Boilerplate-free promises in Node

One of the few frustrations I’ve had as I’ve been writing more Node-based JavaScript lately is that deeply-nested code comes quickly. Async-by-default is a powerful philosophy, but callback hell is real.

It’s well-established that the promise pattern helps to reduce nested code, but you still end up writing a lot of boilerplate just to handle a promise:

var Promise = require('bluebird');

// before
function renameSomething (callback) {
    fs.rename('foo', 'bar', callback);
}

// after
function renameSomething_promise () {
    return new Promise(function (resolve, reject) {
        fs.rename('foo', 'bar', function (err) {
            if (err) {
                reject();
            } else {
                resolve();
            }
        });
    });
}

Half the power of promises is that they’re chainable, so you end up writing a ton of short functions like the above, and the reject/resolve boilerplate gets old fast.

Then I discovered that the Bluebird promise implementation has an excellent utility function called promisify. It works on any async function that follows the Node convention of having the callback be the last argument, which takes an error as its first argument.

var Promise = require('bluebird');

function renameSomethingPromise () {
    var rename = Promise.promisify(fs.rename);
    return rename('foo', 'bar');
}

This will return a new Promise, resolve or reject it based on the callback error argument, and pass back whatever value is returned in the following argument. All with the addition of only one line of code per async function (or less if you’re reusing promisified functions).

Promise.promisify can do more, like converting an entire module and resolving promises where the callback returns multiple success arguments.

Node’s community leaders have already stated that callbacks will remain the de facto standard for asynchronous code, so this trick is bound to come in handy for a good while.

UPDATE: It’s come to my attention, via reddit user _Blaster_Master, that passing an object to Promise.promisify (not a single function) has been deprecated in favor of Promise.promisifyAll, which creates new functions on a provided object rather than overwriting existing functions. Use that when you promisify entire modules!