promise-callbacks

promise-callbacks

This package helps you work with a codebase that uses promises instead of callbacks in most-but-not-all places. It differs from most other callback-to-promise libraries out there by preferring a deferred variant of a Promise with a Node callback-compliant defer method. As such, its difference is most significantly that it focuses on interoperating with callbacks at the call site.

It also uses native promises not Bluebird etc.

This is because it's 2017, and this package assumes that you'll convert all your own code to use native promises (especially now that recent versions of Chrome and Node 7.6.0 natively support async/await) and the API calls left over will be 3rd-party libraries that you really don't want to patch, due to not having access to library classes and/or the general hackiness of monkey-patching (just try to trace this).

Hopefully these 3rd-party libraries will get their acts together in the relatively near future. In the meantime, there's promise-callbacks to keep it simple.

Installation

yarn add promise-callbacks

or

npm install promise-callbacks --save

The minimum requirement is a native Promise implementation, though you'll get the most out of this if you're using Chrome minus 2 or Node 7.6.0 or higher for async/await.

Usage

Converting a callback to a promise

const { deferred } = require('promise-callbacks');

function respondWithDelay(done) {
  setTimeout(() => done(null, 'hi'), 2000);
}

async function foo() {
  const promise = deferred();
  respondWithDelay(promise.defer());
  console.log(await promise);
}

What happened there is that promise.deferred() took the result of respondWithDelay, as a callback, and resolved/rejected the associated Promise.

Variadic arguments

To support callbacks that provide several values, you have two options: as an array - where you can destructure into your own variables, or as an object, with a similar outcome.

const { deferred } = require('promise-callbacks');

function manyValues(done) {
  setTimeout(() => {
    done(null, 'several', 'values', 'here');
  }, 2000);
}

async function asArray() {
  const promise = deferred({variadic: true});
  respondWithDelay(promise.defer());
  const [first, second, third] = await promise;
  console.log(`${first} ${second} ${third}`);
}

async function asObject() {
  const promise = deferred({variadic: ['first', 'second', 'third']});
  respondWithDelay(promise.defer());
  const {first, second, third} = await promise;
  console.log(`${first} ${second} ${third}`);
}

Converting a callback API to a promise API

The promisify function is based off of Node 8's util.promisify. It works on versions of Node prior to 8, and has special support for callbacks with multiple values, and has utilities to create a copy of an object with promise-returning methods.

For a function

const { promisify } = require('promise-callbacks');

function respondWithDelay(done) {
  setTimeout(() => done(null, 'hi'), 2000);
}

const respondWithDelayPromised = promisify(respondWithDelay);

async function foo() {
  console.log(await respondWithDelayPromised());
}

For an object

const { promisify } = require('promise-callbacks');
const fs = require('fs');

// Note that readFile and writeFile are internally bound to fs, so they can interact with the
// original context object as they expect.
const { readFile, writeFile } = promisify.methods(fs, ['readFile', 'writeFile']);

readFile('input')
  .then((content) => writeFile('output', content))
  .catch((err) => console.error('err', err));

// If you know all the methods of the object are asynchronous, use promisify.all:
const api = {
  respondWithDelay
};

const promiseAPI = promisify.all(api);

async function foo() {
  console.log(await promiseAPI.respondWithDelay());
}

Converting a promise to a callback

const { asCallback } = require('promise-callbacks');

asCallback(Promise.resolve(true), (err, res) => {
  console.log(res); // true
});

Straightforward. Or, if you don't mind just a little bit of monkey-patching:

const { patchPromise } = require('promise-callbacks');

// Call this once, when your application starts up,
// to add `asCallback` to `Promise.prototype`.
patchPromise();

// Thereafter:
Promise.resolve(true).asCallback((err, res) => {
  console.log(res); // true
});

Real-world example

example/app.js demonstrate these APIs' use in the context of a web server. Do yarn run example to start it.

Shout-outs

asCallback is inspired by Bluebird.