diff --git a/src/env.mjs b/src/env.ts
similarity index 90%
rename from src/env.mjs
rename to src/env.ts
index 5ef617cac..63c0d517b 100644
--- a/src/env.mjs
+++ b/src/env.ts
@@ -10,7 +10,7 @@ const env = import.meta.env;
 const envVarPrefix = 'VITE';
 const DEBUG = env.MODE === 'development';
 
-const getEnv = name => {
+const getEnv = (name: string): string | undefined => {
   const fullName = `${envVarPrefix}_${name}`;
   return env[fullName];
 };
diff --git a/src/utils.js b/src/utils.ts
similarity index 62%
rename from src/utils.js
rename to src/utils.ts
index 8d70f8440..44a4fdf25 100644
--- a/src/utils.js
+++ b/src/utils.ts
@@ -1,4 +1,5 @@
 import clsx from 'clsx';
+import type {IntlShape} from 'react-intl';
 
 import {getEnv} from './env';
 
@@ -8,12 +9,12 @@ const VERSION = getEnv('VERSION');
 
 export const PREFIX = 'openforms';
 
-export const getFormattedDateString = (intl, dateString) => {
+export const getFormattedDateString = (intl: IntlShape, dateString: string) => {
   if (!dateString) return '';
   return intl.formatDate(new Date(dateString));
 };
 
-export const getFormattedTimeString = (intl, dateTimeString) => {
+export const getFormattedTimeString = (intl: IntlShape, dateTimeString: string) => {
   if (!dateTimeString) return '';
   return intl.formatTime(new Date(dateTimeString));
 };
@@ -21,17 +22,17 @@ export const getFormattedTimeString = (intl, dateTimeString) => {
 /**
  * Prefix a name/string/identifier with the Open Forms specific prefix.
  */
-export const applyPrefix = name => {
+export const applyPrefix = (name: string) => {
   return `${PREFIX}-${name}`;
 };
 
-export const getBEMClassName = (base, modifiers = []) => {
+export const getBEMClassName = (base: string, modifiers: string[] = []) => {
   const prefixedBase = applyPrefix(base);
   const prefixedModifiers = modifiers.map(mod => applyPrefix(`${base}--${mod}`));
   return clsx(prefixedBase, ...prefixedModifiers);
 };
 
 // usage: await sleep(3000);
-export const sleep = ms => new Promise(resolve => setTimeout(resolve, ms));
+export const sleep = (ms: number) => new Promise(resolve => setTimeout(resolve, ms));
 
 export const getVersion = () => VERSION || 'unknown';
diff --git a/tsconfig.json b/tsconfig.json
index 3464dd4ea..485d53111 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -24,6 +24,7 @@
       "@/sb-decorators": ["../.storybook/decorators.tsx"]
     },
     "types": [
+      "vite/client",
       "vitest/globals"
     ]
   },