mongoDB collection.findOne() returns undefined

Question

I have a function in a file that I'm using to get users, the console.log() in the function works, but when using it from another file it returns undefined.

    const db = client.db('TestDatabase');
    const collection = db.collection('users');

    exports.getUser = function(_id) {
        collection.findOne({
            _id: _id
        }, (err, data) => {
            if (err) {
                console.log(err);
                return;
            }
            //console.log(data); // Logs the proper data.
            return data; // Return data?
        });
    }

File im using it from:

const database = require('./db.js');

let user = database.getUser(_id);
console.log(user); // This logs "undefined"

Answer 1

尝试给出集合名称；

db.getCollection("<COLLECTION_NAME").find({})....

Answer 2

I'm suspicious of the use of a callback and await . There's no need for the callback function in getUser() if you're going to change how you handle the response later.

In the case that you're trying to use, I would recommend getting rid of the callback and simply wrapping your calls to getUser() in a try/catch so you don't lose your error handling, like so:

 function findOne(query) { return Promise.resolve(query) } function getUser(_id) { return findOne({ _id }) } async function main() { try { let user = await getUser( 'myId' ) console.log(user) } catch (error) { console.error(error) } } main()

The other option is to pass a callback into your getUser() function, and pass that along in your call to findOne() instead of defining it there.

Answer 3

You need to make getUser return a promise if you want to call it using await . Returning a value from your callback function doesn't return anything to the caller.

These days, findOne returns a promise when you don't pass a callback. So you can simply do something like this.

exports.getUser = function(_id) {
    return collection.findOne({
        _id
    });
}

Answer 4

The OP tries to use the function as async, but it must be qualified that way and must return a promise...

exports.getUser = async function(_id) {
  return new Promise((resolve, reject) => {
    collection.findOne({_id: _id}, (err, data) => {
      err ? reject(err) : resolve(data)
    })
  })
}