読み書き両用ストリームのひな形

dup.js

// writeStreamとして受け取るデータは素通し
var stream = require('stream')
,   util   = require('util')
;

function DuplexStream () {
    this.readable = true;
    this.writable = true;
    this.source;
    this.on('pipe', function (src) { this.source = src }.bind(this));
}
util.inherits(DuplexStream, stream.Stream);

var dp = DuplexStream.prototype;

function write (data_, _enc) {
    var data = data_;
    if (Buffer.isBuffer(data)){
        data = data.toString(_enc || 'utf8');
        data = data.toUpperCase();
    }
    process.stdout.write(data);
}

dp.write = function (data_, _enc) {
    write(data_, _enc);
    this.emit('data', data_, _enc);
};
dp.end = function (data_, _enc) {
    write(data_, _enc);
    this.emit('end', data_, _enc);
};

dp.resume = function () { this.source && this.source.resume() };
dp.pause  = function () { this.source && this.source.pause() };

module.exports = function () {
    return new DuplexStream();
};

dup.pipe.js

#!/usr/bin/env node

var dp = require('./dup');
var dpStream    = dp();
var readStream  = process.stdin;
var writeStream = process.stdout;

readStream.pipe(dpStream).pipe(writeStream);