async function return empty array, does not consider the await?

Question

Hope you are all staying safe.

So I am trying to get all the post of the friends of a current user but it does not work, I always receive a empty array, it is like the function send the array to the response before exercuting all of the loop.

So what I am doing is

getting all the friends of the current user
mapping over all his friends to get all the post of each friends
Then I map over all his post and a push each post in an array
Finally, I want the function to send the array that contains each post when all the loop is over but it does not work, I always get a empty array. I am king of new to the promise and async/await wolrd but I don't get it.

The function looks like this

exports.getFollowerPost = async (req, res) => {
    const id_user  = req.params.userUid;

    let friendsReference = db.collection('Friends').doc(id_user);

    let data = [];

    await  friendsReference.get().then((result) => {

        if (!result.exists) {
            console.log('No such document!');
            res.send('no docs exist')
          } 
          else {
              let friends  = result.data();

              let friendsOfUser = friends.follow;

               friendsOfUser.map((follow)  => {

                let postUser =  db.collection('Post').doc(follow);

                 postUser.get().then(item => {

                    const eachPost = item.data().post;

                    eachPost.map(eachPostUser => {
                        data.push(eachPostUser);
                        console.log("add post")
                    })
                }).catch(error => {
                    console.log(error)
                })
              })          
          }   
    })
    .catch(error =>  {
        res.status(400).send(error)
    })

    res.send(data) // always send a empty array ? why 
}

Julio Ojeda · Accepted Answer · 2020-04-02 18:10:45Z

3

Try this one.

exports.getFollowerPost = async (req, res) => {
  const id_user = req.params.userUid;

  let friendsReference = db.collection('Friends').doc(id_user);

  let data = [];

  try {
    const result = await friendsReference.get();

    if (!result.exists) {
      console.log('No such document!');
      res.send('no docs exist')
    }
    else {
      let friends = result.data();

      let friendsOfUser = friends.follow;

      friendsOfUser.map(async (follow) => {

        let postUser = db.collection('Post').doc(follow);

        const item = await postUser.get()

        const eachPost = item.data().post;

        eachPost.map(eachPostUser => {
          data.push(eachPostUser);
          console.log("add post")
        })


      })
    }
  } catch (error) {
    res.status(400).send(error)
  }

  res.send(data) // always send a empty array ? why 
}

edited Apr 2, 2020 at 18:10

answered Apr 2, 2020 at 16:28

Julio Ojeda

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4 Comments

Matti Greff Over a year ago

You have to remove that bottom catch, but yeah besides that it looks good.

Philippe Vincent Over a year ago

Very appreciate to the two of you for helping me . Thanks Julio for your code. I tried it but it did not work, I add the error, "await can only be use in async function", at first I was wondering where this error came from because I had clearly declare the async at the beginning of the function. I realize the problem was the map.(item => {}) that create a sync arrow function. I have replace it with a simple for loop and now it is working. Thanks

Matti Greff Over a year ago

Now that I think about it you will want to stick to the then style promises for that loop. that or push all of the promises to an array and use Promise.all.

Matti Greff Over a year ago

The way it is currently it only is operating on one async action at a time, which could considerably slow down the speed of this guy.

Matti Greff · Accepted Answer · 2020-04-02 16:28:48Z

1

You are mixing the then and async/await methods for handling promises. Your first step would be sticking to just using await.

const result = await friendsReference.get()

So basically you are awaiting then, which is a handler of a promise, not a promise though so it doesn't wait for it to finish.

edited Apr 2, 2020 at 16:28

answered Apr 2, 2020 at 16:22

Matti Greff

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