Firebase 中的异步函数甚至在函数体完成执行之前就解决了 Promise

Question

所以我有一个异步函数，它接受一组自定义对象作为参数，然后遍历每个对象并从 firestore 获取一些数据。 Firestore 中的数据被转换为另一个自定义对象，并添加到另一个自定义对象数组中。这些对象的长度由函数返回。

async function getCompsReceived(theInterestedPeople: InterestedPerson[]) {

  const whatsNewObjects: WhatsNewObject[] = []

  theInterestedPeople.forEach(async (person) => {

    const documentsSnapshot = await 
    db.collection('Users').doc(person.uid).collection('complimentsReceived')
      .orderBy('receivedTime', 'desc')
      .limit(documentLimit)
      .get()

  if (documentsSnapshot.empty) {
    console.log(`${person.userName} has no compsReceived`)
    return
  }


  await documentsSnapshot.forEach((newCompReceived: { receiverUid: string; receiverUserName: string; 
    complimentsReceivedContent: string | null; hasImage: boolean | null; senderUid: string | null; 
      senderUserName: string | null; noOfLikes: number | null; receivedTime: number | null; 
        complimentId: string | null} ) => {
         //Add each new Compliment to the WhatsNewObjects Array
         const whatsNewDoc = new WhatsNewObject(
           newCompReceived.receiverUid,
           newCompReceived.receiverUserName,
           true,
           newCompReceived.complimentsReceivedContent,
           newCompReceived.hasImage,
           newCompReceived.senderUid,
           newCompReceived.senderUserName,
           newCompReceived.noOfLikes,
           newCompReceived.receivedTime,
           newCompReceived.complimentId,
           'PERSON_COMPLIMENT_RECEIVED'
         )

    whatsNewObjects.push(whatsNewDoc)

  })

  console.log(`length of whatsNewObjects after adding ${person.userName}'s compsReceived is      
    ${whatsNewObjects.length}`)

})

console.log(`returning the length of WhatsNewObjects at getCompsReceived which is ${whatsNewObjects.length}}`)
    return whatsNewObjects.length

}

问题是这个函数总是返回 0 并且在 Firebase 控制台上打印日志语句，我看到函数的主体在函数已经返回一个类型为 number 的 Promise 值之后被执行。

有人可以帮助我如何让函数在返回 whatsNewObjects.length 之前等待主体执行？

Answer 1

您不应该使用forEach 来迭代异步代码，因为它不使用await。它只会一劳永逸。请改用for..of：

async function getCompsReceived(theInterestedPeople: InterestedPerson[]) {

    const whatsNewObjects: WhatsNewObject[] = []

    for (const person of theInterestedPeople) {
        const documentsSnapshot = await db.collection('Users').doc(person.uid).collection('complimentsReceived')
            .orderBy('receivedTime', 'desc')
            .limit(documentLimit)
            .get();

        if (documentsSnapshot.empty) {
            console.log(`${person.userName} has no compsReceived`)
            return;
        }

        documentsSnapshot.forEach((newCompReceived: {
            receiverUid: string; receiverUserName: string;
            complimentsReceivedContent: string | null; hasImage: boolean | null; senderUid: string | null;
            senderUserName: string | null; noOfLikes: number | null; receivedTime: number | null;
            complimentId: string | null
        }) => {
            //Add each new Compliment to the WhatsNewObjects Array
            const whatsNewDoc = new WhatsNewObject(
                newCompReceived.receiverUid,
                newCompReceived.receiverUserName,
                true,
                newCompReceived.complimentsReceivedContent,
                newCompReceived.hasImage,
                newCompReceived.senderUid,
                newCompReceived.senderUserName,
                newCompReceived.noOfLikes,
                newCompReceived.receivedTime,
                newCompReceived.complimentId,
                'PERSON_COMPLIMENT_RECEIVED'
            );
            whatsNewObjects.push(whatsNewDoc);

        })
        console.log(`length of whatsNewObjects after adding ${person.userName}'s compsReceived is ${whatsNewObjects.length}`);
    }
    console.log(`returning the length of WhatsNewObjects at getCompsReceived which is ${whatsNewObjects.length}}`);
    return whatsNewObjects.length;
}

Answer 2

问题与forEach 有关，即使您在回调函数中使用async/await，它实际上也是异步运行的。 您只需将forEach 更改为任何类型的for 迭代器即可解决此问题。

for (let index = 0; index < theInterestedPeople.length; index++) {
    const person = array[index];
    ....your code
}
// or
for (const person of theInterestedPeople) {
    
}
// or