https://www.jacksonhewitt.com/Static/js/polyfills/promise-finally.js

1if (typeof Promise !== 'function') {
2	throw new TypeError('A global Promise is required');
3}
4
5if (typeof Promise.prototype.finally !== 'function') {
6	var speciesConstructor = function (O, defaultConstructor) {
7		if (!O || (typeof O !== 'object' && typeof O !== 'function')) {
8			throw new TypeError('Assertion failed: Type(O) is not Object');
9		}
10		var C = O.constructor;
11		if (typeof C === 'undefined') {
12			return defaultConstructor;
13		}
14		if (!C || (typeof C !== 'object' && typeof C !== 'function')) {
15			throw new TypeError('O.constructor is not an Object');
16		}
17		var S = typeof Symbol === 'function' && typeof Symbol.species === 'symbol' ? C[Symbol.species] : undefined;
18		if (S == null) {
19			return defaultConstructor;
20		}
21		if (typeof S === 'function' && S.prototype) {
22			return S;
23		}
24		throw new TypeError('no constructor found');
25	};
26
27	var shim = {
28		finally(onFinally) {
29			var promise = this;
30			if (typeof promise !== 'object' || promise === null) {
31				throw new TypeError('"this" value is not an Object');
32			}
33			var C = speciesConstructor(promise, Promise); // throws if SpeciesConstructor throws
34			if (typeof onFinally !== 'function') {
35				return Promise.prototype.then.call(promise, onFinally, onFinally);
36			}
37			return Promise.prototype.then.call(
38				promise,
39				x => new C(resolve => resolve(onFinally())).then(() => x),
40				e => new C(resolve => resolve(onFinally())).then(() => { throw e; })
41			);
42		}
43	};
44	Object.defineProperty(Promise.prototype, 'finally', { configurable: true, writable: true, value: shim.finally });
45}