infer return type of functions

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)
 }