Recreating Async / Await

GetBook.js

// Types ///////////////////////////////////////////////////////////////////

type BookId = string
type BookTitle = string
type BookAuthor = string
type BookRating = number
type AuthorName = string
type AuthorId = number
type Author = {
  name: AuthorName,
  id: AuthorId
}
type Book = {
  id: BookId,
  title: BookTitle,
  authorId: AuthorId
}
type BookInfo = {
  author: BookAuthor,
  rating: BookRating
}
type BookWithInfo = Book & BookInfo
type RatingId = number
type RatingValue = number
type Rating = {
  id: RatingId,
  value: RatingValue
}
type Database = {
  books: {[BookId]: Book},
  authors: {[AuthorId]: Author},
  ratings: {[RatingId]: Rating}
}

const database: Database = {
  books: {
    123: {
      id: 123,
      authorId: 345,
      ratingId: 678,
      title: 'Book Title'
    }
  },
  authors: {
    345: {
      name: 'author name',
      id: 345
    }
  },
  ratings: {
    678: {
      id: 678,
      value: 4.5
    }
  }
}

// Level 1
const fetchBook = (title: BookTitle) => Promise.resolve(Object.values(database.books).find(book => book.title === title))
const fetchAuthor = (id: AuthorId) => Promise.resolve(database.authors[id])
const fetchRating = (id: BookId) => Promise.resolve(database.ratings[id])

// Level 2
const getBook = (title: Promise<BookTitle>) => title.then(title => fetchBook(title))
const getAuthor = (book: Promise<Book>) => book.then(book => fetchAuthor(book.authorId))
const getRating = (book: Promise<Book>) => book.then(book => fetchRating(book.ratingId))

const assembleBookWithInfo = ([book, author, rating]: [Book, Author, Rating]): BookWithInfo => ({
  ...book,
  author,
  rating
})

// Level 3
const getBookWithInfo = (title): Promise<BookWithInfo> => {
  const book = getBook(title)
  const author = getAuthor(book)
  const rating = getRating(book)

  return Promise.all([ book, author, rating ])
    .then(assembleBookWithInfo)
}

// Level 4
getBookWithInfo(Promise.resolve('Book Title'))