具有可选扩展的通用打字稿

如何使用扩展进行输入检查，使通用 ApiResBook 类型与可选道具一起使用？

Sandbox https://www.typescriptlang.org/play/index.html#code/PTAEFkEMEsDtQC4E8AOBTAzqSW6gCaQKQBGOaoAtKBgBYD2ArgDb4DkCoJFAxrZLADmafAFgAUMnSgAQvXoBrUAF5QAbwmhQ0fAC4aCAE5xBmxNATM0+jEZNmAbtAzQSV-SXlWBZyIwQMhjZ2QhIAvhISUhQAgv6BKupmOsHGoeJasJAAttYGaabiEeISIKAAkrC0aIZoCCKgAGaG9NmIqJhNLW3ZMLBRHbLyCgBi0GisWKoKaEj0jUOKA9JxAfSGYxP4U6AzcwurgZHiZdFN66DCsDVEJqBoAB7oPPX4TXV8oLUYKPSwGBRmq1QDEAArlZaxFDQABKmDkigAPGZQdAeDN8AjRuNJvcHvVYNtFtithgADQotEYw7rTa4x4Eok0jY4okAH1AjEJaEacAaqi5+B5fPwFPEAD5EhoMlxhvpUejEViyaAFRisXTtpKAGRJGVaPxrQwAfnlVJEzM1WAZaEJWEtrKwxtVVMRzJVaot8Vpjsl+kFwuuYhlxWKpTAjQ+tDxORQVlA805GEgwgkOCQsB4TS5L2gfyasAAFABKPVmHh-WzvBB8MHlRKIgCiTzQLxEABEiJBEgIkOLC2YDdCYrm-qkTGKtFpPPgkPpe2Zi-Lus40Ii1AQu-pm89Xp3iKAwqAORuai0gohDIwKGFJcpJdKp186oxDPBrgB3VUrgGFwvfehmAcNBS3vPUny0SMa1oQsAANIGhWCVRnJBi0HCCADoAltP9DEgD9QMlPCPwwgArDA-hLND9Uw7Ci0LFBIEMAE9HUTdiG3Fs23wfduyPE97kMc99CMa9D0I58KKAtAGKYlji2oiCpwwngiD4P8zwkx8lKnCt-kAtAMOYehBELTT0KfMJFKnKyzDDGUym+X5-goM5nDYs91hEq8KEPY91EIDi2M8RR9A3FJ8juI9ovLStOCcysKFUfCYE4KDa3BN1oThDAsURNgdDYcV+zYDBsgCNgVTUKzQHCY56pOMB6lsDBIWJAB5WBmCQRIYmy+FhnywrxTarEAHULFoZlev63LBoK9gVTYLJciK44yjoJhWC4CglnEPSqxChROu6-QsROnrVA3I6wu0Vi2AARjYPzigOzgjomgJmTO4ZPqm71DClWVQrYiLHsq7AAdula8jYKAHme6LD3WsBNpYN5uEE89RuGC7GyEi5VD62EBqRNgUBaFB23oTBYAQZtnAQNbJEGcbJuZfHzxmkm5rJwqlop+gqZpjA6YZ2xmbe4Hjq6pBOa8jrZaBm7Qfup6VUNQIwph-Q4cgBGkaPCQpY+yamAQDmCYvNmvoBqUVbUMGnrCI8gA.

我有这样的主要类型，与数据库字段相同：

// Main types as in database - should't be changed
type Book = {
  id: string
  title: string
  visible: boolean
  author: string
}

type Author = {
  id: string
  name: string
}

对于 api 获取响应，我需要通用类型，该类型将根据请求的字段塑造对象

// Inhereted from types from main
type BookFields = keyof Book
type AuthorFields = keyof Author

// type for generating expected fetch response from API
type ApiResBook<
  PickedBookFields extends BookFields,
  PickedAuthorFields extends AuthorFields | undefined = undefined,
> = {
  book: Pick<Book, PickedBookFields> & {
    author?: PickedAuthorFields extends AuthorFields ? Pick<Author, PickedAuthorFields> : undefined
  }
}

// fetch example of usage
async function fn() {

  const fetchAPI = <ExpectedData = any>(
    apiAction: string,
    body: any
  ): Promise<{ data: ExpectedData } | { error: true }> => {
    return new Promise((resolve) => {
      fetch(`api`, body)
        .then((raw) => raw.json())
        .then((parsed: { data: ExpectedData } | { error: true }) => resolve(parsed))
        .catch((err) => {
          console.log(err)
        })
    })
  }

  // response type is { error: true  } | {data: { book: { id: string } } }
  const response = await fetchAPI<ApiResBook<'id'>>('smth', {}) 
}

问题在于通用 ApiResBook 类型，我不知道如何使某些通用类型可选。测试示例包括：

//tests
type BookOnly = ApiResBook<'id'>
type BookWithAuthor = ApiResBook<'id', 'name'>

// should be ok
const bookOnly: BookOnly = { book: { id: '1' } }
const bookWithAuthor: BookWithAuthor = { book: { id: '1', author: { name: 'Max' } } }

// should be error
type BookOnly2 = ApiResBook<'propFoesntExist'>
const bookOnlyError: BookOnly = { book: { id: '1', author: {name: 'Max'} } } 
const bookWithoutAuthorError: BookWithAuthor = {book: {id: '1'}} 

我自己解决了这个问题：/似乎工作正常。

type ApiResBook<
  PickedBookFields extends BookFields,
  PickedAuthorFields extends AuthorFields | undefined = undefined,
> = {
  book: Pick<Book, PickedBookFields> & {
    author: PickedAuthorFields extends AuthorFields ? Pick<Author, PickedAuthorFields> : undefined
  }
}

.