52. create a middleware system

Have you ever used Express Middleware before?

Middleware functions are functions with fixed interface that could be chained up like following two functions.

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

You are asked to create simplified Middleware system:

type Request = objecttype NextFunc =  (error?: any) => voidtype MiddlewareFunc =   (req: Request, next: NextFunc) => voidtype ErrorHandler =   (error: Error, req: Request, next: NextFunc) => voidclass 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  }}

Now we can do something similar with Express

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}

Notice that use() could also accept an ErrorHandler which has 3 arguments. The error handler is triggered if next() is called with an extra argument or uncaught error happens, like following.

const middleware = new Middleware()// throw an error at first functionmiddleware.use((req, next) => {   req.a = 1   throw new Error('sth wrong')    // or `next(new Error('sth wrong'))`})// since error occurs, this is skippedmiddleware.use((req, next) => {   req.b = 2})// since error occurs, this is skippedmiddleware.use((req, next) => {   console.log(req)})// since error occurs, this is calledmiddleware.use((error, req, next) => {   console.log(error)   console.log(req)})middleware.start({})// Error: sth wrong// {a: 1}