Type alias NestedKeys<Obj>

NestedKeys<Obj>: {
    [Key in keyof Obj]-?: undefined extends Obj[Key]
        ? IsNestedField<Exclude<Obj[Key], undefined>> extends never
            ? never
            : Key
        : IsNestedField<Obj[Key]> extends never
            ? never
            : Key
}[keyof Obj] & string

This is an internal type that is used to extract the keys of a user-provided generic type that are nested properties.