readline module and streams

[Ginden]

Currently, readline module interface is rather special. Interface class is EventEmitter acting as stream, but not conforming to Stream interface. It's possible to redirect output to stream, but it's significantly less convenient than just .pipe.

Then I propose one of the following:

• add class readline.Stream subclassing Transform.
  • readline.Interface can be backed by this stream, then no code duplication occurs. Though, I'm not sure about viability of this approach, as Interface contains lots of code related to TTY handling.
• make readline.Interface instances subclasses of Transform stream

Compare:

foo = getReadableStream();
bar = transformStream();
foo
   .pipe(readline.Stream())
   .pipe(bar)
   .pipe(process.stdout)

vs.

foo = getReadableStream();
bar = transformStream();
readline.createInterface({
   input: foo,
   output: bar
}); // We are using factory function for side effects.
bar.pipe(process.stdout);

User-land packages providing this functionality:

• byline (423k downloads per month)

[Fishrock123]

We could do this but I would like to point out that it specifically does not pretend to act like a stream, nor do the docs state that it is any type of stream, it merely acts on a stream and can provide an output stream. That makes it transform-like but does not inherently make it a "transform stream".

[Ginden]

We could do this

I'm willing to make a pull request if there is green light for this.

I believe that Interface instance is conceptually a stream. It reads data in chunks and emits other chunks based on source asynchronously. Making it implement Stream semantics is a change in interface, not in principle of working.

[Fishrock123]

Fair enough. I think I'd prefer if the first option was attempted first, not sure how backwards-compat and API surface overlap would work out for the second option.

[bengl]

@Ginden You should go ahead and do a PR for this (the first option). FWIW, I'd really like to see this happen.

[clakech]

Hello, I would like this in a future nodejs version please ;)

[refack]

Put into https://github.com/nodejs/node/projects/13 backlog

[zavan]

I see that project is closed/archived. Is this being tracked elsewhere?

Edit: Nevermind, I figured out you can do this with pipeline:

import { pipeline } from "node:stream";
import { createInterface } from "node:readline";

pipeline(
  process.stdin,
  (input) => createInterface({ input }),
  process.stdout,
  (err) => {
    if (err) {
      console.error('Pipeline failed.', err);
    } else {
      console.log('Pipeline succeeded.');
    }
  }
);