Replacing callbacks with promises in Node.js

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
}

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).

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.

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.