Converting a callback API to promises.

Remarks#

Promises have state, they start as pending and can settle to:

fulfilled meaning that the computation completed successfully.
rejected meaning that the computation failed.

Promise returning functions should never throw, they should return rejections instead. Throwing from a promise returning function will force you to use both a } catch { and a .catch. People using promisified APIs do not expect promises to throw. If you’re not sure how async APIs work in JS - please see this answer first.

Converting a whole NodeJS module at once

Let’s say you have a library that returns callbacks, for example the fs module in NodeJS:

const fs = require("fs");
fs.readFile("/foo.txt", (err, data) => {
  if(err) throw err;
  console.log(data);
});

We want to convert it to a promise returning API, with bluebird - we can do this using promisifyAll which converts an entire API to use promises:

const Promise = require("bluebird");
const fs = Promise.promisifyAll(require("fs"));
// this automatically adds `Async` postfixed methods to `fs`.
fs.readFileAsync("/foo.txt").then(console.log);

Which lets you use the whole module as promises.

Here are some common ecxamples on how to promisify certain modules:

// The most popular redis module
var Promise = require("bluebird");
Promise.promisifyAll(require("redis"));
// The most popular mongodb module
var Promise = require("bluebird");
Promise.promisifyAll(require("mongodb"));
// The most popular mysql module
var Promise = require("bluebird");
// Note that the library's classes are not properties of the main export
// so we require and promisifyAll them manually
Promise.promisifyAll(require("mysql/lib/Connection").prototype);
Promise.promisifyAll(require("mysql/lib/Pool").prototype);
// Mongoose
var Promise = require("bluebird");
Promise.promisifyAll(require("mongoose"));
// Request
var Promise = require("bluebird");
Promise.promisifyAll(require("request"));
// Use request.getAsync(...) not request(..), it will not return a promise
// mkdir
var Promise = require("bluebird");
Promise.promisifyAll(require("mkdirp"));
// Use mkdirp.mkdirpAsync not mkdirp(..), it will not return a promise
// winston
var Promise = require("bluebird");
Promise.promisifyAll(require("winston"));
// rimraf
var Promise = require("bluebird");
// The module isn't promisified but the function returned is
var rimrafAsync = Promise.promisify(require("rimraf"));
// xml2js
var Promise = require("bluebird");
Promise.promisifyAll(require("xml2js"));
// jsdom
var Promise = require("bluebird");
Promise.promisifyAll(require("jsdom"));
// fs-extra
var Promise = require("bluebird");
Promise.promisifyAll(require("fs-extra"));
// prompt
var Promise = require("bluebird");
Promise.promisifyAll(require("prompt"));
// Nodemailer
var Promise = require("bluebird");
Promise.promisifyAll(require("nodemailer"));
// ncp
var Promise = require("bluebird");
Promise.promisifyAll(require("ncp"));
// pg
var Promise = require("bluebird");
Promise.promisifyAll(require("pg"));

Converting a single NodeJS function

You can convert a single function with a callback argument to a Promise-returning version with Promise.promisify, so this:

const fs = require("fs");
fs.readFile("foo.txt", (err, data) => {
   if(err) throw err;
   console.log(data);
});

becomes:

const promisify = require("bluebird");
const readFile = promisify(require("fs").readFile));
readFile("foo.txt").then(console.log); // promisified version

Converting any other callback API

In order to convert any callback API to promises assuming the promisify and promisifyAll version doesn’t fit - you can use the promise constructor.

Creating promises generally means specifying when they settle - that means when they move to the fulfilled (completed) or rejected (errored) phase to indicate the data is available (and can be accessed with .then).

new Promise((fulfill, reject) => { // call fulfill/reject to mark the promise
   someCallbackFunction((data) => {
      fulfill(data); // we mark it as completed with the value
   })
});

As an example, let’s convert setTimeout to use promises:

function delay(ms) { // our delay function that resolves after ms milliseconds
  return new Promise((resolve, reject) => { // return a new promise
    setTimeout(resolve, ms); // resolve it after `ms` milliseconds have passed
  })
}
// or more concisely:
const delay = ms => new Promise(r => setTimeout(r, ms));

We can now use it like a regular promise returning function:

delay(1000).then(() => console.log("One second passed")).
            then(() => delay(1000)).
            then(() => console.log("Another second passed"));