ES Proposal spec-compliant shim for Promise.prototype.finally
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README.md

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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