Bluebird.js Promise Library Cheat Sheet

Bluebird is a high-performance, feature-packed Promise library for JavaScript and Node.js.

Method Chaining Overview

promise
  .then(okFn, errFn)
  .spread(okFn, errFn)        // * Unpacks array into arguments
  .catch(errFn)
  .catch(TypeError, errFn)    // * Filtered error catch
  .finally(fn)
  .map(function (e) { ... })  // * Iterate concurrently
  .each(function (e) { ... }) // * Iterate sequentially

Methods marked with * are Bluebird-specific extensions beyond the standard ES6 Promise specification.

---

Multiple Arguments & Spreading

Promise.resolve(['foo', 'bar'])
  .spread((item1, item2) => {
    console.log(item1, item2); // 'foo', 'bar'
  });

See: Promise.spread

---

Combining Fixed Promises (Promise.join)

Coordinates multiple independent promises with a fixed signature:

Promise.join(
  getPictures(),
  getMessages(),
  getTweets(),
  (pics, msgs, tweets) => {
    return { pics, msgs, tweets };
  }
);

See: Promise.join

---

Array Collection Methods

• Promise.all([p]): Resolves when all promises resolve, or rejects immediately on first error.
• Promise.some([p], count): Resolves when count promises settle successfully.
• Promise.any([p]): Resolves as soon as any single promise settles (equivalent to some(p, 1)).
• Promise.map(array, fn, { concurrency: 5 }): Maps items with controlled concurrency.

Promise.map(urls, url => fetch(url), { concurrency: 3 })
  .then(responses => {
    // Process responses with max 3 simultaneous requests
  });

---

Object Promises (Promise.props)

Resolves an object containing promise values into an object with resolved properties:

Promise.props({
  photos: getPhotos(),
  posts: getPosts()
}).then(result => {
  console.log(result.photos, result.posts);
});

---

Promisification

Convert legacy Node.js callback-style functions ((err, result) => ...) into promises:

const fs = require('fs');
const Promise = require('bluebird');

// Promisify a single function
const readFile = Promise.promisify(fs.readFile);

// Promisify an entire module
Promise.promisifyAll(fs);
// Now fs.readFileAsync is available

---

Coroutines (Generators)

const loginUser = Promise.coroutine(function* (email, password) {
  const user = yield User.findOne({ email }).exec();
  const valid = yield user.comparePassword(password);
  if (!valid) throw new Error('Invalid credentials');
  return user;
});