实现一个koa

koa

const http = require("http"); // 组合中间件
function compose(middlewareList) {
  return function(ctx) {
    function dispatch(i) {
      const fn = middlewareList[i];
      try {
        return Promise.resolve(fn(ctx, dispatch.bind(null, i + 1)));
      } catch (err) {
        return Promise.reject(err);
      }
    }
    return dispatch(0);
  };
}
class LikeKoa2 {
  constructor() {
    this.middlewareList = [];
  }

  // 核心方法
  use(fn) {
    this.middlewareList.push(fn);
    return this;
  }

  // 处理中间件的 http 请求
  handleRequest(ctx, middleWare) {
    // 这个 middleWare 就是 compose 函数返回的 fn
    // 执行 middleWare(ctx) 其实就是执行中间件函数，然后再用 Promise.resolve 封装并返回
    return middleWare(ctx);
  }

  // 将 req res 组合成为 ctx
  createContext(req, res) {
    // 简单模拟 koa 的 ctx ，不管细节了
    const ctx = {
      req,
      res
    };
    return ctx;
  }

  callback() {
    const fn = compose(this.middlewareList);

    return (req, res) => {
      const ctx = this.createContext(req, res);
      return this.handleRequest(ctx, fn);
    };
  }

  listen(...args) {
    const server = http.createServer(this.callback());
    return server.listen(...args);
  }
}
module.exports = LikeKoa2;

test

const Koa = require('./myKoa.js');
const app = new Koa();// logger

app.use(async (ctx, next) => {
  await next();
  const rt = ctx['X-Response-Time'];
  console.log(`${ctx.req.method} ${ctx.req.url} - ${rt}`);
});
  
app.use(async (ctx, next) => {
  const start = Date.now();
  await next();
  const ms = Date.now() - start;
  ctx['X-Response-Time'] = `${ms}ms`;});// 
  
app.use(async ctx => {
  ctx.res.end('hello world')
});

app.listen(8000);