Nodejs MySQL connection query return value to function call

Question

I am trying get value from database. Trying it out with a demo example. But I am having problem to synchronize the calls, tried using callback function. I am beginner in node.js, so don't know if this is the right way.

FILE 1 : app.js

var data;

var db = require('./db.js');

var query = 'SELECT 1 + 1 AS solution';

var r = db.demo(query, function(result) { data = result; });

console.log( 'Data : ' + data);

FILE 2 : db.js

var mysql      = require('./node_modules/mysql');

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

module.exports.demo = function(queryString, callback) {
    try {
        connection.connect();
        console.log('Step 1');

        connection.query(queryString, function(err, rows, fields) {
            console.log('Step 2');
            if (err) {
                console.log("ERROR : " + err);
            }
            console.log('The solution is: ', rows[0].solution);

            callback(rows[0].solution);

            return rows[0].solution;
        });
        callback();

        connection.end();
        console.log('Step 3');
    }
    catch(ex) {
        console.log("EXCEPTION : " + ex);
    }
};

OUTPUT :

Step 1
Step 3
Data : undefined
Step 2
The solution is:  2

Referred to this question also, but it didnt solve my problem : nodeJS return value from callback

robertklep · Accepted Answer · 2013-03-26 11:41:23Z

3

The issue is this:

var r = db.demo(query, function(result) { data = result; });

console.log( 'Data : ' + data);

The console.log will run before the callback function gets called, because db.demo is asynchronous, meaning that it might take some time to finish, but all the while the next line of the code, console.log, will be executed.

If you want to access the results, you need to wait for the callback function to be called:

var r = db.demo(query, function(result) { 
  console.log( 'Data : ' + result);
});

This is how most code dealing with I/O will function in Node, so it's important to learn about it.

answered Mar 26, 2013 at 11:41

robertklep

205k38 gold badges421 silver badges412 bronze badges

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5 Comments

Aks1357 Over a year ago

Thank you very much. Its executing the way it should, but i have a doubt, by using the above approach we can only perform actions on result we got in the scope of the callback function, right..?

Aks1357 Over a year ago

if i want to perform action on data after the function call wil it work..?

robertklep Over a year ago

No, it won't, but that's how Node works. There are libraries which mitigate this sort of workflow, like q.

Farhat Aziz Over a year ago

can I use await like var r = await db.demo(query, function(result) { data = result; });

robertklep Over a year ago

@FarhatAziz no, db.demo() is not async and doesn't return a promise.

Johnny Saldana · Accepted Answer · 2022-01-30 15:57:53Z

0

This is my solution

in a file db.js


require('dotenv').config();
const mysql = require('mysql');

const con = mysql.createConnection({
  user: process.env.SQL_USER,
  host: process.env.SQL_HOST,
  database: process.env.SQL_DB,
  password: process.env.SQL_PSWD,
  port: 3306,
});

async function connect() {
  try {
    await con.connect();
    console.log("Connected to MySql!");
  } catch (err) {
    console.log(err);
  }
}

module.exports = { connect };

Then in another file index.js

const db = require("./data/db");

db.connect();

answered Jan 30, 2022 at 15:57

Johnny Saldana

294 bronze badges

1 Comment

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