Cannot get return from module in nodejs + node-mysql

Question

I got the following code on my localhost:

/model/company.js

var mysql      = require('mysql');
var connection = mysql.createConnection({
  host     : 'localhost',
  user     : 'root',
  password : 'admin'
});

module.exports = {
    get: function(){
        connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
            return rows;
        })
    }       
}

/routes/company.js

var Company    = require('../models/company')


var express = require('express');
var router = express.Router();

router.route('/companies').get(function(req, res){

    console.log(Company.get());
    //res.json(Company.get());


})

I already tried some things, but I think this is how it should be. But my console.log returns me undefined. I don't know what I'm doing wrong.

If I do:

 connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
                console.log(rows)
            })

It works.

What do I need to do (or study)?

score 2 · Accepted Answer · edited May 23 '17 at 12:01

Your get function returns undefined because you don't specify a return value. You specify a return value for your callback function, but your outer function doesn't know or care about that. If you want the behavior you expect, you need to pass in a callback to your get function to have access to the rows variable.

Try this:

 module.exports = {
    get: function(callback){
        connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
            // A 'node-style' callback will usually be callback(error, value)
            callback(null, rows);
        })
    }       
}

Your console.log will work if you do this:

Company.get(function(error, rows){
  console.log(rows);
}

For a general overview of asynchronous behavior in javascript, check out this answer.

score 1 · Answer 2 · answered May 18 '15 at 20:12

The reason is because your get function is calling an asynchronous function inside it. Where you're calling return rows, it's actually returning the rows to the anonymous function not the get function.

This is basically what you have:

module.exports = {
    get: function(){
        connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
            //This return is returning THIS function --^
            return rows;
        });

        return; //You're not returning anything. So it's undefined like just calling return.
    }       
}

What you want:

/model/company.js:

...

module.exports = {
    get: function(callback){
        connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
            callback(rows);
        })
    }       
}

/routes/company.js:

...

router.route('/companies').get(function(req, res){
    Company.get(function (rows) {
        console.log(rows);
    });
})

score -1 · Answer 3 · answered May 18 '15 at 20:10

-1

You need to return the json as follows

get: function(){
        connection.query('SELECT 1 + 1 AS solution', function(err, rows, fields) {
            res.json(rows);
        })
    }

answered May 18 '15 at 20:10

Nagasimha Iyengar

461
5
8

2

If this is the case, he'd also need to pass in the `res` object. In general, it's probably not a good practice to have your database module know about how the response object is going to work. – Brennan May 18 '15 at 20:11

Cannot get return from module in nodejs + node-mysql

3 Answers3