diff --git a/db/bill.ts b/db/bill.ts
index 8c47808..6baf960 100644
--- a/db/bill.ts
+++ b/db/bill.ts
@@ -18,10 +18,10 @@ export interface Bill {
 
 export interface BillDocument
   extends Bill,
-    mongoose.SchemaTimestampsConfig,
-    mongoose.Document {}
+  mongoose.SchemaTimestampsConfig,
+  mongoose.Document { }
 
-export interface BillModel extends mongoose.Model<BillDocument> {}
+export interface BillModel extends mongoose.Model<BillDocument> { }
 
 const BillSchema = new mongoose.Schema<BillDocument>(
   {
@@ -29,7 +29,7 @@ const BillSchema = new mongoose.Schema<BillDocument>(
       type: Number,
       required: true,
       validate: {
-        validator: function (v: number) {
+        validator: function(v: number): boolean {
           const bill = this as BillDocument
 
           return v <= bill.milageEnd
@@ -43,7 +43,7 @@ const BillSchema = new mongoose.Schema<BillDocument>(
       required: true,
 
       validate: {
-        validator: function (v: number) {
+        validator: function(v: number): boolean {
           const bill = this as BillDocument
 
           return v >= bill.milageStart
@@ -77,12 +77,12 @@ const BillSchema = new mongoose.Schema<BillDocument>(
   }
 )
 
-BillSchema.virtual('milage').get(function () {
+BillSchema.virtual('milage').get(function(): number {
   const bill = this as BillDocument
   return bill.milageEnd - bill.milageStart
 })
 
-BillSchema.virtual('total').get(function () {
+BillSchema.virtual('total').get(function(): number {
   const bill = this as BillDocument
 
   return getBillTotal(bill)
diff --git a/db/booking.ts b/db/booking.ts
index 2fb585e..4dc8f77 100644
--- a/db/booking.ts
+++ b/db/booking.ts
@@ -22,8 +22,8 @@ export interface Booking {
 
 export interface BookingDocument
   extends Booking,
-    mongoose.Document,
-    mongoose.SchemaTimestampsConfig {}
+  mongoose.Document,
+  mongoose.SchemaTimestampsConfig { }
 
 export interface BookingModel extends mongoose.Model<BookingDocument> {
   findBookedDays(uuidsToIngore?: string[]): Promise<string[]>
@@ -52,10 +52,10 @@ const BookingSchema = new mongoose.Schema<BookingDocument>(
       required: true,
       get: dateFormatBackend,
       validate: {
-        validator: function (v: Date) {
+        validator: function(v: Date): boolean {
           return v >= nowInTz()
         },
-        message: (props: { value: Date }) => `${props.value} is in the past`,
+        message: (props: { value: Date }): string => `${props.value} is in the past`,
       },
     },
     endDate: {
@@ -63,27 +63,27 @@ const BookingSchema = new mongoose.Schema<BookingDocument>(
       required: false,
       get: dateFormatBackend,
       validate: {
-        validator: function (v: Date) {
+        validator: function(v: Date): boolean {
           return v >= nowInTz()
         },
-        message: (props: { value: Date }) => `${props.value} is in the past`,
+        message: (props: { value: Date }): string => `${props.value} is in the past`,
       },
     },
     days: {
       type: [String],
       required: true,
       validate: {
-        validator: async function (days: string[]) {
+        validator: async function(days: string[]): Promise<boolean> {
           const booking = this as Booking
           const uuid = booking.uuid && [booking.uuid]
           const bookedDays = await getBookedDays(uuid)
 
-          const doubleBookedDays = days.filter((day: string) =>
+          const doubleBookedDays = days.filter((day: string): boolean =>
             bookedDays.includes(day)
           )
           return doubleBookedDays.length === 0
         },
-        message: (props: { value: string[] }) =>
+        message: (props: { value: string[] }): string =>
           `At least one day is of ${props.value.join(',')} is already booked`,
         type: VALIDATION_ERRORS.AT_LEAST_ONE_DAY_BOOKED,
       },
@@ -105,7 +105,7 @@ const BookingSchema = new mongoose.Schema<BookingDocument>(
   }
 )
 
-BookingSchema.pre('validate', function (next: () => void) {
+BookingSchema.pre('validate', function(next: () => void): void {
   const booking = this as BookingDocument
   booking.days = getDays({
     startDate: new Date(booking.startDate),
@@ -114,7 +114,7 @@ BookingSchema.pre('validate', function (next: () => void) {
   next()
 })
 
