Replacing callbacks with promises in Node.js

Question

I have a simple node module which connects to a database and has several functions to receive data, for example this function:

---

dbConnection.js:

import mysql from 'mysql';

const connection = mysql.createConnection({
  host: 'localhost',
  user: 'user',
  password: 'password',
  database: 'db'
});

export default {
  getUsers(callback) {
    connection.connect(() => {
      connection.query('SELECT * FROM Users', (err, result) => {
        if (!err){
          callback(result);
        }
      });
    });
  }
};

The module would be called this way from a different node module:

---

app.js:

import dbCon from './dbConnection.js';

dbCon.getUsers(console.log);

I would like to use promises instead of callbacks in order to return the data. So far I've read about nested promises in the following thread: Writing Clean Code With Nested Promises , but I couldn't find any solution that is simple enough for this use case. What would be the correct way to return result using a promise?

Answer 1

Using the Promise class

I recommend to take a look at MDN's Promise docs which offer a good starting point for using Promises. Alternatively, I am sure there are many tutorials available online.:)

Note: Modern browsers already support ECMAScript 6 specification of Promises (see the MDN docs linked above) and I assume that you want to use the native implementation, without 3rd party libraries.

As for an actual example...

The basic principle works like this:

1. Your API is called
2. You create a new Promise object, this object takes a single function as constructor parameter
3. Your provided function is called by the underlying implementation and the function is given two functions - resolve and reject
4. Once you do your logic, you call one of these to either fullfill the Promise or reject it with an error

This might seem like a lot so here is an actual example.

exports.getUsers = function getUsers () {
  // Return the Promise right away, unless you really need to
  // do something before you create a new Promise, but usually
  // this can go into the function below
  return new Promise((resolve, reject) => {
    // reject and resolve are functions provided by the Promise
    // implementation. Call only one of them.

    // Do your logic here - you can do WTF you want.:)
    connection.query('SELECT * FROM Users', (err, result) => {
      // PS. Fail fast! Handle errors first, then move to the
      // important stuff (that's a good practice at least)
      if (err) {
        // Reject the Promise with an error
        return reject(err)
      }

      // Resolve (or fulfill) the promise with data
      return resolve(result)
    })
  })
}

// Usage:
exports.getUsers()  // Returns a Promise!
  .then(users => {
    // Do stuff with users
  })
  .catch(err => {
    // handle errors
  })

Using the async/await language feature (Node.js >=7.6)

In Node.js 7.6, the v8 JavaScript compiler was upgraded with async/await support . You can now declare functions as being async , which means they automatically return a Promise which is resolved when the async function completes execution. Inside this function, you can use the await keyword to wait until another Promise resolves.

Here is an example:

exports.getUsers = async function getUsers() {
  // We are in an async function - this will return Promise
  // no matter what.

  // We can interact with other functions which return a
  // Promise very easily:
  const result = await connection.query('select * from users')

  // Interacting with callback-based APIs is a bit more
  // complicated but still very easy:
  const result2 = await new Promise((resolve, reject) => {
    connection.query('select * from users', (err, res) => {
      return void err ? reject(err) : resolve(res)
    })
  })
  // Returning a value will cause the promise to be resolved
  // with that value
  return result
}

Answer 2

With bluebird you can use Promise.promisifyAll (and Promise.promisify ) to add Promise ready methods to any object.

var Promise = require('bluebird');
// Somewhere around here, the following line is called
Promise.promisifyAll(connection);

exports.getUsersAsync = function () {
    return connection.connectAsync()
        .then(function () {
            return connection.queryAsync('SELECT * FROM Users')
        });
};

And use like this:

getUsersAsync().then(console.log);

or

// Spread because MySQL queries actually return two resulting arguments, 
// which Bluebird resolves as an array.
getUsersAsync().spread(function(rows, fields) {
    // Do whatever you want with either rows or fields.
});

Adding disposers

Bluebird supports a lot of features, one of them is disposers, it allows you to safely dispose of a connection after it ended with the help of Promise.using and Promise.prototype.disposer . Here's an example from my app:

function getConnection(host, user, password, port) {
    // connection was already promisified at this point

    // The object literal syntax is ES6, it's the equivalent of
    // {host: host, user: user, ... }
    var connection = mysql.createConnection({host, user, password, port});
    return connection.connectAsync()
        // connect callback doesn't have arguments. return connection.
        .return(connection) 
        .disposer(function(connection, promise) { //Disposer is used when Promise.using is finished. connection.end(); });
}

Then use it like this:

exports.getUsersAsync = function () {
    return Promise.using(getConnection()).then(function (connection) {
            return connection.queryAsync('SELECT * FROM Users')
        });
};

This will automatically end the connection once the promise resolves with the value (or rejects with an Error ).

