Thinking

koa-compose

let middlewares = []
middlewares.push((next) => {
  console.log(1)
  next()
  console.log(1.1)
})

middlewares.push((next) => {
  console.log(2)
  next()
  console.log(2.1)
})

middlewares.push((next) => {
  console.log(3)
  next()
  console.log(3.1)
})

compose(middlewares)() // 1 2 3 3.1 2.1 1.1

export function compose(middlewares: Middleware[]) {
  if (!Array.isArray(middlewares))
    throw new TypeError('Middleware stack must be an array!')

  for (const fn of middlewares) {
    if (typeof fn !== 'function')
      throw new TypeError('Middleware must be composed of functions!')
  }

  return () => {
    return dispatch(0)
    function dispatch(i: number): Promise<unknown> {
      const fn = middlewares[i]
      if (!fn) return Promise.resolve()

      try {
        return Promise.resolve(fn(dispatch.bind(null, i + 1)))
      } catch (error) {
        return Promise.reject(error)
      }
    }
  }
}

type Middleware = (next: () => Promise<unknown>) => Promise<unknown>