middleware_edit from koa

middleware.js

class Middleware {
  constructor() {
    this._middleware = []
  }

  use(...fns) {
    fns.forEach(fn => {
      if (typeof fn !== 'function') {
        throw new TypeError('Middleware must be function!')
      }
      this._middleware.push(fn)
    })
    return this
  }
  
  
  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!')
    }

    return function _compose(state, 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]
        if (i === middleware.length) fn = next
        if (!fn) return Promise.resolve(state)
        try {
          return Promise.resolve(fn(state, dispatch.bind(null, i + 1)));
        } catch (err) {
          return Promise.reject(err)
        }
      }
    }
  }

  async start(state) {
    const fn = this.compose(this._middleware)
    await fn(state)
  }
}

middleware_example.js

// test run middleware
async function test() {
  const reducer = new Middleware()
  const state = {
    count: 1
  }
  const m2 = a => {
    console.log(a)
    return async (state, next) => {
      console.log('millde func')
      await next()
    }
  }
  reducer.use(m2('func'))
  reducer.use(async (state, next) => {
    state.name = 'm'
    console.log('middle before 1')
    await next()
  })
  reducer.use(async (state, next) => {
    const p = new Promise(resolve => {
      setTimeout(() => {
        state.count++
        console.log('settimeout')
        resolve(state)
      }, 100)
    })
    console.log('middle 1 before await')
    await p
    console.log('middle 1')
    await next()
  })
  reducer.use(async (state, next) => {
    state.name = 'test'
    console.log('middle 2')
    await next()
  })
  reducer.use(async (state, next) => {
    state.name = 'm'
    console.log('middle 3')
    await next()
  })
  reducer.use((state, next) => {
    state.name = 'm'
    console.log('middle after 3')
    next()
  })
  await reducer.start(state)
  console.log('result')
  console.log(state)
  return state
}

async function runTest(params) {
  const result = await test()
  console.log('xxxx redulst')
  console.log(result)
}

runTest()