Problem creating an object within forEach loop javascript

Question

I have been trying to create a JSON object with the data that I pull from MongoDB database.

Last line res.status(200).json(userData) seems to return a response before the data processing is over, so I get an empty object without processed data as a response. Any ideas on how to solve this problem?

             // chats are defined before, but excluded here to make a code easier to read
             let userData = {};
             chats.forEach(function(chat){
               let chatId = chat.id;
               let userIds = chat['userIds'];
               UserAccountingData.find({userId: {$in : userIds}}, function(err, userAccountingData){
                  if(err){
                    console.log(err);
                    res.status(404).json('User data not found.');
                    return;
                  } else {
                  userAccountingData.forEach(function(data){
                    console.log({
                      imageUrl: data.imageUrl,
                      firstName: data.firstName,
                      lastName: data.lastName
                    });
                    userData[data.userId] = {
                      imageUrl: data.imageUrl,
                      firstName: data.firstName,
                      lastName: data.lastName
                    };
                  });
                }
               });
             });
             res.status(200).json(userData);

Console.log shows that there is a data coming from the database:

{ imageUrl: 'www.test.de', firstName: 'Fender', lastName: 'Fen' }
{ imageUrl: 'www.test.de', firstName: 'Baveler', lastName: 'Bav' }

Thank you for your time

It is just a chat object id in form of a string looking like this: "5b1232142512c6166c04e655" — makeze, Jan 12 '20 at 14:39
As your `UserAccountingData.find` seems to be an asynchronous call. It won't get filled by the time you are returning the response. Try to fill the `userData` object and return the data as a promise. — Nikhil Goyal, Jan 12 '20 at 14:42

score 1 · Accepted Answer · answered Jan 12 '20 at 14:53

This is because the UserAccountingData.find runs asynchronously. So we need to add async/await logic to this code.

First, define the find function.

const findUserAccountingData = (userIds) => new Promise((resolve, reject) => {
    UserAccountingData.find({ userId: { $in: userIds } }, function (err, userAccountingData) {
        if (err) {
            return reject(err);
        }

        resolve(userAccountingData)
    })
});

Next, modify the original code like below.

let userData = {};
try {
    for (let chat of chats) {
        let chatId = chat.id;
        let userIds = chat['userIds'];
        const userAccountingData = await findUserAccountingData(userIds)
        userAccountingData.forEach(function (data) {
            console.log({
                imageUrl: data.imageUrl,
                firstName: data.firstName,
                lastName: data.lastName
            });
            userData[data.userId] = {
                imageUrl: data.imageUrl,
                firstName: data.firstName,
                lastName: data.lastName
            };
        });
    }
} catch (error) {
    console.log(error);
    // res.status(404).json('User data not found.');   // error doesn't occur when user data not exists.
    res.status(500).json(JSON.stringify(error));
}
res.status(200).json(userData);

Finally you need to mark the calling function as async.

The current code runs serially. We can modify this code for improving performance. That is, we can write the code working exactly like Promise.all() — TopW3, Jan 12 '20 at 14:57
It works like supposed now. I did not know this technique. Thank you for the extra comments, I will do reading on this topic. — makeze, Jan 12 '20 at 15:10

Problem creating an object within forEach loop javascript

1 Answers1