AI solution for JavaScript Coding Question on BFE.dev
52. create a middleware system

One possible solution to this coding question is:

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

  use(func) {
    this.middlewareList.push(func)
  }

  start(req) {
    let index = 0

    const next = (error) => {
      const middleware = this.middlewareList[index]
      index++

      if (!middleware) {
        // no more middleware to execute
        return
      }

      try {
        if (error) {
          // there was an error, trigger the error handler
          if (middleware.length === 3) {
            // middleware is an error handler, call it
            middleware(error, req, next)
          } else {
            // middleware is not an error handler, skip it
            next(error)
          }
        } else {
          // call the middleware normally
          middleware(req, next)
        }
      } catch (err) {
        // there was an uncaught error, trigger the error handler
        if (this.middlewareList[index] && this.middlewareList[index].length === 3) {
          this.middlewareList[index](err, req, next)
        } else {
          next(err)
        }
      }
    }

    // start the middleware chain
    next()
  }
}

Here, we define the Middleware class which has a list of middleware functions called middlewareList. The use method simply adds a middleware function or error handler to this list.

The start method takes a req object and begins the middleware chain by calling the first middleware function with req and a next function. The next function takes an optional error argument and is used to trigger the next middleware function. If a middleware function throws an error or calls next with an error argument, we skip all remaining middleware functions and call the next error handling middleware function (if any). If there are no more middleware functions or error handling middleware functions, the chain ends.

We can use this class as demonstrated in the original prompt: create a new instance of Middleware, add some middleware functions and an error handling middleware function to it, and call start with a req object. Each middleware function can modify the req object and/or call next to invoke the next middleware function in the chain.

Note that this is a simplified implementation of middleware system and may not have some features that professional middleware systems have.