promise-nodeify

Call a Node-style callback with the resolution value or rejection cause of a Promise without the common pitfalls.

Introductory Example

var promiseNodeify = require('promise-nodeify');

// Function which returns a Promise
function returnsPromise() {
  return new Promise(function(resolve, reject) {
    resolve(42);
  });
}

// Function which takes an optional node-style callback
function takesCallback(callback) {
  var promise = returnsPromise();
  // if callback is not a function, promise is returned as-is
  // otherwise callback will be called when promise is resolved or rejected
  // promise will not cause unhandledRejection if callback is a function
  return promiseNodeify(promise, callback);
}

Features

The important features of nodeify as compared to naive implementations:

• Creates Error for falsey rejection causes. Since Promises may resolve or reject without providing a value or cause, the callback would have no way to distinguish success from failure. This module ensures the error argument is always truthy, substituting an Error when the rejection cause is falsey (and passing the original value as the .cause property, as bluebird does).
• Exceptions thrown by callback cause uncaughtException as they would for other callbacks (unlike passing callback to .then, which causes unhandledRejection or swallows them).
• The callback handles the promise rejection, preventing unhandledRejection (unlike if the promise were ignored and callback invoked directly).
• Reduces confusion by only returning a Promise when no callback is given (as opposed to returning the promise argument, which creates uncertainty about unhandledRejections and multiple threads of control - or returning the result passing the callback to .then, which resolves to the callback result).

Behavior Comparison

This module provides similar behavior to several popular promise libraries in a promise-library-agnostic way which only requires the ES6 promise functionality subset. However, these existing implementations differ in subtle ways. A brief comparison:

Behavior	this module	bluebird #asCallback	es-nodeify	nodeify	then #nodeify	Un-thenify1	when.js .bindCallback
returns (with function)	undefined	this Promise2	undefined	Promise<undefined>	undefined	undefined	when(promise)
returns (with falsey)	promise	promise	promise	Promise<undefined>	promise	undefined with unhandledRejection	when(promise)
returns (non-function)	promise	promise	undefined with unhandledRejection	promise	promise	undefined with unhandledRejection	when(promise) with uncaughtException
callback exception	uncaughtException	uncaughtException	unhandledRejection	uncaughtException	uncaughtException	unhandledRejection	uncaughtException
falsey cause	Error with .cause	Error with .cause3	Error	falsey cause	falsey cause	TypeError	falsey cause
reject argument length	1	1	1	1	1	1	2
resolve argument length	2	undefined ? 1 : 24	2	2	2	2	2
extra argument	ignored	options5	ignored	ignored	this of callback	ignored	ignored

Notes:

1. Un-thenify serves a similar purpose, but wraps the Promise-returning function rather than taking the Promise as an argument.
2. Temporarily reverted in petkaantonov/bluebird#151 and restored in petkaantonov/bluebird#168
3. In response to petkaantonov/bluebird#434
4. In response to petkaantonov/bluebird#170
5. Supports the spread boolean option to pass Array values as separate arguments to callback.

Performance Comparison

These benchmarks were done using the benchmark/index.js script on an Intel(R) Core(TM) i5-3320M CPU @ 2.60GHz with Node v4.3.1 on Linux and the following module versions:

Module	Version
benchmark	2.1.0
bluebird	3.3.3
cli-table	0.3.1
es-nodeify	1.0.0
microtime	2.0.0
native-promise-only	0.8.1
nodeify	1.0.0
pinkie-promise	2.0.0
promise	7.1.1
q	1.4.1
rsvp	3.2.1
unthenify	1.0.1
when	3.7.7

