proxy-polyfill VS window.fetch polyfill

Didn't know about it, looks like Proxy can be polyfilled in RN: https://github.com/GoogleChrome/proxy-polyfill

Proxy polyfill: assuming you are referring to this [0], since I haven't seen anything else like this, then I'll paste here what the readme says:

> The polyfill supports just a limited number of proxy 'traps'. It also works by calling seal on the object passed to Proxy. This means that the properties you want to proxy must be known at creation time.

i.e. that's not a polyfill. It's a polyfill for a subset of the thing, maybe that's useful for somebody, but it's useless for the use cases I had for Proxy so far.

Shipping an entire regex engine with your app: right, that's the only way to do something like that. Not that that's actually the same thing though, I can't just load this and use lookarounds as normal, i.e. it's not a polyfill.

For all practical purposes these features are not polyfillable. If your idea of a polyfill includes not actually polyfilling the entire thing or shipping an entire engine with your app then sure, anything is polyfillable.

[0]: https://github.com/GoogleChrome/proxy-polyfill

Browsers without ES6 Proxy support can use the proxy-polyfill.

Most js libraries use XMLHttpRequest and so provide HTTP_X_REQUESTED_WITH: XMLHttpRequest, but neither Chrome's implementation nor Github's polyfill of the new fetch uses a similar header. So how can one detect that the request is AJAX?

In this case, Github offers a great fetch polyfill for browsers: https://github.com/github/fetch

You can polyfill fetch() if that's a concern:

https://github.com/github/fetch

I'm using this fetch polyfill in Redux with redux-promise-middleware.