duplex

streams.js

var through = require('through'),
    duplex = require('duplexer')

function dosomething() {
    var input = through(),
        output = through(),
        stream = duplex(input, output)

    output.write('two')
    input.pipe(output)

    return stream
}

var source = through()

source.pipe(dosomething()).pipe(process.stdout)

source.write('one')
// one