This post was written for Nodejitsu blog and is crossposted here. Nodejitsu loved what I was doing at and is now supporting the project.

I’m always on a look out for interesting modules and unusual solutions. When working with promises in Node.js, there’s a constant friction between callbacks that is Node’s defacto interface and your desire to use promises throughout your entire project. The now famous Bluebird (Github: petkaantonov/bluebird, License: MIT) library has a good collection of helper methods to assist you in going to and from callback world.

Over time I began to feel the need for a simple helper function to automagically integrate a callback style function into promises control flow. Calling promisify each time on a function isn’t super fun and promisifyAll feels a little odd to me and so yesterday I set out to look for a module that implements something like this:

  .call(fs.readFile, 'path/to/file.txt', 'utf8')

Not surprisingly somebody else has already thought of this. I found the songbird (GitHub: duereg/songbird, License: MIT) module by Matt that implements a very clean interface for converting any callback style function to a return a promise instead. songbird does what is generally considered a no-no – it extends Object and Function prototype which gives you a magic promise property on every function. songbird also wraps the Bluebird library so you don’t have to install both. Lets check it out:

npm install songbird


var Promise = require('songbird');
var fs = require('fs');

// callback API
fs.readFile('./package.json', 'utf8', function(err, pkg) {
  console.log('callbacks', pkg);

// songbird API
fs.promise.readFile('./package.json', 'utf8').then(function(pkg) {
  console.log('promise', pkg);

    fs.promise.readFile('./package.json', 'utf8'),
    fs.promise.readFile('./songbird.js', 'utf8')
  .spread(function(pkg, songbird) {
    console.log('package.json length is', pkg.length);
    console.log('songbird.js length is', songbird.length);

Closing thoughts

While I think it’s a great idea to mix in the promise property to Object and Function, however with great power comes great responsibility. I strongly urge against using songbird in modules that you would distribute on npm because it would have a very big side effect on anyone who dares to install your code. However, when used on a project that isn’t made available publicly, songbird would be a great asset.

Check out source code for this article on GitHub and an interactive example on Runnable. Let us know what you think! It’s not every day you encounter a module that boldly extends Object and Function.