Issues arise when attempting to instantiate an object within a forEach loop in JavaScript

I've been working on creating a JSON object using data pulled from a MongoDB database.

The issue I'm facing is that the last line res.status(200).json(userData) seems to send a response before the data processing is complete, resulting in an empty object being returned without any processed data. Any suggestions on how to resolve this issue?

// The 'chats' variable is defined elsewhere
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 confirms that data is being retrieved from the database:

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

Thank you for your assistance.

Answer №1

The reason for this issue is that the UserAccountingData.find function operates asynchronously. To fix this, we must incorporate async/await logic into the code.

To start, define the find function as shown below.

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

        resolve(userAccountingData)
    })
});

Next, make adjustments to the original code like this.

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(500).json(JSON.stringify(error)); // Handle error by returning status 500 with error details
}
res.status(200).json(userData);

Lastly, remember to mark the calling function as async.

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