Nodeify Resolved Promise

Performance (in operations per second) of calling nodeify on a resolved promise (larger is better):

ops/sec	bluebird	native	npo	pinkie	q	rsvp	then	when
bluebird#nodeify	1,922,721.987	TypeError	TypeError	TypeError	TypeError	TypeError	TypeError	TypeError
es-nodeify	1,345,702.588	506,103.345	510,887.217	534,013.961	68,915.816	1,974,250.737	2,096,468.119	1,756,177.934
nodeify	147,481.019	251,414.264	251,535.145	253,880.998	58,504.098	1,355,812.482	1,102,467.756	1,160,226.624
promiseNodeify	1,586,092.279	481,842.79	452,529.247	455,657.062	66,045.273	2,108,607.126	2,370,823.723	1,942,722.539
then#nodeify	136,716.987	202,670.23	225,297.257	231,042.286	56,384.953	764,719.55	1,320,158.92	739,062.155
unthenify	100,638.922	79,097.99	80,488.25	78,298.365	40,683.82	103,125.162	100,618.139	101,887.997
when.bindCallback	823.326	856.669	842.975	834.864	748.669	847.556	850.316	839.995

Nodeify Rejected Promise

Performance (in operations per second) of calling nodeify on a rejected promise (larger is better):

ops/sec	bluebird	native	npo	pinkie	q	rsvp	then	when
bluebird#nodeify	1,889,496.469	TypeError	TypeError	TypeError	TypeError	TypeError	TypeError	TypeError
es-nodeify	1,247,981.228	520,349.959	455,337.77	466,964.692	64,703.247	2,182,281.005	2,062,330.035	1,889,184.935
nodeify	147,454.87	325,956.476	326,958.556	325,971.637	53,878.098	1,232,726.201	952,338.091	926,626.949
promiseNodeify	1,170,756.604	465,186.326	478,343.59	489,024.094	62,905.801	2,097,277.371	1,928,682.943	1,497,451.328
then#nodeify	131,588.987	241,627.02	246,557.24	245,427.553	49,655.492	684,232.864	1,178,175.996	634,041.464
unthenify	96,359.916	82,291.679	82,507.055	83,324.584	38,842.741	96,432.332	97,113.05	99,892.099
when.bindCallback	822.083	837.698	848.358	851.348	789.546	854.184	844.102	851.644

Installation

NPM

This package can be installed using npm by running:

npm install promise-nodeify

Browser

This package can be installed using bower by running:

bower install promise-nodeify

Without Package Manager

This module is also available with a UMD loader, both minified and un-minified, in the dist directory. They can be downloaded, self-hosted, or loaded from a CDN. To use the RawGit CDN, use the following (X)HTML:

<script src="https://cdn.rawgit.com/kevinoid/promise-nodeify/v0.1.0/dist/promise-nodeify.min.js"></script>

Recipes

Delegate to Promise.prototype.nodeify

If the behavior differences discussed in the Behavior Comparison section (and any future differences which may occur) are not significant to your use case and you are interested in taking advantage of the potential performance benefit of the implementation provided by the promise library, use the .delegated function:

// Using .delegated delegates to .nodeify on the promise argument when present
var promiseNodeify = require('promise-nodeify').delegated;

function returnsPromise() {
  return new Promise(function(resolve, reject) {
    resolve(42);
  });
}

function takesCallback(callback) {
  var promise = returnsPromise();
  return promiseNodeify(promise, callback);
}

Polyfill Promise.prototype.nodeify

To polyfill the .nodeify (or .asCallback) method for a Promise library, assign the .nodeifyThis function to Promise.prototype.nodeify as follows:

Promise.prototype.nodeify = require('promise-nodeify').nodeifyThis;

function returnsPromise() {
  return new Promise(function(resolve, reject) {
    resolve(42);
  });
}

function takesCallback(callback) {
  var promise = returnsPromise();
  return promise.nodeify(callback);
}

More examples can be found in the test specifications.

API Docs

For a description of the available functions and their arguments, see the API Documentation.

Contributing

Contributions are appreciated. Contributors agree to abide by the Contributor Covenant Code of Conduct. If this is your first time contributing to a Free and Open Source Software project, consider reading How to Contribute to Open Source in the Open Source Guides.

The dist files are only updated for releases, so please don't include them in pull requests.

If the desired change is large, complex, backwards-incompatible, can have significantly differing implementations, or may not be in scope for this project, opening an issue before writing the code can avoid frustration and save a lot of time and effort.

License

This project is available under the terms of the MIT License. See the summary at TLDRLegal.