promise.prototype.finally

What is promise.prototype.finally?

The promise.prototype.finally package provides a polyfill for the `finally` method on the Promise prototype. This method allows for a callback to be executed when the promise is settled, regardless of its outcome (fulfilled or rejected). It is useful for running cleanup code or finalizing operations without caring about the promise's resolution.

What are promise.prototype.finally's main functionalities?

Adding finally support to Promises

This feature allows for the addition of the `finally` method to the Promise prototype, enabling cleanup or final operations to be run regardless of whether the promise was fulfilled or rejected.

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

new Promise((resolve, reject) => {
  // Asynchronous operation
}).finally(() => {
  // Cleanup or final code to run regardless of promise outcome
});

Other packages similar to promise.prototype.finally

bluebird

Bluebird is a comprehensive promise library that includes a `finally` method among its wide array of features. Compared to promise.prototype.finally, Bluebird offers a much broader set of promise-related utilities, optimizations, and conveniences.

q

Q is another popular promise library that supports a `finally` method. It provides a rich set of promise manipulation and control flow utilities. While Q's `finally` method serves a similar purpose to that of promise.prototype.finally, Q is designed as a complete promise library rather than a polyfill.

README

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