type GivenIdType = number;
type StringableIdType = number;

interface MyType {
    givenId: GivenIdType;
    stringableId: StringableIdType;
}

type ToPropertyNull<T, K extends keyof T> = { [P in keyof T]: P extends K ? null : T[P] };
type AddTmpIdId<T, K extends keyof T> = ToPropertyNull<T, K> & { tmpId: string };

type StringableType = Omit<MyType, 'stringableId'> & {
    stringableId: string;
}; // { givenId: GivenIdType, stringableId: string }
type NonGivenIdWithTmpId = AddTmpIdId<
    StringableType,
    'givenId'
>; // { givenId: null, stringableId: string, tmpId: string }

const test = <
    T extends StringableType | NonGivenIdWithTmpId
>(
    v: T
): (Omit<T, 'stringableId'>) & { stringableId: StringableIdType } => ({
    ...v,
    stringableId: Number.parseInt(v.stringableId, 10),
});