looping over an array of mongodb document ids and querying that id and storing the result in a new array gives empty array

Question

Below is the code I am having issues with:

    const users = []
    event.registeredUsers.forEach(userId => {
            User.findOne({ _id: userId }).then(user => {
                console.log(user) // logs a valid user
                users.push(user)
            });
        });

    console.log(users) // logs empty array

The event is a MongoDB document that has registeredUsers field which is an array of MongoDB object Ids. When console logging the user returned in the then block it does shows a valid user document but when res.send(users) after the loop has been finished, it shows empty array in the browser.

Answer 1

You are trying to execute promise based code inside forEach which doesnt work the way you expect it work. Use either for..of or Promise.all

for..of (sequential)

const users = []
for(const userId of event.registeredUsers) {
    User.findOne({ _id: userId }).then(user => {
       console.log(user) // logs a valid user
       users.push(user)
     });
 }

console.log(users) 

Promise.all (in parallel)

Promise.all(event.registeredUsers.map(userId => {
    return User.findOne({ _id: userId }).then(user => {
       console.log(user) // logs a valid user
       return user;
     });
 }).then(users => console.log(users));