✏ EDIT: through2.js

var Transform = require('readable-stream/transform')
  , inherits  = require('util').inherits
  , xtend     = require('xtend')

function DestroyableTransform(opts) {
  Transform.call(this, opts)
  this._destroyed = false
}

inherits(DestroyableTransform, Transform)

DestroyableTransform.prototype.destroy = function(err) {
  if (this._destroyed) return
  this._destroyed = true
  
  var self = this
  process.nextTick(function() {
    if (err)
      self.emit('error', err)
    self.emit('close')
  })
}

// a noop _transform function
function noop (chunk, enc, callback) {
  callback(null, chunk)
}

// create a new export function, used by both the main export and
// the .ctor export, contains common logic for dealing with arguments
function through2 (construct) {
  return function (options, transform, flush) {
    if (typeof options == 'function') {
      flush     = transform
      transform = options
      options   = {}
    }

if (typeof transform != 'function')
      transform = noop

if (typeof flush != 'function')
      flush = null

return construct(options, transform, flush)
  }
}

// main export, just make me a transform stream!
module.exports = through2(function (options, transform, flush) {
  var t2 = new DestroyableTransform(options)

t2._transform = transform

if (flush)
    t2._flush = flush

return t2
})

// make me a reusable prototype that I can `new`, or implicitly `new`
// with a constructor call
module.exports.ctor = through2(function (options, transform, flush) {
  function Through2 (override) {
    if (!(this instanceof Through2))
      return new Through2(override)

this.options = xtend(options, override)

DestroyableTransform.call(this, this.options)
  }

inherits(Through2, DestroyableTransform)

Through2.prototype._transform = transform

if (flush)
    Through2.prototype._flush = flush

return Through2
})

module.exports.obj = through2(function (options, transform, flush) {
  var t2 = new DestroyableTransform(xtend({ objectMode: true, highWaterMark: 16 }, options))

t2._transform = transform

if (flush)
    t2._flush = flush

return t2
})

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