Answer 3

Node.js version 8.0.0+:

You don't have to use bluebird to promisify the node API methods anymore. Because, from version 8+ you can use native util.promisify :

const util = require('util');

const connectAsync = util.promisify(connection.connectAsync);
const queryAsync = util.promisify(connection.queryAsync);

exports.getUsersAsync = function () {
    return connectAsync()
        .then(function () {
            return queryAsync('SELECT * FROM Users')
        });
};

Now, don't have to use any 3rd party lib to do the promisify.

Answer 4

Assuming your database adapter API doesn't output Promises itself you can do something like:

exports.getUsers = function () {
    var promise;
    promise = new Promise();
    connection.connect(function () {
        connection.query('SELECT * FROM Users', function (err, result) {
            if(!err){
                promise.resolve(result);
            } else {
                promise.reject(err);
            }
        });
    });
    return promise.promise();
};

If the database API does support Promises you could do something like: (here you see the power of Promises, your callback fluff pretty much disappears)

exports.getUsers = function () {
    return connection.connect().then(function () {
        return connection.query('SELECT * FROM Users');
    });
};

Using .then() to return a new (nested) promise.

Call with:

module.getUsers().done(function (result) { /* your code here */ });

---

I used a mockup API for my Promises, your API might be different. If you show me your API I can tailor it.

Answer 5

2019:

Use that native module const {promisify} = require('util'); to conver plain old callback pattern to promise pattern so you can get benfit from async/await code

const {promisify} = require('util');
const glob = promisify(require('glob'));

app.get('/', async function (req, res) {
    const files = await glob('src/**/*-spec.js');
    res.render('mocha-template-test', {files});
});

Answer 6

When setting up a promise you take two parameters, resolve and reject . In the case of success, call resolve with the result, in the case of failure call reject with the error.

Then you can write:

getUsers().then(callback)

callback will be called with the result of the promise returned from getUsers , ie result

Answer 7

Using the Q library for example:

function getUsers(param){
    var d = Q.defer();

    connection.connect(function () {
    connection.query('SELECT * FROM Users', function (err, result) {
        if(!err){
            d.resolve(result);
        }
    });
    });
    return d.promise;   
}

Answer 8

Below code works only for node -v > 8.x

I use this Promisified MySQL middleware for Node.js

read this article Create a MySQL Database Middleware with Node.js 8 and Async/Await

database.js

var mysql = require('mysql'); 

// node -v must > 8.x 
var util = require('util');


//  !!!!! for node version < 8.x only  !!!!!
// npm install util.promisify
//require('util.promisify').shim();
// -v < 8.x  has problem with async await so upgrade -v to v9.6.1 for this to work. 



// connection pool https://github.com/mysqljs/mysql   [1]
var pool = mysql.createPool({
  connectionLimit : process.env.mysql_connection_pool_Limit, // default:10
  host     : process.env.mysql_host,
  user     : process.env.mysql_user,
  password : process.env.mysql_password,
  database : process.env.mysql_database
})


// Ping database to check for common exception errors.
pool.getConnection((err, connection) => {
if (err) {
    if (err.code === 'PROTOCOL_CONNECTION_LOST') {
        console.error('Database connection was closed.')
    }
    if (err.code === 'ER_CON_COUNT_ERROR') {
        console.error('Database has too many connections.')
    }
    if (err.code === 'ECONNREFUSED') {
        console.error('Database connection was refused.')
    }
}

if (connection) connection.release()

 return
 })

// Promisify for Node.js async/await.
 pool.query = util.promisify(pool.query)



 module.exports = pool

You must upgrade node -v > 8.x

you must use async function to be able to use await.

example:

   var pool = require('./database')

  // node -v must > 8.x, --> async / await  
  router.get('/:template', async function(req, res, next) 
  {
      ...
    try {
         var _sql_rest_url = 'SELECT * FROM arcgis_viewer.rest_url WHERE id='+ _url_id;
         var rows = await pool.query(_sql_rest_url)

         _url  = rows[0].rest_url // first record, property name is 'rest_url'
         if (_center_lat   == null) {_center_lat = rows[0].center_lat  }
         if (_center_long  == null) {_center_long= rows[0].center_long }
         if (_center_zoom  == null) {_center_zoom= rows[0].center_zoom }          
         _place = rows[0].place


       } catch(err) {
                        throw new Error(err)
       }