53. 实现middleware

你是否使用过 Express Middleware ?

Middleware函数是指可以串联起来的，有特定interface的函数。

app.use('/user/:id', function (req, res, next) {
  next()
}, function (req, res, next) {
  next(new Error('sth wrong'))
})

请实现一个简单的middleware 系统!

type Request = object

type NextFunc =  (error?: any) => void

type MiddlewareFunc = 
  (req: Request, next: NextFunc) => void

type ErrorHandler = 
  (error: Error, req: Request, next: NextFunc) => void

class Middleware {
  use(func: MiddlewareFunc | ErrorHandler) {
    // do any async operations
    // call next() to trigger next function
  }
  start(req: Request) {
    // trigger all functions with a req object
  }
}

有了上述之后，我们可以支持类似Express的用法。

const middleware = new Middleware()

middleware.use((req, next) => {
   req.a = 1
   next()
})

middleware.use((req, next) => {
   req.b = 2
   next()
})


middleware.use((req, next) => {
   console.log(req)
})

middleware.start({})
// {a: 1, b: 2}

注意到 use() 支持ErrorHandler，ErrorHandler有3个参数，在发生Error或者next()被含参数调用的时候触发，比如这样。

const middleware = new Middleware()

// throw an error at first function
middleware.use((req, next) => {
   req.a = 1
   throw new Error('sth wrong') 
   // or `next(new Error('sth wrong'))`
})

// since error occurs, this is skipped
middleware.use((req, next) => {
   req.b = 2
})

// since error occurs, this is skipped
middleware.use((req, next) => {
   console.log(req)
})

// since error occurs, this is called
middleware.use((error, req, next) => {
   console.log(error)
   console.log(req)
})

middleware.start({})
// Error: sth wrong
// {a: 1}