diff --git a/src/utils/DirectoryUtils.js b/src/utils/DirectoryUtils.ts
similarity index 81%
rename from src/utils/DirectoryUtils.js
rename to src/utils/DirectoryUtils.ts
index 577a6441f8..255ae0e3fd 100644
--- a/src/utils/DirectoryUtils.js
+++ b/src/utils/DirectoryUtils.ts
@@ -14,9 +14,12 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { IInstance } from "matrix-js-sdk/src/client";
+import { Protocols } from "../components/views/directory/NetworkDropdown";
+
 // Find a protocol 'instance' with a given instance_id
 // in the supplied protocols dict
-export function instanceForInstanceId(protocols, instanceId) {
+export function instanceForInstanceId(protocols: Protocols, instanceId: string): IInstance {
     if (!instanceId) return null;
     for (const proto of Object.keys(protocols)) {
         if (!protocols[proto].instances && protocols[proto].instances instanceof Array) continue;
@@ -28,7 +31,7 @@ export function instanceForInstanceId(protocols, instanceId) {
 
 // given an instance_id, return the name of the protocol for
 // that instance ID in the supplied protocols dict
-export function protocolNameForInstanceId(protocols, instanceId) {
+export function protocolNameForInstanceId(protocols: Protocols, instanceId: string): string {
     if (!instanceId) return null;
     for (const proto of Object.keys(protocols)) {
         if (!protocols[proto].instances && protocols[proto].instances instanceof Array) continue;
diff --git a/src/utils/HostingLink.js b/src/utils/HostingLink.ts
similarity index 91%
rename from src/utils/HostingLink.js
rename to src/utils/HostingLink.ts
index 134e045ca2..f8c0f12c3f 100644
--- a/src/utils/HostingLink.js
+++ b/src/utils/HostingLink.ts
@@ -17,7 +17,7 @@ limitations under the License.
 import SdkConfig from '../SdkConfig';
 import { MatrixClientPeg } from '../MatrixClientPeg';
 
-export function getHostingLink(campaign) {
+export function getHostingLink(campaign: string): string {
     const hostingLink = SdkConfig.get().hosting_signup_link;
     if (!hostingLink) return null;
     if (!campaign) return hostingLink;
@@ -27,7 +27,7 @@ export function getHostingLink(campaign) {
     try {
         const hostingUrl = new URL(hostingLink);
         hostingUrl.searchParams.set("utm_campaign", campaign);
-        return hostingUrl.format();
+        return hostingUrl.toString();
     } catch (e) {
         return hostingLink;
     }
diff --git a/src/utils/KeyVerificationStateObserver.js b/src/utils/KeyVerificationStateObserver.ts
similarity index 86%
rename from src/utils/KeyVerificationStateObserver.js
rename to src/utils/KeyVerificationStateObserver.ts
index 023cdf3a75..e4fae3f3c8 100644
--- a/src/utils/KeyVerificationStateObserver.js
+++ b/src/utils/KeyVerificationStateObserver.ts
@@ -17,14 +17,14 @@ limitations under the License.
 import { MatrixClientPeg } from '../MatrixClientPeg';
 import { _t } from '../languageHandler';
 
-export function getNameForEventRoom(userId, roomId) {
+export function getNameForEventRoom(userId: string, roomId: string): string {
     const client = MatrixClientPeg.get();
     const room = client.getRoom(roomId);
     const member = room && room.getMember(userId);
     return member ? member.name : userId;
 }
 
-export function userLabelForEventRoom(userId, roomId) {
+export function userLabelForEventRoom(userId: string, roomId: string): string {
     const name = getNameForEventRoom(userId, roomId);
     if (name !== userId) {
         return _t("%(name)s (%(userId)s)", { name, userId });
diff --git a/src/utils/MegolmExportEncryption.js b/src/utils/MegolmExportEncryption.ts
similarity index 90%
rename from src/utils/MegolmExportEncryption.js
rename to src/utils/MegolmExportEncryption.ts
index 8e7ee2005d..47c395bfb7 100644
--- a/src/utils/MegolmExportEncryption.js
+++ b/src/utils/MegolmExportEncryption.ts
@@ -26,17 +26,17 @@ const subtleCrypto = window.crypto.subtle || window.crypto.webkitSubtle;
  * Make an Error object which has a friendlyText property which is already
  * translated and suitable for showing to the user.
  *
- * @param {string} msg   message for the exception
+ * @param {string} message message for the exception
  * @param {string} friendlyText
- * @returns {Error}
+ * @returns {{message: string, friendlyText: string}}
  */
-function friendlyError(msg, friendlyText) {
-    const e = new Error(msg);
-    e.friendlyText = friendlyText;
-    return e;
+function friendlyError(
+    message: string, friendlyText: string,
+): { message: string, friendlyText: string } {
+    return { message, friendlyText };
 }
 