-BookingSchema.static('findBookedDays', async function (
+BookingSchema.static('findBookedDays', async function(
   uuidsToIngore: string[] = []
 ): Promise<string[]> {
   const model = this as BookingModel
diff --git a/db/index.ts b/db/index.ts
index b4e0679..78d2e12 100644
--- a/db/index.ts
+++ b/db/index.ts
@@ -1,12 +1,12 @@
 import * as mongoose from 'mongoose'
 import BookerModel, { Booker } from './booker'
-import BookingModel, { Booking } from './booking'
+import BookingModel, { Booking, BookingDocument } from './booking'
 import BillModel, { Bill } from './bill'
 import { BOOKING_STATUS } from './enums'
 
 let connectedPromise: Promise<typeof mongoose>
 
-function connect() {
+function connect(): Promise<typeof mongoose> {
   if (connectedPromise) {
     return
   }
@@ -20,12 +20,12 @@ function connect() {
   return connectedPromise
 }
 
-export async function getBookedDays(uuidsToIngore?: string[]) {
+export async function getBookedDays(uuidsToIngore?: string[]): Promise<string[]> {
   await connect()
   return BookingModel.findBookedDays(uuidsToIngore)
 }
 
-export async function getBookingByUUID(uuid: string) {
+export async function getBookingByUUID(uuid: string): Promise<BookingDocument> {
   await connect()
   return BookingModel.findOne({ uuid })
 }
@@ -33,7 +33,7 @@ export async function getBookingByUUID(uuid: string) {
 export async function getBookings({
   status = [BOOKING_STATUS.CONFIRMED, BOOKING_STATUS.REQUESTED],
   startDateGreaterThan = '2000-01-01T00:00:00Z',
-}: { status?: BOOKING_STATUS[]; startDateGreaterThan?: string } = {}) {
+}: { status?: BOOKING_STATUS[]; startDateGreaterThan?: string } = {}): Promise<BookingDocument[]> {
   await connect()
   return await BookingModel.find({
     status: { $in: status },
diff --git a/helpers/array.ts b/helpers/array.ts
index c89a615..8ac2e20 100644
--- a/helpers/array.ts
+++ b/helpers/array.ts
@@ -1,4 +1,4 @@
-export function getNextSmaller<T>(array: T[], pivot: T) {
+export function getNextSmaller<T>(array: T[], pivot: T): T {
   if (!array || !Array.isArray(array) || !array.length) {
     return null
   }
@@ -9,7 +9,7 @@ export function getNextSmaller<T>(array: T[], pivot: T) {
     .find((item) => item < pivot)
 }
 
-export function getNextBigger<T>(array: T[], pivot: T) {
+export function getNextBigger<T>(array: T[], pivot: T): T {
   if (!array || !Array.isArray(array) || !array.length) {
     return null
   }
diff --git a/helpers/bill.ts b/helpers/bill.ts
index d809072..bea0b2e 100644
--- a/helpers/bill.ts
+++ b/helpers/bill.ts
@@ -1,11 +1,11 @@
 import { MILAGE_TARIFS } from '../db/enums'
 import { AdditionalCost } from '../db/bill'
 
-function roundToCent(amount: number) {
+function roundToCent(amount: number): number {
   return Math.round(amount * 100) / 100
 }
 
-export function getMilageCosts(tarif: MILAGE_TARIFS, km: number): number {
+export function getMilageCosts({ tarif, km }: { tarif: MILAGE_TARIFS; km: number }): number {
   if (tarif === MILAGE_TARIFS.NOCHARGE) {
     return 0
   }
@@ -56,7 +56,7 @@ export function getBillTotal({
   milage?: number
   additionalCosts: AdditionalCost[]
 }): number {
-  const milageCosts = getMilageCosts(tarif, milage)
+  const milageCosts = getMilageCosts({ tarif, km: milage })
   const additionalCostsSum = additionalCosts
     .map(({ value }) => value)
     .reduce((acc: number, value: number) => acc + value, 0)
diff --git a/helpers/date.ts b/helpers/date.ts
index a4cccfd..99bd5a4 100644
--- a/helpers/date.ts
+++ b/helpers/date.ts
@@ -4,7 +4,7 @@ import { utcToZonedTime } from 'date-fns-tz'
 const FRONTEND_FORMAT = 'dd.MM.yyyy'
 const BACKEND_FORMAT = 'yyyy-MM-dd'
 
-export function daysFormatFrontend(days: string[]) {
+export function daysFormatFrontend(days: string[]): string {
   if (days.length === 0) {
     return ''
   }
@@ -24,7 +24,7 @@ export function getDays({
 }: {
   startDate: Date
   endDate: Date
-}) {
+}): string[] {
   let currentDay = new Date(startDate.getTime())
   const days = [dateFormatBackend(currentDay)]
 
