用到的知识点：
1、bind函数
2、函数递归调用自身
3、promise
'use strict'
/**
* Expose compositor.
*/
module.exports = compose
/**
* Compose `middleware` returning
* a fully valid middleware comprised
* of all those which are passed.
*
* @param {Array} middleware
* @return {Function}
* @api public
*/
function compose (middleware) {
if (!Array.isArray(middleware)) throw new TypeError('Middleware stack must be an array!')
for (const fn of middleware) {
if (typeof fn !== 'function') throw new TypeError('Middleware must be composed of functions!')
}
/**
* @param {Object} context
* @return {Promise}
* @api public
*/
return function (context, next) {
// last called middleware #
let index = -1
return dispatch(0)
function dispatch (i) {
if (i <= index) return Promise.reject(new Error('next() called multiple times'))
index = i
let fn = middleware[i]
console.log(fn)
if (i === middleware.length) fn = next
if (!fn) return Promise.resolve()
try {
return Promise.resolve(fn(context, dispatch.bind(null, i + 1)));
} catch (err) {
return Promise.reject(err)
}
}
}
}
test
function one(ctx,next){
console.log('第一个');
var d=2
next().then(function(ct){
console.log(ct)
d=222
})
console.log(d)
return 21
}
function two(ctx,next){
console.log('第二个');
return next();
}
function three(ctx,next){
console.log('第三个');
return 'three';
}
// 传入中间件函数组成的数组队列，合并成一个中间件函数
const middlewares = compose([one, two, three]);
// 执行中间件函数,函数执行后返回的是Promise对象
middlewares().then(function (result){
console.log('队列执行完毕',result);
})