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

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);
}

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).

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:

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.

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:

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

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

This package can be installed using npm by running:

npm install promise-nodeify

This package can be installed using bower by running:

bower install promise-nodeify

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>

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);
}

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.

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

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.

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