NodeJS callback after multiple async-functions in for-loop

Question

I get a document from a mongodb which contains an array with comments for that document. In the comment is the _id of the user which wrote the comment.

I now need to ge

Answer 1

You can try like the following way

const d = {
    "_id" : ObjectId("5c18c1cbc47e5e29d42e4b0e"),
    "completed" : false,
    "completedAt" : null,
    "comment" : [ 
        {
            "_id" : ObjectId("5c18c95e328c8319ac07d817"),
            "comment" : "This is a comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }, 
        {
            "_id" : ObjectId("5c18fb578de5741f20a4e2bd"),
            "comment" : "Another comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }
    ]
}

d.comment.forEach( async (obj, index) => {
    await new Promise((res) => {
            obj.counter = index;
            res();
    })
});

console.log(d);

For reference please take a look on following link Asycn/Await using forEach

Answer 2

There are serveral approaches you can use here based on your convenience

Using async await

let article = await Article.findOne({ _id: articleid }).lean().exec()

await Promise.all(
  article.comment.map(async(obj) => {
    const user = await User.findOne({ _id: obj.user })
    obj.username = user.username
  })
)

console.log(article)

Using $lookup aggregation 3.6

Since mongodb has its own powerfull $lookup aggregation operator to join multiple collection and probably the better approach without any iteration

Article.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(articleid) }},
  { "$unwind": "$comment" },
  { "$lookup": {
    "from": "users",
    "let": { "userId": "$comment.user" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": ["$$userId", "$_id"] }}}
    ],
    "as": "comment.user"
  }},
  { "$unwind": "$comment.user" },
  { "$group": {
    "_id": "$_id",
    "comment": { "$push": "$comment" },
    "completed": { "$first": "$completed" },
    "completedAt": { "$first": "$completedAt" }
  }}
])

Using $lookup aggregation 3.4

Article.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(articleid) }},
  { "$unwind": "$comment" },
  { "$lookup": {
    "from": "users",
    "localField": "comment.user",
    "foreignField": "_id",
    "as": "comment.user"
  }}
  { "$unwind": "$comment.user" },
  { "$group": {
    "_id": "$_id",
    "comment": { "$push": "$comment" },
    "completed": { "$first": "$completed" },
    "completedAt": { "$first": "$completedAt" }
  }}
])