// 题目需求
let middleware = []
middleware.push((next) => {
console.log(1)
next()
console.log(1.1)
})
middleware.push((next) => {
console.log(2)
next()
console.log(2.1)
})
middleware.push((next) => {
console.log(3)
next()
console.log(3.1)
})
let fn = compose(middleware)
fn()
/*
1
2
3
3.1
2.1
1.1
*/
//实现compose函数
function compose(middlewares) {
}
/**
* Compose `middleware` returning
* a fully valid middleware comprised
* of all those which are passed.
*
* @param {Array} middleware
* @return {Function}
* @api public
*/
function compose (middleware) {
// 校验传入的参数是数组，校验数组中每一项是函数
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) {
// 一个函数中多次调用报错
// await next()
// await next()
if (i <= index) return Promise.reject(new Error('next() called multiple times'))
index = i
// 取出数组里的 fn1, fn2, fn3...
let fn = middleware[i]
// 最后 相等，next 为 undefined
if (i === middleware.length) fn = next
// 直接返回 Promise.resolve()
if (!fn) return Promise.resolve()
try {
return Promise.resolve(fn(context, dispatch.bind(null, i + 1)))
} catch (err) {
return Promise.reject(err)
}
}
}
}
