几十行代码实现简易版koa～

application.js

const http = require("http")
// 洋葱模型
function compose(middleware){
    return function(ctx,next){
        let index = -1
        return dispatch(0)
        function dispatch(i){
            if(i <= index ){
                return Promise.reject(new Error("what!!"))
            }
                index = i
                fn =middleware[i]
                if (i === middleware.length){fn = next }
                if(!fn){
                    return Promise.resolve()
                }
                try {
                    return Promise.resolve(fn(ctx,dispatch.bind(null,++i)))

                } catch (error) {
                    return Promise.reject(error)

                }
            }
        }
}
class Koa {
    constructor(options){
        this.options = options
        this.middleware = []
    }
    use(fn){
        if(typeof fn !== "function"){
            throw new Error("not a function")
        }
        this.middleware.push(fn)
    }
    listen(){
        let server =  http.createServer(this.callback())
        server.listen(...arguments)
   }
    callback(){
        let fnMiddleWare = compose(this.middleware)
        let handleRequest = (req,res)=>{
             this.handleRequest(fnMiddleWare,req,res)
        }
        return handleRequest
    }
    handleRequest(fnMiddleWare,req,res){
        let ctx = this.createContext(req,res)
        fnMiddleWare(ctx).then(this.handleResponse).catch(this.handleError)
    }
    handleResponse(){}
    handleError(){}
    createContext(req,res){
        //不会破坏原对象，方便扩展
        let ctx = Object.create({})
        ctx.req = req;
        ctx.res =res;
        return ctx
    }
}
module.exports = Koa

使用：

const koa = require("./koa/application")
const app = new koa()
app.use(async (ctx,next)=>{
    ctx.res.write("111")
    await next()
    ctx.res.write("333")
    ctx.res.end()
})
app.use(async (ctx,next)=>{
    ctx.res.write("222")
    await next()
})
app.listen(3000);