1.使用中間件

• 創(chuàng)建文件夾
  mkdir useragent && cd useragent
• 安裝插件

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);

• 運(yùn)行服務(wù)器
  node index.js
• 打開瀏覽器
  輸入 127.0.0.1:3000,結(jié)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.創(chuàng)建中間件
每一個(gè)中間件會(huì)接收ctx和next兩個(gè)參數(shù),next是下一個(gè)回調(diào)的Promise,而ctx是koa封裝的上下文,這個(gè)對(duì)象包含響應(yīng)和請(qǐng)求的所有方法,當(dāng)我們想添加一些方法的時(shí)候,可以直接過載到ctx對(duì)象上.
我們嘗試創(chuàng)建一個(gè)在控制臺(tái)中輸出當(dāng)前訪問url的中間件

• 創(chuàng)建log.js

module.exports = options => {
  if (!options.format) {
    console.error("需要傳遞format函數(shù)");
  }
  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);

• 啟動(dòng)服務(wù)器
  node index.js
• 訪問
  http://127.0.0.1:3000/home
  結(jié)果如下:
  ========= /home=========

通常自定義的中間件需要產(chǎn)地配置項(xiàng),所以開發(fā)的中間件插件通常是兩層函數(shù),一層函數(shù)來傳遞配置項(xiàng),一層是kos.js中間件,注意一定要調(diào)用next,否則無法調(diào)用后續(xù)中間件.