how to return result in a list use Node.JS querying mongoDB

Question

I am new to JS. here is my function, so when I use this, it will return an empty list.

if I replace resultArray.push(result); with console.log(result); it will correctly show me the result.

I am expecting to see the function return the query result inside a list.

function queryMongo (key) {

    var url = "mongodb://user:pw/task?replicaSet=bdb";
    var resultArray = [];

    // connect to the mongoDB
    MongoClient.connect(url, { useUnifiedTopology: true }, function(err, db) {
        if (err) throw err;
        var dbo = db.db("task");
        var query = { 'device' : key };
        var cursor = dbo.collection('commands').find(query);
        cursor.forEach(function(result){
            if (err) throw err;
            resultArray.push(result);
        }, function(){
            db.close();
        })
    });
    return resultArray;
}

There isn't actually a question. Please try reformulating your message to let the community know what you actually expect from them and also removing "please help me" as this doesn't add value to your issue. I would also invite you making it nicer to read by using the code formatting feature. — rels
– rels, Commented Mar 25, 2020 at 0:22

rels · Accepted Answer · 2020-03-25 00:40:49Z

As shown in here one way would be to get the whole dataset at once:

async function queryMongo(key) {
  var url = 'mongodb://user:pw/task?replicaSet=bdb'

  // connect to the mongoDB
  return new Promise(function (resolve, reject) {
    MongoClient.connect(url, { useUnifiedTopology: true }, function (err, db) {
      if (err) throw err
      var dbo = db.db('task')
      var query = { device: key }
      // Fetch all results
      dbo
        .collection('commands')
        .find(query)
        .toArray(function (err, items) {
          if (err) {
            return reject(err)
          }
          resolve(items)
          db.close()
        })
    })
  })
}

async function doWork() {
  // without "async" keyword it's not possible to "await" the result
  console.log(await queryMongo('some-key'))
}

doWork();

Please take following advices into consideration:

Don't connect/disconnect at each function call aside if you're performing queryMongo() once in a while
The reason your code was not working is because you were returning the result before the async call was actually finished. Hence when doing queryMongo() result was straightly []. Indeed, establishing the connection to mongo and performing the query takes "time" and NodeJS will continue performing code execution while this is happening. I would advise to read a bit around the event loop to understand this mechanic a bit better.

Mike Kravtsov · Accepted Answer · 2020-03-25 01:12:42Z

1

I think that the main problem is that all db calls are asynchronous. You can't just return the value, you can pass to a callback or return Promise.

const MongoClient = require('mongodb').MongoClient;

function queryMongo(key, callback) {
  const url = 'mongodb://localhost:27017';

  MongoClient.connect(url, { useUnifiedTopology: true }, function(err, db) {
    if (err) throw err;
    const dbo = db.db('tasks');
    const query = { device: key };

    dbo
      .collection('commands')
      .find(query)
      .toArray((err, doc) => {
        if (err) throw err;
        db.close();
        callback(doc);
      });
  });
}

queryMongo(12, doc => {

  /* do something */
  console.log(doc);
});

answered Mar 25, 2020 at 1:12

Mike Kravtsov

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