To write error handlers using Express.js, you can follow almost the same template as a middleware function, for example:

this.app.use((err, req, res, next) => {
  console.error(err.stack)
  res.status(500).send('My custom error handler with 500 error...')
})

Also, you can define as many error handlers as you want, for instance, one for logging, another one for client errors, and another one for a catch-all error, as follows:

this.app.use(errorHandler.logging)
this.app.use(errorHandler.clientErrorHandler)
this.app.use(errorHandler.errorHandler)

For logging purposes, we want to catch the log and pass the error on the next() function, as follows:

export let logging = (err: Error, req: Request, res: Response, next: NextFunction) => {
  logger.error(err)
  next(err)
}

On client error, if the error is of the xhr type, we will send a 500 error to the requester, and if it is not, we will again pass the error on the next() function, as follows:

export let clientError = (err: Error, req: Request, res: Response, next: NextFunction) => {
  if (req.xhr) {
    res.status(500).send({ error: err.message })
  } else {
    next(err)
  }
}

Finally, if none of the previous steps catch the error, the default error handler will catch everything and send a 500 error, as follows:

export let errorHandler = (err: Error, req: Request, res: Response, next: NextFunction) => {
  res.status(500).send({ error: err.message })
}