proposal-promise-finally | ECMAScript Proposal , specs , and reference | Reactive Programming library

The TC39 proposal-promise-finally, which is now part of the ES2018 specification, lists the following key points also paraphrased on MDN to describe exactly what the method does.

promise.finally(func) is similar to promise.then(func, func), but is different in a few critical ways:

• When creating a function inline, you can pass it once, instead of being forced to either declare it twice, or create a variable for it
• A finally callback will not receive any argument, since there's no reliable means of determining if the promise was fulfilled or rejected. This use case is for precisely when you do not care about the rejection reason, or the fulfillment value, and so there's no need to provide it.
• Unlike Promise.resolve(2).then(() => {}, () => {}) (which will be resolved with undefined), Promise.resolve(2).finally(() => {}) will be resolved with 2.
• Similarly, unlike Promise.reject(3).then(() => {}, () => {}) (which will be resolved with undefined), Promise.reject(3).finally(() => {}) will be rejected with 3.

However, please note: a throw (or returning a rejected promise) in the finally callback will reject the new promise with that rejection reason.

In other words, a concise polyfill using a Promise implementation that conforms to the Promises/A+ specification is as follows (based on the answers by @Bergi and @PatrickRoberts).