fetch-readablestream passes through res.body from the underlying fetch implementation, so that won't help with having .body.getReader() be defined, will it?

It only does that if the underlying fetch implementation actually supports Response.body. Otherwise, it will use either XHR with response type moz-chunked-arraybuffer (for "true" streaming support on older Firefox versions), or XHR with responseText (for "pseudo-streaming" support on all browsers, but with fairly bad performance due to string concatenation). Check the source code for more details.

Since React Native uses this package as its implementation of fetch, wouldn't landing it here address that the next time React Native core upgrades the dep? Or is there something lower level that would need to change?

A fetch polyfill is merely a wrapper around XMLHttpRequest. It can't "magically" add streaming support: the underlying platform needs to provide some low-level primitive that can be used to stream a response (such as moz-chunked-arraybuffer or responseText). React Native only implements XMLHttpRequest, so if that does not have any way to do streaming, then a fetch polyfill built on top of that won't be able to do streaming either.

If React Native wanted to support streaming, they would need to add something to their XMLHttpRequest implementation that a fetch polyfill can pick up. However, since there is no standardized API for streaming in XHR, this would require a non-standard extension to their XHR API, which is unlikely to be integrated into a general-purpose fetch polyfill like this one.

The "proper" way would be for React Native to implement fetch themselves on top of native APIs, rather than using a fetch polyfill built on top of XHR.

That said, I had a quick look at the source code for their XHR, and it would seem like they should support "pseudo-streaming" with responseText. So perhaps fetch-readablestream could automatically use that to polyfill Response.body when run inside React Native? It wouldn't be very efficient though, I think. 🤷‍♂

Implement response.body as a stream #746

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