對于node.js而言，目前接觸的比較多的就是Express框架，對于Express框架的中間件理解就是，一層一層逐級往下，類似流水線上的每一個工序，如下

const express = require('express');
const app = new express();

app.use(function(){
  console.log('Here is the first one';)
})

app.use(function(){
  console.log('Here is the second one')
})

//輸出結果為
// Here is the first one
// Here is the second one

由于當時理解洋蔥模型時候，不夠理解透徹，當時認為只是簡單的layer through layer，每一個layer只是單單為一個二維平面，但是實際上，Koa的洋蔥模型，每一層layer相當于一個球面，當貫穿整個模型時，實際上每一個球面(sophere)會穿透兩次，在首先理解這個點時，先熟悉一下ES6的 yield 語法

class Test{
  constructor(){
    this.state = this.fn1();
    this.state.next();
    this.state.next();
  }
  *fn1(){
    console.log('This is fn1, phase 1');
    yield* this.fn2();
    console.log('This is fn1, phase 2');
  }
  *fn2(){
    console.log('This is fn2, phase 1');
    yield* this.fn3();
    console.log('This is fn2, phase 2');
  }
  *fn3(){
    console.log('This is fn3, phase 1');
    console.log('This is fn3, phase 2');
  }
}

const test = new Test();

// 此時的yield* 代表會執(zhí)行后面的函數(shù)
// 輸出結果為
//This is fn1, phase 1
//This is fn2, phase 1
//This is fn3, phase 1
//This is fn3, phase 2
//This is fn2, phase 2
//This is fn1, phase 2
//典型的洋蔥結構

對于Koa

var koa = require('koa');
var app = koa();

app.use(function* f1(next) {
    console.log('f1: pre next');
    yield next;
    console.log('f1: post next');
});

app.use(function* f2(next) {
    console.log('  f2: pre next');
    yield next;
    console.log('  f2: post next');
});

app.use(function* f3(next) {
    console.log('    f3: pre next');
    this.body = 'hello world';
    console.log('    f3: post next');
});

//執(zhí)行熟悉如下
f1: pre next
  f2: pre next
    f3: pre next
    f3: post next
  f2: post next
f1: post next

每個中間件會執(zhí)行兩次，這就是Koa中間件的核心思想