node/lib/_stream_duplex.js

// a duplex stream is just a stream that is both readable and writable.
// Since JS doesn't have multiple prototypal inheritance, this class
// prototypally inherits from Readable, and then parasitically from
// Writable.

'use strict';

module.exports = Duplex;

const util = require('util');
const Readable = require('_stream_readable');
const Writable = require('_stream_writable');

util.inherits(Duplex, Readable);

var keys = Object.keys(Writable.prototype);
for (var v = 0; v < keys.length; v++) {
  var method = keys[v];
  if (!Duplex.prototype[method])
    Duplex.prototype[method] = Writable.prototype[method];
}

function Duplex(options) {
  if (!(this instanceof Duplex))
    return new Duplex(options);

  Readable.call(this, options);
  Writable.call(this, options);

  if (options && options.readable === false)
    this.readable = false;

  if (options && options.writable === false)
    this.writable = false;

  this.allowHalfOpen = true;
  if (options && options.allowHalfOpen === false)
    this.allowHalfOpen = false;

  this.once('end', onend);
}

// the no-half-open enforcer
function onend() {
  // if we allow half-open state, or if the writable side ended,
  // then we're ok.
  if (this.allowHalfOpen || this._writableState.ended)
    return;

  // no more data can be written.
  // But allow more writes to happen in this tick.
  process.nextTick(this.end.bind(this));
}