Returned object is undefined, using dynamicHelpers

As @Exploit says in his comment, your tags function makes an async call to query.exec, which will finish after your tags function has returned. Express' helpers and dynamicHelpers can't be async, so you need to refactor this somehow. One way would be to put it on req in an earlier middleware or route, and then have a simple dynamicHelper which returns that:

dynamicHelpers.js:

exports.tags = function(req, res) {    return req.tags;};

Middleware (you might not want to do this for all routes, you could look into e.g. Route specific middleware or Route param pre-conditions):

app.use(function(req, res, next) {    var environment = require('../environment');    var service = require('../service');        service.init(environment);    var model = service.useModel('tag');    var query = model.Tag.find({});    query.exec(function (err, tags) {        if (err) {            return next(err);        }        req.tags = tags;        next();    });});

It's kinda dirty but you can do this to wait for tags to be set. The solution Linus G Thiel provided is better though because this will block your application.

tags: function(req, res) {    var environment = require('../environment');    var service = require('../service');    service.init(environment);    var model = service.useModel('tag');    var query = model.Tag.find({});    var _tags = false;    query.exec(function (err, tags) {        if (err) {            console.log(err);            // do something        }        console.log(tags);        _tags = tags;    });    while(_tags == false);    return _tags;}

Have you tried it like this?

exports = function(app) {    app.dynamicHelpers({        tags: function(req, res) {            var environment = require('../environment');            var service = require('../service');            service.init(environment);            var model = service.useModel('tag');            var query = model.Tag.find({});            query.exec(function (err, tags) {            if (err) {                console.log(err);                // do something            }            console.log(tags);            return tags;            });        }    });}require("helpers")(app);

You need to defined the middleware before the other routes

module.exports = function(app, express, next){    app.configure(function() {        app.use(function(req, res, next) {            var environment = require('./environment');            var service = require('./service');                service.init(environment);            var model = service.useModel('tag');            var query = model.Tag.find({});            query.exec(function (err, tags) {                if (err) {                    return next(err);                }                req.tags = tags;                next();            console.log(req.tags); <--- works fine            });        });        // lots of more app.use functions (eg. express.cookieParser());)        // ...    });};