For Topicbox (which doesn’t use React, but that’s irrelevant), I decided a good signal to check was Object.values; if it’s missing, we include core-js. Thus, older browsers (IE, Safari≤10.0, iOS≤10.2, Chrome≤53, Firefox≤46 and Edge≤13?) will pull that in, but recent versions of all browsers won’t need the weight (the check is very light, and the overhead roughly `Object.values||require('core-js')`), and you have a solid and dependable ES6+ base to build on.

I assume you're then dynamically loading core-js when it's required? Dynamic module loading is something we've avoided so far, maybe we need to start looking into that.

This article may interest you: https://philipwalton.com/articles/loading-polyfills-only-whe...

Also on being able to use new syntax: https://philipwalton.com/articles/deploying-es2015-code-in-p...

They're only required if you target IE10 or lower.

Does that mean React is only using the save subset of Map/Set methods that are supported by IE11? That'd one great news and should be documented.

Some older versions of Safari as well I believe since Safari had shipped a broken implementation prior to becoming ES2015 compliant.

If you have specific data about what was broken please share in an issue report so we can amend the docs if necessary. Thanks!

Here is one issue where I saw some comments on the broken Map implementation: https://github.com/google/traceur-compiler/issues/1810

Admittedly, this is an older browser at this point (Safari 11 formally shipped yesterday), but I remember it biting a workplace I was at 2 1/2 years ago & we had to polyfill Map and Set. I believe this was fixed in Safari 9.

I would like to report it on GitHub, but unfortunately I think it would violate my employment contract until I complete the process to get approval to do open source work.

Good realization. We were already using Map/Set polyfills in our apps so didn't notice the cost as much. I imagine it should be possible to polyfill with a much smaller footprint; maybe we can find another to recommend. As Dan notes Map+Set polyfills are also not needed in newer browsers.