Promise finally not supported about native-promise-only

[getify]

NPO adds ES6 promises to pre-ES6 environments that have no promises at all. It's not designed or intended to be a polyfill on top of native promises for further post-ES6 additions to the promise API.

There are a number of polyfill options for recent amendments to promises (like finally), which can be used on top of native-promise-only. In the case of finally, it's actually quite easy to polyfill in a simple/naive way:

// adapted from: https://gist.github.com/spiralx/58675999ca5b28453538d30859761947
if (!Promise.prototype.finally) {
  Promise.prototype.finally = function finally(callback){
    return this.then(
      value => this.constructor.resolve(callback()).then(() => value),
      reason => this.constructor.resolve(callback()).then(() => { throw reason })
    );
  }
}

You could probably just copy-n-paste that snippet in the code bundle where you're already including native-promise-only and I bet it would work fine (haven't tried it myself, tbf).

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There are other polyfills that are a little bit more robust and strictly spec adherent (for extreme spec nuances) if you're worried about the far corner cases. For example, there's this very popular one: https://www.npmjs.com/package/promise.prototype.finally Note that this one can optionally include a separate polyfill for Promise, so you could switch from native-promise-only completely.