typescript-example.ts

type UserEmail = NonNullable<UserEmailInput>;

// 等於 (將 Type Alias 型別化名拆開)
type UserEmail = NonNullable<string | null | undefined>;

// 等於 (將 union 聯集 各自拆開)
type UserEmail =
  | NonNullable<string>
  | NonNullable<null>
  | NonNullable<undefined>;

// 等於 (將 NonNullable 回寫成原本的定義)
type UserEmail =
  | (string extends null | undefined ? never : string)
  | (null extends null | undefined ? never : null)
  | (undefined extends null | undefined ? never : undefined);

// 等於
type UserEmail = string | never | never ;

// 等於 (never 是所有 type 的 subtype，所以聯集時可以省略)
type UserEmail = string;