node.js - nodejs trouble while updating array asynchronously -

i have problem asynchrony of nodejs. hope can me.

exports.deviceinfo = function(req, res, next) {   var nets = req.body.networks;    db.collection("device", function(err, deviceconf) {     var detected_beacons = [];      if (!err) {       (var = 0; < nets.length; i++) {         deviceconf           .find({             "data.number": nets[i]           })           .toarray(function(errfind, saver) {             if (!errfind && saver) {               saver.foreach(function(disp) {                 var detected = {};                 detected.address = disp.address;                 detected_beacons.push(detected);                 console.log(detected_beacons);               });             }           });       }       console.log(detected_beacons);       res.json(detected_beacons);     } else {       console.error(err);       res.status(500);       res.json({         message: "couldn't connect database"       });     }   }); }; 

if write logs, see problem. detected_beacons being returned before clause. want returned after for in order full of data. have read callbacks , promises don't know how use them in code.

try code. cannot loop concerning callback functions.

exports.deviceinfo= function(req,res,next){ var nets = req.body.networks; db.collection("device", function(err, deviceconf){     var detected_beacons = [];     var netsize = nets.length;     var beaconsloop = function(i) {         deviceconf.find({"data.number" : nets[i]}).toarray(function(errfind, saver){                 if (!errfind && saver){                     var saversize = saver.length;                     saver.foreach(function(disp){                         var detected = {};                         detected.address = disp.address;                         detected_beacons.push(detected);                         saversize--;                         if (saversize == 0) {                             if (i != netsize - 1) {                                 i++;                                 beaconsloop(i);                             } else {                                 res.json(detected_beacons);                             }                         }                     });                 }             });     };      if (!err){         if (netsize > 0) beaconsloop(0);     } else {         console.error(err);         res.status(500);         res.json({message:"couldn't connect database"});     } }); 

};