@@ -40,22 +40,22 @@ export function getDays({
   return days
 }
 
-function dateFormat(date: Date, formatString: string) {
+function dateFormat(date: Date, formatString: string): string {
   if (!date) {
     return null
   }
   return format(date, formatString)
 }
 
-export function dateFormatBackend(date: Date) {
+export function dateFormatBackend(date: Date): string {
   return dateFormat(date, BACKEND_FORMAT)
 }
 
-export function dateFormatFrontend(date: Date) {
+export function dateFormatFrontend(date: Date): string {
   return dateFormat(date, FRONTEND_FORMAT)
 }
 
-function dateParse(input: string, formatString: string) {
+function dateParse(input: string, formatString: string): Date {
   const date = parse(input, formatString, new Date())
   if (date.getTime() !== NaN) {
     return date
@@ -64,11 +64,11 @@ function dateParse(input: string, formatString: string) {
   return null
 }
 
-export function dateParseFrontend(input: string) {
+export function dateParseFrontend(input: string): Date {
   return dateParse(input, FRONTEND_FORMAT)
 }
 
-export function nowInTz(timezone = 'Europe/Berlin') {
+export function nowInTz(timezone = 'Europe/Berlin'): Date {
   const now = new Date()
   return utcToZonedTime(now, timezone)
 }
diff --git a/helpers/ical.ts b/helpers/ical.ts
index d942000..61c704c 100644
--- a/helpers/ical.ts
+++ b/helpers/ical.ts
@@ -38,7 +38,7 @@ Buchungs-Link: ${getBaseURL()}/booking/${booking.uuid}
   return value
 }
 
-export function generateBookedCalendar(bookings: Booking[]) {
+export function generateBookedCalendar(bookings: Booking[]): string {
   const events = bookings.map((booking) => ({
     productId: 'app.vercel.pfadi-bussle/ics',
     calName: 'Pfadi-Bussle Buchungen',
diff --git a/helpers/mail.ts b/helpers/mail.ts
index 0c3b291..e0b0601 100644
--- a/helpers/mail.ts
+++ b/helpers/mail.ts
@@ -31,7 +31,7 @@ Pfadi Bussle Wart
 Tel. 0151/212 253 62
 `
 
-function getReceivedBookingBookerText(booking: Booking) {
+function getReceivedBookingBookerText(booking: Booking): string {
   return `Hallo liebe/r ${booking.booker.name},
 
 Vielen Dank für Deine Buchungsanfrage zum ${daysFormatFrontend(booking.days)}!
@@ -48,7 +48,7 @@ ${footer}
 `
 }
 
-function getBookingConfirmedText(booking: Booking) {
+function getBookingConfirmedText(booking: Booking): string {
   return `Hallo liebe/r ${booking.booker.name},
 
 deine Buchunganfrage zum ${daysFormatFrontend(
@@ -64,7 +64,7 @@ einsehen und stornieren.
 ${footer}
 `
 }
-function getBookingRejectedText(booking: Booking) {
+function getBookingRejectedText(booking: Booking): string {
   return `Hallo liebe/r ${booking.booker.name},
 
 es tut uns leid aber deine Buchungsanfrage zum ${daysFormatFrontend(
@@ -78,7 +78,7 @@ ${footer}
 `
 }
 
-function getReceivedBookingAdminText(booking: { uuid: string }) {
+function getReceivedBookingAdminText(booking: { uuid: string }): string {
   return `Hallo lieber Admin,
 
 es ging folgende Buchung ein: ${getBaseURL()}/admin/booking/${booking.uuid}
@@ -86,7 +86,7 @@ es ging folgende Buchung ein: ${getBaseURL()}/admin/booking/${booking.uuid}
 MfG`
 }
 
-export async function sendReceivedBookingAdminMail(booking: Booking) {
+export async function sendReceivedBookingAdminMail(booking: Booking): Promise<void> {
   try {
     await sendMail({
       to: [{ email: ADMIN_EMAIL }],
@@ -102,7 +102,7 @@ export async function sendReceivedBookingAdminMail(booking: Booking) {
   }
 }
 
-export async function sendReceivedBookingBookerMail(booking: Booking) {
+export async function sendReceivedBookingBookerMail(booking: Booking): Promise<void> {
   try {
     await sendMail({
       to: [{ email: booking.booker.email, name: booking.booker.name }],
@@ -118,7 +118,7 @@ export async function sendReceivedBookingBookerMail(booking: Booking) {
   }
 }
 
-export async function sendBookingConfirmed(booking: Booking) {
+export async function sendBookingConfirmed(booking: Booking): Promise<void> {
   try {
     await sendMail({
       to: [{ email: booking.booker.email, name: booking.booker.name }],
@@ -143,7 +143,7 @@ export async function sendBookingConfirmed(booking: Booking) {
   }
 }
 
