Undefined value after returning an array of values from a MySQL query in a different file

Question

I'm using the Mysql connector because I need to get some data from my database, but I'm having the following issue:

I have two files server.js and dbConnection.js and I have a return in the dbConnection.js file which should return an array and it should show it in the server.js file. However, it prints out an undefined array. I don't understand what I'm doing wrong since I also tried to print out the array before reurning it in the dbConnection.js file and it's shown with the data.

server.js:

const express = require('express');
const dbConnection = require('./dbConnection.js');

app.get('/', function (req, res) {
    const val1 = new Promise((resolve, reject) => {
        dbConnection
            .getData()
            .then(data => {
        resolve(data)
                })
        .catch(err => reject('error'))
    });

    Promise.all([val1])
    .then(data => {
        console.log(data) //here it prints out [undefined]
    });        
});

dbConnection.js:

const mysql = require('mysql');
const con = mysql.createConnection({
    host: "localhost",
    user: "root",
    database: "db1"
});

const getData = async ()=> {
    var array = new Array();
    const sql1 = "SELECT * FROM table1 WHERE active=1";
    con.query(sql1, function (err, result, fields) {
        if (err) throw err;
        array = [];
        for(var i=0; i<result.length; i++) {
            array.push(result[i].active);                        
        }
        console.log(array) //here it prints out the array with its values
        return array;
    });
}
module.exports.getData = getData;

Edit: Maybe this will be helpful in order to figure out what's happening. I have just tried this and it prints out an empty array []:

const mysql = require('mysql');
var array = new Array();
const con = mysql.createConnection({
    host: "localhost",
    user: "root",
    database: "db1"
});

const getData = async ()=> {
    const sql1 = "SELECT * FROM table1 WHERE active=1";
    con.query(sql1, function (err, result, fields) {
        if (err) throw err;
        //array = [];
        for(var i=0; i<result.length; i++) {
            array.push(result[i].active);

        }
        console.log(array) //here it prints out its data
        //return array;
    });
    console.log(array); //here it prints out []
}

module.exports.getData = getData;

When I print the array out in the dbConnection.js file:

When I print it out in the server.js file:

Why is this happening and how to fix it?

Thanks in advance.

Did you check the value in "data" before you resolve in server.js? What is the data at that instant? — Sreehari, Apr 15 '19 at 10:59
I have just tried it and without returning in ```dbConnection.js```, its value is [ undefined ] too... Weird. — , Apr 15 '19 at 11:03
@Sreehari I have added to my main post something that I have just tried and it might result helpful in order to figure out what's happening. Check it out if you can. I don't get it. It prints out an empty array. — , Apr 15 '19 at 11:23
The place where it has printed out empty array is expected as it is outside of "con.query" asynchronous handler. — Sreehari, Apr 15 '19 at 11:33
Gotcha, so that is why it does not return the array properly I assume? — , Apr 15 '19 at 11:37

Sreehari · Accepted Answer · 2019-04-15T11:59:54.550

Use Async/Await with promises. You cannot use the syntax with callback. You have to change your dbConnection.js as below. You have to promisify your callback.

function myQuery(){ 
    return new Promise(function(resolve, reject){
        var array = new Array();
        const sql1 = "SELECT * FROM table1 WHERE active=1";
        con.query(sql1, function (err, result, fields) {
            if (err) throw err;
            array = [];
            for(var i=0; i<result.length; i++) {
                array.push(result[i].active);                        
            }
            console.log(array) //here it prints out the array with its values
            resolve(array);
        });
    })
}

const getData = async ()=> {
    var array= await myQuery();
    return array;       
}
module.exports.getData = getData;

Thanks. I have just tried this, but there is an error on your code I assume since it crashes on New Promise: https://i.imgur.com/bgcyjZ8.png and my code editor underlines it in red color: https://i.imgur.com/uui9dJl.png — , Apr 15 '19 at 11:56

Undefined value after returning an array of values from a MySQL query in a different file

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