Koa.js lite

example.js

const Koa = require('./koa');
const app = new Koa();

// logger
app.use(async function (ctx, next) {
  const start = new Date();
  await next();
  const ms = new Date() - start;
  console.log(`${ctx.method} ${ctx.url} - ${ms}`);
});

// response
app.use(ctx => {
  ctx.body = 'Hello World';
});

app.listen(8888, function() {
  console.log('listen at 8888');
});

koa.js

const invokeMiddlewares = async function(context, fns) {
  const length = fns.length;
  let current = 0;

  const next = function() {    
    return new Promise((resolve, reject) => {
      const fn = fns[current++];
      if (current > length) return resolve();
      try {
        resolve(fn(context, next));
      } catch(error) {
        reject(error);
      }
    });
  };

  return next();
};

module.exports = class Koa {
  constructor() {
    this.middlewares = [];
    this.server = null;
  }

  use(fn) {
    this.middlewares.push(fn);
  }

  createContext(req, res) {
    const ctx = {
      body: null,
      method: req.method,
      url: req.url,
      req,
      res
    };
    return ctx;
  }

  async handleRequest(req, res) {
    const context = this.createContext(req, res);
    await invokeMiddlewares(context, this.middlewares);
    
    let body = context.body;
    if (typeof body === 'string') {
      return res.end(body);
    }
    body = JSON.stringify(body);
    res.end(body);
  }

  listen(port, callback) {
    const http = require('http');
    const server = this.server = http.createServer((req, res) => {
      this.handleRequest(req, res);
    });
    server.listen(port, callback);
  }
};