-export async function sendBookingRejected(booking: Booking) {
+export async function sendBookingRejected(booking: Booking): Promise<void> {
   try {
     await sendMail({
       to: [{ email: booking.booker.email, name: booking.booker.name }],
@@ -175,7 +175,7 @@ async function sendMail({
     type?: string
     filename: string
   }[]
-}) {
+}): Promise<void> {
   const data = {
     personalizations: [
       {
diff --git a/lib/authenticate.ts b/lib/authenticate.ts
index 9df5003..e6aea45 100644
--- a/lib/authenticate.ts
+++ b/lib/authenticate.ts
@@ -6,7 +6,7 @@ export function authenticateAdmin({
 }: {
   username: string
   password: string
-}) {
+}): boolean {
   if (username !== 'admin') {
     return false
   }
diff --git a/pages/api/admin/booking/[uuid]/bill.ts b/pages/api/admin/booking/[uuid]/bill.ts
index c01ad40..9463659 100644
--- a/pages/api/admin/booking/[uuid]/bill.ts
+++ b/pages/api/admin/booking/[uuid]/bill.ts
@@ -2,7 +2,7 @@ import { Bill } from '../../../../../db/bill'
 import { createBill, patchBill } from '../../../../../db/index'
 import withSession, { isAdminSession } from '../../../../../lib/session'
 
-export default withSession(async function billHandler(req, res) {
+export default withSession(async function billHandler(req, res): Promise<void> {
   if (!isAdminSession(req)) {
     res.status(403).send({ message: 'Not Authorized' })
     return
diff --git a/pages/api/admin/booking/[uuid]/index.ts b/pages/api/admin/booking/[uuid]/index.ts
index 7393d13..49699de 100644
--- a/pages/api/admin/booking/[uuid]/index.ts
+++ b/pages/api/admin/booking/[uuid]/index.ts
@@ -7,7 +7,7 @@ import {
 } from '../../../../../helpers/mail'
 import { BOOKING_STATUS } from '../../../../../db/enums'
 
-export default withSession(async function bookingHandler(req, res) {
+export default withSession(async function bookingHandler(req, res): Promise<void> {
   if (!isAdminSession(req)) {
     res.status(403).send({ message: 'Not Authorized' })
     return
diff --git a/pages/api/admin/login.ts b/pages/api/admin/login.ts
index 9319b1a..897cec3 100644
--- a/pages/api/admin/login.ts
+++ b/pages/api/admin/login.ts
@@ -1,7 +1,7 @@
 import { authenticateAdmin } from '../../../lib/authenticate'
 import withSession from '../../../lib/session'
 
-async function loginHandler(req: any, res: any) {
+async function loginHandler(req: any, res: any): Promise<void> {
   const { method } = req
 
   switch (method) {
diff --git a/pages/api/booking/[uuid]/index.ts b/pages/api/booking/[uuid]/index.ts
index 7dc49d3..0db5ee4 100644
--- a/pages/api/booking/[uuid]/index.ts
+++ b/pages/api/booking/[uuid]/index.ts
@@ -6,7 +6,7 @@ import { getBookingByUUID } from '../../../../db/index'
 export default async function userHandler(
   req: NextApiRequest,
   res: NextApiResponse
-) {
+): Promise<void> {
   const {
     method,
     query: { uuid: uuids },
diff --git a/pages/api/booking/index.ts b/pages/api/booking/index.ts
index a6632f8..4d9ff83 100644
--- a/pages/api/booking/index.ts
+++ b/pages/api/booking/index.ts
@@ -10,7 +10,7 @@ import {
 export default async function userHandler(
   req: NextApiRequest,
   res: NextApiResponse
-) {
+): Promise<void> {
   const { method } = req
 
   let booking: Booking
diff --git a/pages/api/daysbooked.ics.ts b/pages/api/daysbooked.ics.ts
index 5694956..cc693d2 100644
--- a/pages/api/daysbooked.ics.ts
+++ b/pages/api/daysbooked.ics.ts
@@ -5,7 +5,7 @@ import { generateBookedCalendar } from '../../helpers/ical'
 export default async function useHandler(
   req: NextApiRequest,
   res: NextApiResponse
-) {
+): Promise<void> {
   const { method } = req
 
   switch (method) {
diff --git a/pages/api/daysbooked.ts b/pages/api/daysbooked.ts
index 9bbde4d..8b4efd4 100644
--- a/pages/api/daysbooked.ts
+++ b/pages/api/daysbooked.ts
@@ -4,7 +4,7 @@ import { getBookedDays } from '../../db/index'
 export default async function useHandler(
   req: NextApiRequest,
   res: NextApiResponse
-) {
+): Promise<void> {
   const { method } = req
 
   switch (method) {