koa中间件实现

const add = (x,y) => x+y
const square = z => z *z


//中间件思想，把多个函数进行融合
//  (...args)  意思是返回新函数
// const compose = (fn1,fn2) => (...args) => fn2(fn1(...args))
// const compose = (...[first,...other]) => (...args) => {
//     let ret = first(...args)
//     other.forEach(fn=>{
//         ret = fn(ret)
//     })
//     return ret
// }

function compose(middlewares){
    return function(){
        //dispatch(0) 从第一个开始执行
        return dispatch(0)
        //dispatch 里的i表示执行的是第几个方法
        function dispatch(i){
            let fn = middlewares[i]
            if(!fn){
                return Promise.resolve()
            }
            return Promise.resolve(
                fn(function next(){
                    //执行下一个函数
                    return dispatch(i+1)
                })
            )
        }
    }
}

async function fn1(next){
    console.log("fn1")
    await next()
    console.log('end fn1')
}
async function fn2(next){
    console.log("fn2")
    await delay()
    await next()
    console.log('end fn2')
}
async function fn3(next){
    console.log("fn3")
}
function delay(){
    return new Promise((resolve,reject)=>{
        setTimeout(() => {
            resolve()
        }, 2000);
    })
}

const middlewares = [fn1,fn2,fn3]
const finalFn = compose(middlewares)
finalFn()


// const fn =compose(add,square,square)
// const fn = (x,y) => square(add(x,y))
// console.log(fn(1,2))   

最后用node 文件名查看结果