infer return type of functions

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