How to return a value from a mysql SELECT query in node.js

callback, node-mysql, node.js

Solution

Your two functions should be something like -

function fetchID(data, callback) {
        connection.query('SELECT id_user FROM USERS WHERE username = ?', data.username, function(err, rows) {
            if (err) {
                callback(err, null);
            } else 
                callback(null, rows[0].id_user);
        });
}

and then

var user_id;

fetchID(data, function(err, content) {
    if (err) {
        console.log(err);
        // Do something with your error...
    } else {
        user_id = content;
    }
});

Here in the callback function, the returned variable `content` will hold the value for `user_id`.

EDIT

I have not solved the exact problem as you had described above.

But in following example, I have shown that, the callback mechanism is working -

First (Table creation and insert some dummy data)-

use test;
create table users (id int(11) primary key,username varchar(100));
insert into users values(1, "John");
insert into users values(2, "Sham");

Now I have made your post method as get and tested in browser.

Following is the full class tested in my localhost -

var application_root = __dirname,
    express = require("express"),
    mysql = require('mysql');
var app = express();
var connection = mysql.createConnection({
  host : 'localhost',
  user : 'root',
  password : 'admin',
  database: "test"
});
app.get('/getuser', function(req, res) {
  //get data from the request
    var data = {
        username: req.query.username
    };
    function fetchID(data, callback) {
        connection.query('SELECT id FROM users WHERE username = ?',        
               data.username, function(err, rows) {
            if (err) {
                callback(err, null);
            } else 
                callback(null, rows[0].id);
        });
    }
    var user_id;
    fetchID(data, function(err, content) {
        if (err) {
        console.log(err);
        res.send(err);  
        // Do something with your error...
        } else {
        user_id = content;
        console.log(user_id);
        res.send("user id is -" + user_id);
        }
    });
})
app.listen(1212);

Now these requests will produce this output - `http://127.0.0.1:1212/getuser?username=john` => `user id is -1` and `http://127.0.0.1:1212/getuser?username=sham` => `user id is -2`

Hope this code example will help you to understand the callback in node.js.

Thanks

Problem

I'm still very new to Node.js, and i'm trying to understand how callbacks work. So, here is my problem : I should've put more code : POST : ``` app.post('/register', function(req, res) { //get data from the request var data = { username: req.body.username, email: req.body.email, password: req.body.password }; function fetchID(callback) { connection.query('SELECT id_user FROM USERS WHERE username = ?', data.username, function(err, rows) { if (err) { callback(err, null); } else callback(null, rows[0].id_user); }); } var user_id; fetchID(function(err, content) { if (err) { console.log(err); return next("Mysql error, check your query"); } else { user_id = content; console.log(user_id); //undefined } }); console.log(user_id); //undefined var payload = { iss: req.hostname, sub: user_id } console.log(payload.sub); //correct id }) ``` GET : ``` app.get('/todos', function(req, res) { if (!req.headers.authorization) { return res.status(401).send({ message: 'You are not authorized !' }); } var token = req.headers.authorization.split(' ')[1]; var payload = jwt.decode(token, "shhh.."); //additional level of security console.log('sub id is : ' + payload.sub); //undefined if (!payload.sub) { return res.status(401).send({ message: 'Authentication failed !' }); } }) ``` I commented each console.log to be more clear. I need to get the correct id when i check for `if (!payload.sub)` in app.get()

Original source