60行代码写一个超简陋的koa

一个很简陋的koa代码，可以很轻松的了解koa的工作流程

app.js代码

const http = require('http')
const { Stream } = require('stream')
const qs = require('querystring')

const app = {
  middleware: [],
  use(fn) {
    this.middleware.push(fn)
  },
  listen(port = 3000) {
    const server = http.createServer(this.callback.bind(this))
    return server.listen(port)
  },
  async callback(req, res) {
    console.log('start')
    const fn = compose(this.middleware)
    // 这里有很多代理的方法
    const ctx = {
      request: req,
      response: res,
      req,
      res,
      get query() {
        return { ...qs.parse(req.url.split('?')[1] || '') }
      },
    }
    await fn(ctx)
      .then(() => {
        const { body } = ctx
        if (!body) return res.end('null')
        if (Buffer.isBuffer(body)) return res.end(body)
        if (typeof body === 'string') return res.end(body)
        if (body instanceof Stream) return body.pipe(res)
        return res.end(JSON.stringify(body))
      })
      .catch((error) => {
        res.end(error.toString())
      })
    console.log('end')
  },
}

function compose(middleware = []) {
  let index = -1

  return (ctx, next) => {
    return dispatch(0)
    function dispatch(i) {
      if (i <= index) return Promise.reject(new Error('next() called multile times'))
      index = i
      let fn = middleware[i]
      if (i === middleware.length) fn = next
      if (!fn) return Promise.resolve()
      try {
        return Promise.resolve(fn(ctx, dispatch.bind(null, i + 1)))
      } catch (err) {
        return Promise.reject(err)
      }
    }
  }
}

module.exports = app

test.js 代码

// const fs = require('fs')
const app = require('./app')

app.use(async function test1(ctx, next) {
  console.log(1)
  console.log('query', ctx.query)
  await next()
  console.log(6)
})
app.use(async function test2(ctx, next) {
  console.log(2)
  await next()
  console.log(5)
})
app.use(async function test3(ctx, next) {
  await next()
  // ctx.body = fs.createReadStream('asset/a.jpg')
  console.log(4)
})

app.listen(8000)

console.log('http://localhost:8000')