如何将 MongoDB 对象推入全局数组而不覆盖它？

Question

I am having a scoping issue. First, I am searching for the current user and once I find his or her object, I access their friends array. Within that friends array, the usernames of each friend is stored. Then, I attempt to do a forEach loop on that friends array and try to add each found friend object into an array called friendsObjArr. When I test it with one friend and the res.json statement inside the last else statement, it works. However I need it to be outside of the loop, so every friend is pushed. And the problem is, after the loop ends, the data in the array is gone too. Here is my code:

function getFriends(req, res){
    const username = req.params.userId
    let friendsObjArr = []
    User.findOne({username: username}, (err, foundUser) => {
        if (err){
            console.log(err);
        }
        else {
            foundUser.friends.forEach(friend => {
                User.findOne({username: friend}, (err, foundFriend) =>{
                    if (err){
                        console.log(err);
                    }
                    else{
                        console.log(foundFriend);
                        friendsObjArr.push(foundFriend)   
                    }
                })
            });
            res.json(friendsObjArr)
        }
    })
    
}

Answer 1

I would suggest you leverage MongoDB's aggregation pipeline to help you find and group users instead of doing the looping yourself. You may incur performance issue in your js code for iterating objects and querying db everytime.

Here is a MongoDB playground for your reference.