异步函数返回未定义

Question

I really need to brush up on my async await and promises. I would love some advice.

I'm making an async function call to firebase firestore. The function should return a string depending on a single input param.

The feature is for a 1-1 user chat. The function is to create the chat/find existing chat, and return its ID.

Right now, I am getting undefined as the return value of openChat and can't work out why. The function otherwise works, apart from the return.

I have two functions. One is a React class component lifecycle method, the other my firebase async function.

Here is the class component lifecycle method:

async getChatId(userId) {
  let chatPromise = new Promise((resolve, reject) => {
    resolve(openChat(userId))
  })
  let chatId = await chatPromise
  console.log('chatId', chatId) //UNDEFINED
  return chatId
}

async requestChat(userId) {
  let getAChat = new Promise((resolve, reject) => {
    resolve(this.getChatId(userId))
  })
  let result = await getAChat
  console.log('result', result) //UNDEFINED
}

render() {
  return (<button onClick = {() => this.requestChat(userId)}>get id</button>)
}

and here is the async function:

// both my console.log calls show correctly in console
// indicating that the return value is correct (?)

export async function openChat(otherPersonId) {
  const user = firebase.auth().currentUser
  const userId = user.uid

  firestore
    .collection('users')
    .doc(userId)
    .get()
    .then(doc => {
      let chatsArr = doc.data().chats

      let existsArr =
        chatsArr &&
        chatsArr.filter(chat => {
          return chat.otherPersonId === otherPersonId
        })
      if (existsArr && existsArr.length >= 1) {
        const theId = existsArr[0].chatId

        //update the date, then return id

        return firestore
          .collection('chats')
          .doc(theId)
          .update({
            date: Date.now(),
          })
          .then(() => {
            console.log('existing chat returned', theId)
            //we're done, we just need the chat id
            return theId
          })
      } else {
        //no chat, create one

        //add new chat to chats collection
        return firestore
          .collection('chats')
          .add({
            userIds: {
              [userId]: true,
              [otherPersonId]: true
            },
            date: Date.now(),
          })
          .then(docRef => {
            //add new chat to my user document

            const chatInfoMine = {
              chatId: docRef.id,
              otherPersonId: otherPersonId,
            }
            //add chat info to my user doc
            firestore
              .collection('users')
              .doc(userId)
              .update({
                chats: firebase.firestore.FieldValue.arrayUnion(chatInfoMine),
              })

            //add new chat to other chat user document
            const chatInfoOther = {
              chatId: docRef.id,
              otherPersonId: userId,
            }
            firestore
              .collection('users')
              .doc(otherPersonId)
              .update({
                chats: firebase.firestore.FieldValue.arrayUnion(chatInfoOther),
              })
            console.log('final return new chat id', docRef.id)
            return docRef.id
          })
      }
    })
}

If you have any useful tips whatsoever, I would be forever grateful to hear them!

Expected results are a returned string. The string is correctly displayed the console.log of the async function).

Actual results are that the return value of the async function is undefined.

Answer 1

You do not return anything from your openChat function, so that function resolves to undefined .

You have to write:

export async function openChat(otherPersonId) {
  const user = firebase.auth().currentUser
  const userId = user.uid

  return firestore // here you need to return the returned promise of the promise chain
    .collection('users')
    .doc(userId)
    .get()
    /* .... */
}

And those new Promise in getChatId and requestChat do not make much sense. It is sufficient to await the result of openChat(userId) or this.getChatId(userId)

async getChatId(userId) {
  let chatId = await openChat(userId)
  console.log('chatId', chatId) //UNDEFINED
  return chatId
}

async requestChat(userId) {
  let result = await this.getChatId(userId)
  console.log('result', result) //UNDEFINED
}

Answer 2

You should await the results from your firestore calls if you want to return their values, you are already using async functions :

export async function openChat(otherPersonId) {
    const user = firebase.auth().currentUser
    const userId = user.uid

    const doc = await firestore
        .collection('users')
        .doc(userId)
        .get()

    let chatsArr = doc.data().chats

    let existsArr =
        chatsArr &&
        chatsArr.filter(chat => chat.otherPersonId === otherPersonId)
    if (existsArr && existsArr.length >= 1) {
        const theId = existsArr[0].chatId

        //update the date, then return id

        await firestore
            .collection('chats')
            .doc(theId)
            .update({
                date: Date.now(),
            })

        return theId
    } else {

        const docRef = await firestore
            .collection('chats')
            .add({
                userIds: { [userId]: true, [otherPersonId]: true },
                date: Date.now(),
            })

        const chatInfoMine = {
            chatId: docRef.id,
            otherPersonId: otherPersonId,
        }
        //add chat info to my user doc
        firestore
            .collection('users')
            .doc(userId)
            .update({
                chats: firebase.firestore.FieldValue.arrayUnion(chatInfoMine),
            })

        //add new chat to other chat user document
        const chatInfoOther = {
            chatId: docRef.id,
            otherPersonId: userId,
        }
        firestore
            .collection('users')
            .doc(otherPersonId)
            .update({
                chats: firebase.firestore.FieldValue.arrayUnion(chatInfoOther),
            })
        console.log('final return new chat id', docRef.id)
        return docRef.id   
    }
}

You should also directly await your calls to the function :

async getChatId(userId) {
    let chatId = await openChat(userId)
    console.log('chatId', chatId) //UNDEFINED
    return chatId
}

async requestChat(userId) {
  let result = await this.getChatId(userId)
  console.log('result', result) //UNDEFINED
}