1.使用中间件

• 创建文件夹
  mkdir useragent && cd useragent
• 安装插件

cnpm install -S koa koa-useragent

• 新建index.js

const Koa = require("koa");
const app = new Koa();
const userAgent = require("koa-useragent");
app.use(userAgent);
app.use(async (ctx, next) => {
  console.log(require("util").inspect(ctx.userAgent));
}); 
app.listen(3000);

• 运行服务器
  node index.js
• 打开浏览器
  输入 127.0.0.1:3000,结果如下:

{ isAuthoritative: true,
  isMobile: false,
  isTablet: false,
  isiPad: false,
  isiPod: false,
  isiPhone: false,
  isAndroid: false,
  isBlackberry: false,
  isOpera: false,
  isIE: false,
  isEdge: false,
  isIECompatibilityMode: false,
  isSafari: false,
  isFirefox: false,
  isWebkit: false,
  isChrome: true,
  isKonqueror: false,
  isOmniWeb: false,
  isSeaMonkey: false,
  isFlock: false,
  isAmaya: false,
  isPhantomJS: false,
  isEpiphany: false,
  isDesktop: true,
  isWindows: true,
  isLinux: false,
  isLinux64: false,
  isMac: false,
  isChromeOS: false,
  isBada: false,
  isSamsung: false,
  isRaspberry: false,
  isBot: false,
  isCurl: false,
  isAndroidTablet: false,
  isWinJs: false,
  isKindleFire: false,
  isSilk: false,
  isCaptive: false,
  isSmartTV: false,
  isUC: false,
  isElectron: false,
  isFacebook: false,
  isAlamoFire: false,
  silkAccelerated: false,
  browser: 'Chrome',
  version: '70.0.3538.110',
  os: 'Windows 10.0',
  platform: 'Microsoft Windows',
  geoIp: {},
  electronVersion: '',
  source:
   'Mozilla/5.0 (Windows NT 10.0; Win64; x64)
 AppleWebKit/537.36 (KHTML, like Gecko) Chrome/70.0.3538.110 Safari/537.36' }

2.创建中间件
每一个中间件会接收ctx和next两个参数,next是下一个回调的Promise,而ctx是koa封装的上下文,这个对象包含响应和请求的所有方法,当我们想添加一些方法的时候,可以直接过载到ctx对象上.
我们尝试创建一个在控制台中输出当前访问url的中间件

• 创建log.js

module.exports = options => {
  if (!options.format) {
    console.error("需要传递format函数");
  }
  return async (ctx, next) => {
    console.log(options.format(ctx.url));
    await next();
  };
};

• index.js

const Koa = require("koa");
const app = new Koa();
const userAgent = require("koa-useragent");
const log = require("./log");
const config = { format: text => `========= ${text}=========` };
app.use(userAgent);
app.use(log(config));
app.use(async (ctx, next) => {
  console.log(require("util").inspect(ctx.userAgent));
});
app.listen(3000);

• 启动服务器
  node index.js
• 访问
  http://127.0.0.1:3000/home
  结果如下:
  ========= /home=========

通常自定义的中间件需要产地配置项,所以开发的中间件插件通常是两层函数,一层函数来传递配置项,一层是kos.js中间件,注意一定要调用next,否则无法调用后续中间件.