diff --git a/keyserver/src/keyserver.js b/keyserver/src/keyserver.js
--- a/keyserver/src/keyserver.js
+++ b/keyserver/src/keyserver.js
@@ -7,7 +7,7 @@
 import cors from 'cors';
 import crypto from 'crypto';
 import express from 'express';
-import type { $Request, $Response } from 'express';
+import type { $Request, $Response, Router } from 'express';
 import expressWs from 'express-ws';
 import os from 'os';
 import qrcode from 'qrcode';
@@ -145,7 +145,7 @@
     server.use(express.json({ limit: '250mb' }));
     server.use(cookieParser());
 
-    const setupAppRouter = router => {
+    const setupAppRouter = (router: Router<>) => {
       if (areEndpointMetricsEnabled) {
         router.use(logEndpointMetrics);
       }
diff --git a/keyserver/src/push/crypto.js b/keyserver/src/push/crypto.js
--- a/keyserver/src/push/crypto.js
+++ b/keyserver/src/push/crypto.js
@@ -1,5 +1,6 @@
 // @flow
 
+import type { EncryptResult } from '@commapp/olm';
 import apn from '@parse/node-apn';
 import invariant from 'invariant';
 import _cloneDeep from 'lodash/fp/cloneDeep.js';
@@ -49,7 +50,11 @@
 
     let dbPersistCondition;
     if (notificationSizeValidator) {
-      dbPersistCondition = ({ serializedPayload }) => {
+      dbPersistCondition = ({
+        serializedPayload,
+      }: {
+        +[string]: EncryptResult,
+      }) => {
         const notifCopy = _cloneDeep(encryptedNotification);
         notifCopy.payload.encryptedPayload = serializedPayload.body;
         return notificationSizeValidator(notifCopy);
@@ -128,8 +133,11 @@
 
     let dbPersistCondition;
     if (payloadSizeValidator) {
-      dbPersistCondition = ({ serializedPayload }) =>
-        payloadSizeValidator({ encryptedPayload: serializedPayload.body });
+      dbPersistCondition = ({
+        serializedPayload,
+      }: {
+        +[string]: EncryptResult,
+      }) => payloadSizeValidator({ encryptedPayload: serializedPayload.body });
     }
 
     const {
diff --git a/keyserver/src/push/send.js b/keyserver/src/push/send.js
--- a/keyserver/src/push/send.js
+++ b/keyserver/src/push/send.js
@@ -52,6 +52,7 @@
   NotificationTargetDevice,
   TargetedAPNsNotification,
   TargetedAndroidNotification,
+  AndroidNotification,
 } from './types.js';
 import {
   apnPush,
@@ -840,7 +841,7 @@
     messageInfos,
   };
 
-  const notificationSizeValidator = notif =>
+  const notificationSizeValidator = (notif: apn.Notification) =>
     notif.length() <= apnMaxNotificationPayloadByteSize;
 
   if (!shouldBeEncrypted) {
@@ -999,7 +1000,7 @@
     }));
   }
 
-  const notificationsSizeValidator = notif => {
+  const notificationsSizeValidator = (notif: AndroidNotification) => {
     const serializedNotif = JSON.stringify(notif);
     return (
       !serializedNotif ||
diff --git a/keyserver/src/scripts/generate-converter-from-validator.js b/keyserver/src/scripts/generate-converter-from-validator.js
--- a/keyserver/src/scripts/generate-converter-from-validator.js
+++ b/keyserver/src/scripts/generate-converter-from-validator.js
@@ -216,7 +216,7 @@
 const validator = rawThreadInfoValidator;
 const typeName = 'RawThreadInfo';
 const validatorToBeConverted = tID;
-const conversionExpressionString = inputName =>
+const conversionExpressionString = (inputName: string) =>
   `'${ashoatKeyserverID}|' + ${inputName}`;
 main([
   async () => {
diff --git a/keyserver/src/socket/socket.js b/keyserver/src/socket/socket.js
--- a/keyserver/src/socket/socket.js
+++ b/keyserver/src/socket/socket.js
@@ -193,7 +193,7 @@
 
   onMessage = async (
     messageString: string | Buffer | ArrayBuffer | Array<Buffer>,
-  ) => {
+  ): Promise<void> => {
     invariant(typeof messageString === 'string', 'message should be string');
     let clientSocketMessage: ?ClientSocketMessage;
     try {
@@ -824,12 +824,12 @@
   // The Socket will timeout by calling this.ws.terminate()
   // serverRequestSocketTimeout milliseconds after the last
   // time resetTimeout is called
-  resetTimeout = _debounce(
+  resetTimeout: { +cancel: () => void } & (() => void) = _debounce(
     () => this.ws.terminate(),
     serverRequestSocketTimeout,
   );
 
-  debouncedAfterActivity = _debounce(
+  debouncedAfterActivity: { +cancel: () => void } & (() => void) = _debounce(
     () => this.setStateCheckConditions({ activityRecentlyOccurred: false }),
     stateCheckInactivityActivationInterval,
   );
@@ -858,7 +858,7 @@
     this.handleStateCheckConditionsUpdate();
   }
 
-  get stateCheckCanStart() {
+  get stateCheckCanStart(): boolean {
     return Object.values(this.stateCheckConditions).every(cond => !cond);
   }
 
diff --git a/keyserver/src/utils/depth-queue.js b/keyserver/src/utils/depth-queue.js
--- a/keyserver/src/utils/depth-queue.js
+++ b/keyserver/src/utils/depth-queue.js
@@ -18,7 +18,7 @@
     this.mergeFunction = mergeFunction;
   }
 
-  addInfo(info: T) {
+  addInfo(info: T): void {
     const depth = this.getDepth(info);
     if (depth <= this.maxDequeuedDepth) {
       throw new Error(
diff --git a/keyserver/src/utils/olm-utils.test.js b/keyserver/src/utils/olm-utils.test.js
--- a/keyserver/src/utils/olm-utils.test.js
+++ b/keyserver/src/utils/olm-utils.test.js
@@ -8,13 +8,13 @@
   const alphabet =
     'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789 ';
 
-  const randomString = length =>
+  const randomString = (length: number) =>
     Array.from(
       { length },
       () => alphabet[Math.floor(Math.random() * alphabet.length)],
     ).join('');
 
-  const initAccount = (mark_prekey_published = true) => {
+  const initAccount = (mark_prekey_published: boolean = true) => {
     const account = new olm.Account();
     account.create();
     account.generate_prekey();
@@ -26,12 +26,12 @@
   };
 
   const createSession = (
-    aliceSession,
-    aliceAccount,
-    bobAccount,
-    regen = false,
-    forget = false,
-    invalid_sign = false,
+    aliceSession: olm.Session,
+    aliceAccount: olm.Account,
+    bobAccount: olm.Account,
+    regen: boolean = false,
+    forget: boolean = false,
+    invalid_sign: boolean = false,
   ) => {
     const bobOneTimeKeys = JSON.parse(bobAccount.one_time_keys()).curve25519;
     bobAccount.mark_keys_as_published();
@@ -86,7 +86,12 @@
     return aliceSession;
   };
 
-  const testRatchet = (aliceSession, bobSession, bobAccount, num_msg = 1) => {
+  const testRatchet = (
+    aliceSession: olm.Session,
+    bobSession: olm.Session,
+    bobAccount: olm.Account,
+    num_msg: number = 1,
+  ) => {
     let test_text = randomString(40);
     let encrypted = aliceSession.encrypt(test_text);
     expect(encrypted.type).toEqual(0);