How to return a value from a module?

Question

MongoDB

How can I return the value of res to the main file from the module, if possible, throw off the finished code and explain what the problem is, now it returns undefined, preferably promisom
module code

function insertChat(url){

MongoClient.connect(url,options, function(err, client){

  if(err) throw err;

  const db = client.db('chat');
    const collection = db.collection('chatmessage');

  if(err) return console.log(err);

    collection.findOne(function(err, result){
        if(err) throw err; 
        var res =  result.chatmessage;    
        console.log(res);//test
        return res;
        client.close();
    });
});

}

main file code

database = require('./dbtest.js');
var urlDb = 'mongodb://localhost:27017/chat';
console.log(insertChat(urlDb));

Answer 1

You need to use the same pattern that is implemented in the MongoClient.connect(...) function:

function insertChat(url, callback){
  MongoClient.connect(url,options, function(err, client){
    ...
    collection.findOne(function(err, result){
      ...
      callback(res);
    });
  });
}

database = require('./dbtest.js');
var urlDb = 'mongodb://localhost:27017/chat';
insertChat(urlDb, result => console.log(result));