Hi @kateinoigakukun , what is the status of this issue?
Is more detailed information required to understand the issue?
Or did you understand the issue, but decided it was not important?
Is my contribution to the issue welcome or not?

I think what you need is JavaScript's ArrayBuffer <-> Ruby String (ASCII_8BIT) conversion or just add some methods to JS::Object to behave like IO. But I'm still not sure what kind of design is good to be Rubyish.

If you have any particular idea or implementation, I'm open to discussing it.

Hmmm certainly worth thinking about.
I'm not a Rubyist, so it's not a brilliant idea by any means, but I've thought of two ways.

1. specialize ArrayBuffer#to_s and return data converted to ASCII_8BIT

Of course this is destructive, but no one will spare the old behavior since the current to_s always returns [object ArrayBuffer] and is useless.

2. create a new JS.fetch function and convert to ASCII_8BIT if to_rstr: true

Here is how to use it.

ascii_8bit = JS.fetch("<url>", to_rstr: true).await

Thank you for your idea. I'll continue exploring ways including yours.

It seems String class has String#b method to convert the self content to ASCII-8BIT string. JS::Object#b might be a considerable option?

It might not be bad.
However, String#b is probably just String#force_encoding("ASCII-8BIT") internally.
Note that the nature of this issue is conversion, which is more like String#encode, so the nuance may be a bit different.