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)