5 replies

I don't understand error handling middlewares (Express)

I'm learning from fullstackopen.com and now I'm going into error handling middlewares.

This is their demo code for this part : https://github.com/fullstack-hy2020/part3-notes-backend/blob/part3-5/index.js

My understanding is that middlewares need to have

next()

next()

next()

next() for the next middleware to be executed. Why the demo code shows ...

const errorHandler = (error, request, response, next) => {
  console.error(error.message)

  if (error.name === 'CastError') {
    return response.status(400).send({ error: 'malformatted id' })
  }

  next(error)
}

const unknownEndpoint = (request, response) => {
  response.status(404).send({ error: 'unknown endpoint' })
}

app.use(unknownEndpoint)
app.use(errorHandler)

const errorHandler = (error, request, response, next) => {
  console.error(error.message)

  if (error.name === 'CastError') {
    return response.status(400).send({ error: 'malformatted id' })
  }

  next(error)
}

const unknownEndpoint = (request, response) => {
  response.status(404).send({ error: 'unknown endpoint' })
}

app.use(unknownEndpoint)
app.use(errorHandler)

const errorHandler = (error, request, response, next) => {
  console.error(error.message)

  if (error.name === 'CastError') {
    return response.status(400).send({ error: 'malformatted id' })
  }

  next(error)
}

const unknownEndpoint = (request, response) => {
  response.status(404).send({ error: 'unknown endpoint' })
}

app.use(unknownEndpoint)
app.use(errorHandler)

const errorHandler = (error, request, response, next) => {
  console.error(error.message)

  if (error.name === 'CastError') {
    return response.status(400).send({ error: 'malformatted id' })
  }

  next(error)
}

const unknownEndpoint = (request, response) => {
  response.status(404).send({ error: 'unknown endpoint' })
}

app.use(unknownEndpoint)
app.use(errorHandler)

while there's no

next()

next()

next()

next() in

unknownEndpoint

unknownEndpoint

unknownEndpoint

unknownEndpoint middleware. How can

errorHandler

errorHandler

errorHandler

errorHandler middleware be executed then?

GitHub

part3-notes-backend/index.js at part3-5 · fullstack-hy2020/part3-no...

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I don't understand error handling middlewares (Express)

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