-function cryptoFailMsg() {
+function cryptoFailMsg(): string {
     return _t('Your browser does not support the required cryptography extensions');
 }
 
@@ -49,7 +49,7 @@ function cryptoFailMsg() {
  *
  *
  */
-export async function decryptMegolmKeyFile(data, password) {
+export async function decryptMegolmKeyFile(data: ArrayBuffer, password: string): Promise<string> {
     const body = unpackMegolmKeyFile(data);
     const brand = SdkConfig.get().brand;
 
@@ -124,7 +124,11 @@ export async function decryptMegolmKeyFile(data, password) {
  *    key-derivation function.
  * @return {Promise<ArrayBuffer>} promise for encrypted output
  */
-export async function encryptMegolmKeyFile(data, password, options) {
+export async function encryptMegolmKeyFile(
+    data: string,
+    password: string,
+    options?: { kdf_rounds?: number }, // eslint-disable-line camelcase
+): Promise<ArrayBuffer> {
     options = options || {};
     const kdfRounds = options.kdf_rounds || 500000;
 
@@ -196,7 +200,7 @@ export async function encryptMegolmKeyFile(data, password, options) {
  * @param {String} password  password
  * @return {Promise<[CryptoKey, CryptoKey]>} promise for [aes key, hmac key]
  */
-async function deriveKeys(salt, iterations, password) {
+async function deriveKeys(salt: Uint8Array, iterations: number, password: string): Promise<[CryptoKey, CryptoKey]> {
     const start = new Date();
 
     let key;
@@ -229,7 +233,7 @@ async function deriveKeys(salt, iterations, password) {
     }
 
     const now = new Date();
-    logger.log("E2e import/export: deriveKeys took " + (now - start) + "ms");
+    logger.log("E2e import/export: deriveKeys took " + (now.getTime() - start.getTime()) + "ms");
 
     const aesKey = keybits.slice(0, 32);
     const hmacKey = keybits.slice(32);
@@ -271,7 +275,7 @@ const TRAILER_LINE = '-----END MEGOLM SESSION DATA-----';
  * @param {ArrayBuffer} data  input file
  * @return {Uint8Array} unbase64ed content
  */
-function unpackMegolmKeyFile(data) {
+function unpackMegolmKeyFile(data: ArrayBuffer): Uint8Array {
     // parse the file as a great big String. This should be safe, because there
     // should be no non-ASCII characters, and it means that we can do string
     // comparisons to find the header and footer, and feed it into window.atob.
@@ -279,6 +283,7 @@ function unpackMegolmKeyFile(data) {
 
     // look for the start line
     let lineStart = 0;
+    // eslint-disable-next-line no-constant-condition
     while (1) {
         const lineEnd = fileStr.indexOf('\n', lineStart);
         if (lineEnd < 0) {
@@ -297,6 +302,7 @@ function unpackMegolmKeyFile(data) {
     const dataStart = lineStart;
 
     // look for the end line
+    // eslint-disable-next-line no-constant-condition
     while (1) {
         const lineEnd = fileStr.indexOf('\n', lineStart);
         const line = fileStr.slice(lineStart, lineEnd < 0 ? undefined : lineEnd).trim();
@@ -324,7 +330,7 @@ function unpackMegolmKeyFile(data) {
  * @param {Uint8Array} data  raw data
  * @return {ArrayBuffer} formatted file
  */
-function packMegolmKeyFile(data) {
+function packMegolmKeyFile(data: Uint8Array): ArrayBuffer {
     // we split into lines before base64ing, because encodeBase64 doesn't deal
     // terribly well with large arrays.
     const LINE_LENGTH = (72 * 4 / 3);
@@ -347,7 +353,7 @@ function packMegolmKeyFile(data) {
  * @param {Uint8Array} uint8Array The data to encode.
  * @return {string} The base64.
  */
-function encodeBase64(uint8Array) {
+function encodeBase64(uint8Array: Uint8Array): string {
     // Misinterpt the Uint8Array as Latin-1.
     // window.btoa expects a unicode string with codepoints in the range 0-255.
     const latin1String = String.fromCharCode.apply(null, uint8Array);
@@ -360,7 +366,7 @@ function encodeBase64(uint8Array) {
  * @param {string} base64 The base64 to decode.
  * @return {Uint8Array} The decoded data.
  */
-function decodeBase64(base64) {
+function decodeBase64(base64: string): Uint8Array {
     // window.atob returns a unicode string with codepoints in the range 0-255.
     const latin1String = window.atob(base64);
     // Encode the string as a Uint8Array