diff --git a/api/middleware/index.ts b/api/middleware/index.ts
index 2e7134540f1..59fb5108041 100644
--- a/api/middleware/index.ts
+++ b/api/middleware/index.ts
@@ -5,7 +5,7 @@ export async function auth0Verify(
   this: FastifyInstance,
   request: FastifyRequest,
   reply: FastifyReply
-) {
+): Promise<void> {
   await this.authenticate(request, reply);
 }
 
@@ -16,7 +16,7 @@ export function testMiddleware(
   req: MiddieRequest,
   res: MiddieResponse,
   next: NextFunction
-) {
+): void {
   console.log('Test middleware running');
   console.log(req.headers);
   console.log(req.query);
diff --git a/api/utils/index.ts b/api/utils/index.ts
index 17899676d89..fbe8447a59c 100644
--- a/api/utils/index.ts
+++ b/api/utils/index.ts
@@ -1,6 +1,6 @@
 import { randomBytes, createHash } from 'crypto';
 
-export function base64URLEncode(buf: Buffer) {
+export function base64URLEncode(buf: Buffer): string {
   return buf
     .toString('base64')
     .replace(/\+/g, '-')
diff --git a/client/src/components/Header/index.tsx b/client/src/components/Header/index.tsx
index 37a93dcce75..d55e39a397e 100644
--- a/client/src/components/Header/index.tsx
+++ b/client/src/components/Header/index.tsx
@@ -72,7 +72,7 @@ export class Header extends React.Component<
   }
 
   // elementToFocus must be a link in the language menu
-  showLanguageMenu(elementToFocus: HTMLButtonElement | null) {
+  showLanguageMenu(elementToFocus: HTMLButtonElement | null): void {
     this.setState({ isLanguageMenuDisplayed: true }, () =>
       elementToFocus?.focus()
     );