异步数组。map 返回未定义

Question

I can't quite work out why my async map fetch function returns an array of undefined values or "Promise { }" no matter what I do. My getImageRef function works (I've tried to simplify it by stripping some bits out) but how do I get it to return values to the Promise.all in formatBooks?

const inventory = [
    {
    "isbn":"foo",
    "title":"bar",
    "imageURL":"http://web.com/image.jpg"
    }
]
const getImageRef = async book => { //this part works on its own
  await fetch(book.imageURL)  
    .then(res => res.buffer())
    .then(buffer => client.assets.upload('image', buffer))
    .then(assetDocument => {
      let book.imageRef = assetDocument._id
      return book
    })
    .catch(() => {
      console.error('Error: ' + book.image)
      return book
    })
  }

async function formatBooks (inventory) {

  await Promise.all(inventory.map(async book => {
        const docs = await getImageRef(book) // 'docs' returns as undefined

      })
  ).then(function(docs) {   
    uploadBooks(docs)
  })
}
function uploadBooks (documents) {   
  console.log(documents)
}
formatBooks(data)

Answer 1

You need to change

await fetch(book.imageURL)

to

return fetch(book.imageURL)

in the getImageRef() function.

That way, docs will be equal to whatever getImageRef() returns. It's required because await itself does not cause the whole getImageRef() function to return anything. The function returns whatever comes after the return keyword or undefined if it's not provided - and that's where the undefined comes from in your case.