Sending errors

Error codes

When a client makes an HTTP request to the server, the server usually responds with a status code.

Here are some of the more common status codes.

Code	Status	Meaning
200	OK	The operation has succeeded
201	Created	An entity has been created (e.g. rows inserted into the database)
204	No Content	The operation has succeeded, and the client doesn't need to do anything about it
400	Bad Request	The data sent by the client is not valid
401	Unauthorized	In order to access this endpoint, you need to send credentials
403	Forbidden	The credentials you send are not correct for this endpoint
404	Not Found	This resource is not found
500	Internal Server Error	Something went wrong and the server will not explain why

Detailed information on all status codes can be found on MDN.

In particular, we are interested in the error codes, (400-599).

Custom error class

To allow us to throw errors with status codes, we can extend the Error class.

class AppError extends Error {
  constructor(message, code) {
    super(message)
    this.code = code
  }
}

Throwing custom errors

In our models, we can throw errors that arise from handling data, which is where most errors occur.

async findById(id) {
  if (isNaN(id)) {
    throw new AppError('ID must be a number.', 400)
  }

  const query = 'SELECT * FROM users WHERE id = ?'
  const results = await db.raw(query, [id])

  if (!results.length) {
    throw new AppError(`User with id ${id} does exist.`, 404)
  }

  return results[0]
}

Catching errors

We can catch the error in our controller and decide what to do with it. In this case, we pass the error to an error handler.

router.get('/:userId', async (req, res, next) => {
  const { userId } = req.params
  try {
    const user = await User.findById(userId)
    res.json(user)
  } catch (err) {
    next(err)
  }
})

Handling errors

We can send the error code and a useful message to the client by attaching an error handler to the app.

WARNING

We should ensure the message doesn't expose our server's internal workings too much, as this information could be used by hackers. Better not to send the whole error object, but rather take full control of the information which is sent to the client.

app.use((err, req, res, next) => {
  console.error(err)
  if (err instanceof AppError) {
    res.status(err.code).json({ error: err.message })
  } else {
    res.status(500).json({ error: 'Something went wrong.' })
  }
})

TIP

Make sure that this .use() is the last method called on app before .listen(). It must accept all 4 parameters (err, req, res, next) as this is how express infers that the function is an error handler.