through2 本质上是一种transform的流
被封装更好地操作流
var Transform = require('readable-stream/transform'), inherits  = require('util').inherits, xtend     = require('xtend')function DestroyableTransform(opts) {// 继承TransformTransform.call(this, opts)this._destroyed = false
}inherits(DestroyableTransform, Transform)
// 原型接口destory 用来关闭流
DestroyableTransform.prototype.destroy = function(err) {if (this._destroyed) returnthis._destroyed = truevar self = thisprocess.nextTick(function() {if (err)self.emit('error', err)self.emit('close')//events的使用})
}// a noop _transform function
// 空操作
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
// 返回一个导出的函数接口
function through2 (construct) {
// 返回使用的匿名函数return function (options, transform, flush) {if (typeof options == 'function') {flush     = transformtransform = optionsoptions   = {}}// 这种匿名函数我们一般可以用来做二次判断触发if (typeof transform != 'function')transform = noopif (typeof flush != 'function')flush = nullreturn construct(options, transform, flush)}
}// main export, just make me a transform stream!
// 主要出口，使用through2返回一个DestroyTransform实例
module.exports = through2(function (options, transform, flush) {var t2 = new DestroyableTransform(options)t2._transform = transformif (flush)t2._flush = flushreturn t2
})// make me a reusable prototype that I can `new`, or implicitly `new`
// with a constructor call
// 对外暴露一个可以直接 new （或者不加 new）来创建实例的的构造函数
module.exports.ctor = through2(function (options, transform, flush) {function Through2 (override) {if (!(this instanceof Through2))// 这里就是直接自动newreturn new Through2(override)this.options = xtend(options, override)// 添加配置DestroyableTransform.call(this, this.options)}inherits(Through2, DestroyableTransform)Through2.prototype._transform = transformif (flush)Through2.prototype._flush = flushreturn Through2
})// Object模式的简单封装
module.exports.obj = through2(function (options, transform, flush) {var t2 = new DestroyableTransform(xtend({ objectMode: true, highWaterMark: 16 }, options))t2._transform = transformif (flush)t2._flush = flushreturn t2
})

xtend lib //简单的继承 两种，一种可变  一种不可变
module.exports = extendvar hasOwnProperty = Object.prototype.hasOwnProperty;
// 缓存老套路。优化性能
function extend(target) {// 这里有个有意思的地方。target第一个拿进来。在多个参数的情况下。下面从arguments[1]开始取，而下面可以直接使用target，而不用再声明一下变量for (var i = 1; i < arguments.length//这块其实可以缓存长度的; i++) {var source = arguments[i]for (var key in source) {if (hasOwnProperty.call(source, key)) {// 判断当前实例是否存在属性// 有则添加到target上。无则跳过，支持覆盖target[key] = source[key]}}}return target
}不可变。
module.exports = extendvar hasOwnProperty = Object.prototype.hasOwnProperty;function extend() {
// 与原先不同的是，这里使用了一份初始化的对象引用来作为容器承载
// 其余没有不同var target = {}for (var i = 0; i < arguments.length; i++) {var source = arguments[i]for (var key in source) {if (hasOwnProperty.call(source, key)) {target[key] = source[key]}}}return target
}

• throught
• xtend