具有可選擴展的通用打字稿

Question

如何使用擴展進行輸入檢查使通用 ApiResBook 類型與可選道具一起使用？

沙盒。

我有這個主要類型，與數據庫字段相同：

// 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
}

對於 api fetch 響應，我需要通用類型，該類型將根據請求的字段塑造對象

// 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', {}) 
}

問題在於通用 ApiResBook 類型，我不知道如何將一些通用類型設為可選。 測試示例包括：

//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'}} 

Answer 1

自己解決了這個問題：/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
  }
}

沙盒。