Here there is an example to implement the async web server in JS.
https://nodejs.org/en/knowledge/advanced/streams/how-to-use-fs-create-read-stream/

Currently, the server works this way with promises only:

file: aaaAAAAaaaAAaaaa
res: --------------------bbbBBBBbbbBBbbbb

But, it would be better this way with steam:

file: aaaAAAAaaaAAaaaa
res: ---bbbBBBBbbbBBbbbb

The same idea can be applied to Rust, for example.