This is fork of original Promise.prototype.finally with one small addition:
In case of rejection finally receives error (reason) as argument. It allows to decide which error should be passed further in promise chain. In many cases original error is more helpful for debugging than own finally error.

Let's take an example.

Problem:

Promise.resolve()
  .then(() => doStuffWithError())
  .finally(() => cleanupWithError());
  // here we get the stacktrace of cleanup error, not original error that rejected the promise chain

Solution with original finally:

Promise.resolve()
  .then(doStuffWithError)
  .then(result => {
    return cleanupWithError().then(() => result);
  }, err => {
    return cleanupWithError().catch(() => Promise.reject(err));
  });

Solution with modified finally receiving error argument:

Promise.resolve()
  .then(doStuffWithError)
  .finally(err => cleanupWithError().catch(e => Promise.reject(err || e)));

See more details in this discussion.

Installation:

npm i promise.prototype.finally.err

Usage:

require('promise.prototype.finally.err').shim();

promise.prototype.finally

ES Proposal spec-compliant shim for Promise.prototype.finally. Invoke its "shim" method to shim Promise.prototype.finally if it is unavailable or noncompliant. Note: a global Promise must already exist: the es6-shim is recommended.

This package implements the es-shim API interface. It works in an ES3-supported environment that has Promise available globally, and complies with the proposed spec.

Most common usage:

var assert = require('assert');
var promiseFinally = require('promise.prototype.finally');

var resolved = Promise.resolve(42);
var rejected = Promise.reject(-1);

promiseFinally(resolved, function () {
	assert.equal(arguments.length, 0);

	return Promise.resolve(true);
}).then(function (x) {
	assert.equal(x, 42);
});

promiseFinally(rejected, function () {
	assert.equal(arguments.length, 0);
}).catch(function (e) {
	assert.equal(e, -1);
});

promiseFinally(rejected, function () {
	assert.equal(arguments.length, 0);

	throw false;
}).catch(function (e) {
	assert.equal(e, false);
});

promiseFinally.shim(); // will be a no-op if not needed

resolved.finally(function () {
	assert.equal(arguments.length, 0);

	return Promise.resolve(true);
}).then(function (x) {
	assert.equal(x, 42);
});

rejected.finally(function () {
	assert.equal(arguments.length, 0);
}).catch(function (e) {
	assert.equal(e, -1);
});

rejected.finally(function () {
	assert.equal(arguments.length, 0);

	throw false;
}).catch(function (e) {
	assert.equal(e, false);
});

Tests

Simply clone the repo, npm install, and run npm test

Thanks

Huge thanks go out to @matthew-andrews, who provided the npm package name for v2 of this module. v1 is both in the original repo and preserved in a branch