Add polyfill from polyfill.io

[ForbesLindesay]

Note that we lock to the current default feature set (default-3.6). Future versions of create-react-app will want to update this version number, but we wouldn't want users to inadvertently get different polyfills.

[closes#170]

[gaearon]

Future versions of create-react-app will want to update this version number, but we wouldn't want users to inadvertently get different polyfills.

Why would that be bad? They’re only adding, not removing stuff, right?

[ForbesLindesay]

They’re only adding, not removing stuff, right?

Probably/Mostly. They haven't committed to never removing anything from the default set. Although they haven't done so yet, it would have been reasonable at one point for them to include Array.prototype.contains in the default set (it was a long way down the standards path) but an incompatibility was discovered at the last minute and contains was deprecated in favour of includes. At this point it would have been reasonable for them to remove the contains polyfill.

You can see what they say about this under https://polyfill.io/v2/docs/features/#default-sets

[gaearon]

How about we use this set?

https://cdn.polyfill.io/v2/polyfill.js?features=es6,fetch,Symbol&unknown=polyfill
(min: https://cdn.polyfill.io/v2/polyfill.min.js?features=es6,fetch,Symbol&unknown=polyfill)

This seems like the most valuable stuff in React apps, and also protects against issues like #561 with unknown=polyfill.

[gaearon]

It’s also more descriptive than default-3.6.

[gaearon]

Also worth checking: does their Promise polyfill swallow errors.

[JakeChampion]

I don't think you can solve uncaught rejections in userland (polyfills).

[gaearon]

I don't think you can solve uncaught rejections in userland (polyfills).

Why not? It won’t be 100% accurate but we need to err on the side of less head scratching. Advanced Promise users who know what they’re doing with temporarily unhandled rejections will just include another polyfill.

[gaearon]

Also core-js does this.

[gaearon]

We went with embedded fetch and Promise polyfills for now so closing.