javascript - Issue in returning data retrieved from DB queries called in the loop -

i making multiple mongodb queries in loop. , want send results 1 data array.but when simple use return send data return undefined , not wait results of db request. tried use q.moulde same issue.

code:

var getprayerincat = function(data){     var result ;     var finaldata = [];     if(data.length >0){              data.foreach(function(data2){                  var id= data2.id;                  prayer.find({prayercat:id},function(err,prayer){                      var deferred = q.defer()                      if (err) { // ...                          console.log('an error has occurred');                          // res.send(err);                          result= finaldata = err                      } else {                          if(!prayer){                              // console.log(data2.id+'--0');                              data2.prayerscount = 0;                              result = deferred.resolve(finaldata.push(data2))                          } else {                              // console.log(data2.id+'--'+prayer.length);                              data2.prayerscount = prayer.length;                              // console.log(prayer)                              result =  deferred.resolve(finaldata.push(data2))                          } // else data forward                      }                      deferred.promise;                  })                 // deferred.resolve(finaldata);               })              /*if(finaldata.length > 0) { return finaldata;}*/         } } 

finaldata returned undefined.

let's start general rule using promises:

every function asynchronous must return promise

which functions these in case? it's getprayerincat, foreach callback, , prayer.find.

hm, prayer.find doesn't return promise, , it's library function cannot modify it. rule 2 comes play:

create immediate wrapper every function doesn't

in our case that's easy q's node-interfacing helpers:

var find = q.nbind(prayer.find, prayer); 

now have promises around, , no more need deferreds. third rule comes play:

everything async result goes .then callback

…and returns result. hell, result can promise if "something" asynchronous! this, can write complete callback function:

function getprayercount(data2) {     var id = data2.id;     return find({prayercat:id}) //  ^^^^^^ rule 1     .then(function(prayer) { //  ^^^^^ rule 3         if (!prayer)             data2.prayerscount = 0;         else             data2.prayerscount = prayer.length;         return data2; //      ^^^^^^ rule 3b     }); } 

now, have bit more complicated: loop. repeatedly calling getprayercount() multiple promises, asynchronous tasks run in parallel , resolve in unknown order. want wait of them - i.e. promise resolves results when each of tasks has finished.

for such complicated tasks, don't try come own solution:

check api of library

and there find q.all, this. writing getprayerincat breeze now:

function getprayerincat(data) {     var promises = data.map(getprayercount); // don't use foreach,     return q.all(promises); //  ^^^^^^ rule 1 } 

if needed array q.all resolves to, apply rule 3.