polyfill() does not polyfill Promise.prototype.finally() if the environment already has a Promise implementation.

[Jamie Birch]

In polyfill.js, observe line 30: https://github.com/stefanpenner/es6-promise/blob/master/lib/es6-promise/polyfill.js#L30

As I understand it, if a Promise implementation is found on the global object, then the polyfill will use the global object's Promise implementation as-is, not augmenting it with the stage 4 ES9/ES2018 feature proposal provided in this library, Promise.prototype.finally().

Is this intended behaviour, or a bug? Surely if I have polyfilled es6-promise, I should be getting at least all the advertised features of es6-promise?

[Yury]

Do you know if it works in -auto variant?

[Jamie Birch]

@YurySolovyov require('es6-promise/auto') is just an alias for require('es6-promise').polyfill(), so this issue affects both usages.

[Yury]

Thanks, good to know

[radarfox]

Following browser only polyfill setup works for browsers:

• without Promise
• with Promise and without finally
• with both Promise and finally

It also polyfills promises used in Fetch API and other promise based APIs.

import Promise from 'es6-promise';
if (!('Promise' in window)) {
  window.Promise = Promise;
} else if (!('finally' in window.Promise.prototype)) {
  window.Promise.prototype.finally = Promise.prototype.finally;
}

[Paul Grime]

Does the finally method even exist on the ES6Promise object?

In IE 11, I see the following if I include https://cdnjs.cloudflare.com/ajax/libs/es6-promise/4.1.1/es6-promise.min.js in a script tag.

new ES6Promise(function(){}).finally === undefined

If I include https://cdn.jsdelivr.net/bluebird/latest/bluebird.core.min.js I see:

new Promise(function(){}).finally === function(t){return this._passThrough(t,0,l,l)}

So for my application at least, using bluebird seemed to provide a Promise implementation with a finally method.

And the bluebird approach also provides the finally method when I test on MS Edge.

[Michael Mendy]

Thank you!