Asynchronous function doesn't run

Question

Here is a controller content in the context of a nodejs app:

 var express = require('express'),
    directory_router = express.Router(),
    directory_models = require('./directory_models');

directory_router.get('/list', function(req, res) {
    var data = directory_models.read_result(function (err, data) {
        console.log(data); // Need returned data here, but won't run
        res.send(data); // Same
    });
// If I try to access 'data' here it's empty
});

module.exports = directory_router;

And here is the model that returns the data it fetched in the database:

var settings = require('../../settings');

var read = function() {
    var read_result;

    settings.pg.connect(settings.conString, function (err, client, done) {
        client.query('SELECT link_title FROM links', function (err, result) {
            read_result = result.rows[0].link_title;
            console.log('from model');
            return read_result;
        });
    });

};

module.exports.read_result = read;

The problem is in the controller, I can't find anyway to have the 'data' console.log or res to run. And if I put them outside of the function, they run before the data is returned and thus are undefined. How to properly format the model code, to properly be able to access 'data' after result are returned from the model ? And is it recommended to use a module such as 'async' or 'promise' to handle such situation ?

`read` doesn't accept a callback. Therefore the callback you passed to it wasn't used. — Kevin B, Mar 21 '16 at 15:34

score 0 · Accepted Answer · answered Mar 21 '16 at 15:39

Simplest way is to get a callback as a parameter and call then data will be accessed.

var read = function(cb) {
    settings.pg.connect(settings.conString, function (err, client, done) {
        client.query('SELECT link_title FROM links', function (err, result) {
            if (err) {
                return cb(err);
            }

            cb(null, result.rows[0].link_title);
        });
    });
};

score 0 · Answer 2 · answered Mar 21 '16 at 15:42

0

Pass a callback to the read function

answered Mar 21 '16 at 15:42

Abdul R. Taleb-Agha

141
1
9

You may want to explain a little bit more... – Ian Mar 21 '16 at 15:59

score 0 · Answer 3 · answered Mar 21 '16 at 15:46

You need to handle the data in a callback, cause nodejs is async

var express = require('express'),
directory_router = express.Router(),
directory_models = require('./directory_models');

directory_router.get('/list', function(req, res) {
var data = directory_models.read_result(function (err, data) {
    if (err){
       res.status(400).send("cant get data");
    } 
    else{
    console.log(data); // Need returned data here, but won't run
    res.send(data); // Same
    }
});
// If I try to access 'data' here it's empty
});

and inside model you need the callback as a param like :

var settings = require('../../settings');

var read = function(callback) {
var read_result;

settings.pg.connect(settings.conString, function (err, client, done) {
    client.query('SELECT link_title FROM links', function (err, result) {
if(err){ return callback(err,null) }
else{
        read_result = result.rows[0].link_title;
        console.log('from model');
        return callback(null, read_result);
    });
});

};  
module.exports.read_result = read;

Asynchronous function doesn't run

3 Answers3