javascript - forEach over array with .push into another array -

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• why variable unaltered after modify inside of function? - asynchronous code reference 6 answers

i'm working on node / mongoose / express app , i'm having problem getting array update while using foreach. i'm not 100% sure i'm missing. have synchronous vs asynchronous? may have return wrong.

router.route('/:slug/episodes')  .get(function(req, res) {     var episodearray = [];     var episodedetails = null;     show.findone({ 'slug': req.params.slug }, function(err, show) {         if (err) {             res.send(err);         }         var episodes = show.episodes         episodes.foreach(function(episodeid, index) {             episode.findbyid(episodeid, function(err, episode) {                 if (err) {                     res.send(err);                 }                 episodearray.push(episode);             });         });         res.send(episodearray)     }); }); 

episodearray isn't getting episode added in , ends being full of null values.

your code suffering misunderstanding of how async works. first , foremost should read felix kling link @elclanrs posted here. contributors tend tired of answering same async question on , over. haven't quite gotten tired yet, i'll bite purposes of explaining async, i'll suggest option might better way solve problem.

the async solution: there many ways await array of asynchronous operations complete. async.queue popular choice. how works push set of pending operations queue , tell queue wait until results have been received, @ point perform res.send(). code this:

var async = require('async');  show.findone({     'slug': req.params.slug }, function(err, show) {         if (err) {             res.send(err);         }          var episodearray = [];         var queue = async.queue(function(episodeid, callback) {             episode.findbyid(episodeid, function(err, episode) {                 if (err) {                     throw err;                 }                 episodearray.push(episode);                 callback();             });         });          // note foreach synchronous.         // tasks pushed queue before drain()         episodes.foreach(function(episodeid, index) {             queue.push(episodeid);         });          queue.drain = function() {             res.send(episodearray);         };     }); 

this not best way solve problem, purpose of demonstrating how fix existing code.

as long episodes array not obscenely huge, far better way query episodes might use mongodb's $in operator, so:

show.findone({     'slug': req.params.slug }, function(err, show) {         if (err) {             res.send(err);         }          episode.find({             _id: {                 $in: show.episodes             }         }, function(err, episodes) {                 if (err) {                     throw err;                 }                 res.send(episodes);             });     }); 

edit:

if want go bit deeper, should mention mongoose supports promises, can use make code less nested , repetitive, this:

show.findone({     slug: req.params.slug }) .then(function(show) {     return episode.find({         _id: {             $in: show.episodes         }     }); }) .then(function(episodes) {     res.send(episodes); }) // errors returned above short circuit middleware next() here .error(next);