[EventStatus.CANCELLED] =
-    [];
+const ALLOWED_TRANSITIONS: Record<EventStatus, EventStatus[]> = {
+    [EventStatus.ENCRYPTING]: [
+        EventStatus.SENDING,
+        EventStatus.NOT_SENT,
+    ],
+    [EventStatus.SENDING]: [
+        EventStatus.ENCRYPTING,
+        EventStatus.QUEUED,
+        EventStatus.NOT_SENT,
+        EventStatus.SENT,
+    ],
+    [EventStatus.QUEUED]: [
+        EventStatus.SENDING,
+        EventStatus.CANCELLED,
+    ],
+    [EventStatus.SENT]: [],
+    [EventStatus.NOT_SENT]: [
+        EventStatus.SENDING,
+        EventStatus.QUEUED,
+        EventStatus.CANCELLED,
+    ],
+    [EventStatus.CANCELLED]: [],
+};
 
 // TODO i18n
 function memberNamesToRoomName(names: string[], count = (names.length + 1)) {
diff --git a/src/scheduler.ts b/src/scheduler.ts
index b83a57eba..d0436bd83 100644
--- a/src/scheduler.ts
+++ b/src/scheduler.ts
@@ -284,7 +284,7 @@ export class MatrixScheduler<T = ISendEventResponse> {
     }
 }
 
-function debuglog(...args) {
+function debuglog(...args: any[]) {
     if (DEBUG) {
         logger.log(...args);
     }
diff --git a/src/store/index.ts b/src/store/index.ts
index ad25a1c7e..f4bf214a7 100644
--- a/src/store/index.ts
+++ b/src/store/index.ts
@@ -51,14 +51,14 @@ export interface IStore {
      * Set the sync token.
      * @param {string} token
      */
-    setSyncToken(token: string);
+    setSyncToken(token: string): void;
 
     /**
      * No-op.
      * @param {Group} group
      * @deprecated groups/communities never made it to the spec and support for them is being discontinued.
      */
-    storeGroup(group: Group);
+    storeGroup(group: Group): void;
 
     /**
      * No-op.
@@ -79,7 +79,7 @@ export interface IStore {
      * No-op.
      * @param {Room} room
      */
-    storeRoom(room: Room);
+    storeRoom(room: Room): void;
 
     /**
      * No-op.
@@ -98,7 +98,7 @@ export interface IStore {
      * Permanently delete a room.
      * @param {string} roomId
      */
-    removeRoom(roomId: string);
+    removeRoom(roomId: string): void;
 
     /**
      * No-op.
@@ -110,7 +110,7 @@ export interface IStore {
      * No-op.
      * @param {User} user
      */
-    storeUser(user: User);
+    storeUser(user: User): void;
 
     /**
      * No-op.
@@ -140,13 +140,13 @@ export interface IStore {
      * @param {string} token The token associated with these events.
      * @param {boolean} toStart True if these are paginated results.
      */
-    storeEvents(room: Room, events: MatrixEvent[], token: string, toStart: boolean);
+    storeEvents(room: Room, events: MatrixEvent[], token: string, toStart: boolean): void;
 
     /**
      * Store a filter.
      * @param {Filter} filter
      */
-    storeFilter(filter: Filter);
+    storeFilter(filter: Filter): void;
 
     /**
      * Retrieve a filter.
@@ -168,13 +168,13 @@ export interface IStore {
      * @param {string} filterName
      * @param {string} filterId
      */
-    setFilterIdByName(filterName: string, filterId: string);
+    setFilterIdByName(filterName: string, filterId: string): void;
 
     /**
      * Store user-scoped account data events
      * @param {Array<MatrixEvent>} events The events to store.
      */
-    storeAccountDataEvents(events: MatrixEvent[]);
+    storeAccountDataEvents(events: MatrixEvent[]): void;
 
     /**
      * Get account data event by event type
diff --git a/src/store/indexeddb-local-backend.ts b/src/store/indexeddb-local-backend.ts
index c5c25fc19..2e5ef0cfd 100644
--- a/src/store/indexeddb-local-backend.ts
+++ b/src/store/indexeddb-local-backend.ts
@@ -66,7 +66,7 @@ function selectQuery<T>(
 ): Promise<T[]> {
     const query = store.openCursor(keyRange);
     return new Promise((resolve, reject) => {
-        const results = [];
+        const results: T[] = [];
         query.onerror = () => {
             reject(new Error("Query failed: " + query.error));
         };
@@ -238,7 +238,7 @@ export class LocalIndexedDBStoreBackend implements IIndexedDBBackend {
             const range = IDBKeyRange.only(roomId);
             const request = roomIndex.openCursor(range);
 
-            const membershipEvents = [];
+            const membershipEvents: IEvent[] = [];
             // did we encounter the oob_written marker object
             // amongst the results? That means OOB member
             // loading already happened for this room
diff --git a/src/utils.ts b/src/utils.ts
index d24f1601d..b18aadf49 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -90,23 +90,20 @@ export function removeElement<T>(
     array: T[],
     fn: (t: T, i?: number, a?: T[]) => boolean,
     reverse?: boolean,
-) {
-    let i;
-    let removed;
+): boolean {
+    let i: number;
     if (reverse) {
         for (i = array.length - 1; i >= 0; i--) {
             if (fn(array[i], i, array)) {
-                removed = array[i];
                 array.splice(i, 1);
-                return removed;
+                return true;
             }
         }
     } else {
         for (i = 0; i < array.length; i++) {
             if (fn(array[i], i, array)) {
-                removed = array[i];
                 array.splice(i, 1);
-                return removed;
+                return true;
             }
         }
     }
@@ -276,31 +273,6 @@ export function deepSortedObjectEntries(obj: any): [string, any][] {
     return pairs;
 }
 
-/**
- * Copy properties from one object to another.
- *
- * All enumerable properties, included inherited ones, are copied.
- *
- * This is approximately equivalent to ES6's Object.assign, except
- * that the latter doesn't copy inherited properties.
- *
- * @param {Object} target  The object that will receive new properties
- * @param {...Object} source  Objects from which to copy properties
- *
- * @return {Object} target
- */
-export function extend(...restParams) {
-    const target = restParams[0] || {};
-    for (let i = 1; i < restParams.length; i++) {
-        const source = restParams[i];
-        if (!source) continue;
-        for (const propName in source) { // eslint-disable-line guard-for-in
-            target[propName] = source[propName];
-        }
-    }
-    return target;
-}
-
 /**
  * Inherit the prototype methods from one constructor into another. This is a
  * port of the Node.js implementation with an Object.create polyfill.