I have this class that represents JSON incoming from POST request.

class MilkCarton {
  company: string;
  price: number;
  expiredAt?: string;
  // ...20 more properties
}

Before I store it in my mongoDB database, I want to change the type of expiredAt to Date, so I have another class that represents the schema of the database.

class MilkCartonSchema: {
  company: string;
  price: number;
  expiredAt?: Date;
  // ...20 more properties
}

expiredAt can be null, I want to create an object that copies the object of type MilkCarton but has its expiredAt converted to Date

prepareMilkForDb(milk: MilkCarton): MilkCartonSchema {
  const preparedMilk = {
    ...milk,
  }

  if (milk.expiredAt) {
    preparedMilk.expiredAt = new Date(milk.expiredAt)
  }

  return preparedMilk;
}

But I run into an error because preparedMilk has already inferred its type and has expiredAt as string, it can't be turned into Date from TypeScript perspective if it's string. But I want it to turn into Date, what is the approach to do that correctly in TypeScript?

EDIT: I ended up going with:

prepareMilkForDb(milk: MilkCarton): MilkCartonSchema {
  const preparedMilk = {
    ...milk,
    ...(milk.expiredAt && {
      expiredAt: new Date(milk.expiredAt)
    }),
  }

  return preparedMilk;
}

This works, but isn't exactly what I wished for, if I had another example with 3 different deep nested ISO string properties that need converting to Date (and backwards) it would be very dirty to do this workaround.