javascript - Nodejs MySQL connection query return value to function call - Stack Overflow

admin管理员组

文章数量:1330343

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

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

• javascript
• mysql
• node.js

Share Improve this question Follow edited May 23, 2017 at 11:53 CommunityBot 111 silver badge asked Mar 26, 2013 at 11:21 Aks1357Aks1357 1,07211 gold badge99 silver badges1919 bronze badges

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 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.

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();

本文标签： javascriptNodejs MySQL connection query return value to function callStack Overflow