TypeScript: Reference subtype of a nullable type definition

From some code generate tool, I got some type definition similar to

type ParentType = {
  viewer : {
    userByUuid: {
      username: string | null;
      legalName: string | null;
    } | null;
  } | null;
}

And I want to be able to refer to the type similar to

type User = {
    username: string | null;
    legalName: string | null;
} | null;

I know if the ParentType is not nullable, I can use a way similar to

type User = ParentType['viewer']['userByUuid']

But how can I handle this type reference for the nullable type?

Thanks!

>Solution :

You can use conditional types to extract the non null part,

type ParentType = {
    viewer : {
      userByUuid: {
        username: string | null;
        legalName: string | null;
      } | null;
    } | null;
  }

type NonNullableType<T> = T extends null ? never : T
type Viewer = NonNullableType<ParentType["viewer"]>
type User = NonNullableType<Viewer["userByUuid"]>