Mongoose error handling

The validation constraints added to the user schema fields will throw error messages, if violated when user data is saved to the database. To handle these validation errors and other errors that the database may throw when we make queries to it, we will define a helper method to return a relevant error message that can be propagated in the request-response cycle as appropriate.  

We will add the getErrorMessage helper method in the server/helpers/dbErrorHandler.js file. This method will parse and return the error message associated with the specific validation error or other error that occurred while querying MongoDB using Mongoose.

mern-skeleton/server/helpers/dbErrorHandler.js:

const getErrorMessage = (err) => {
  let message = ''
  if (err.code) {
      switch (err.code) {
          case 11000:
          case 11001:
              message = getUniqueErrorMessage(err)
              break
          default:
              message = 'Something went wrong'
      }
  } else {
      for (let errName in err.errors) {
          if (err.errors[errName].message)
          message = err.errors[errName].message
      }
  }
  return message
}

export default {getErrorMessage}

Errors that are not thrown because of a Mongoose validator violation will contain an error code and in some cases need to be handled differently. For example, errors caused due to a violation of the unique constraint will return a different error object than Mongoose validation errors. The unique option is not a validator but a convenient helper for building MongoDB unique indexes, and thus we will add another getUniqueErrorMessage method to parse the unique constraint related error object and construct an appropriate error message.

mern-skeleton/server/helpers/dbErrorHandler.js:

const getUniqueErrorMessage = (err) => {
  let output
  try {
      let fieldName =   
      err.message.substring(err.message.lastIndexOf('.$') + 2,                                             
      err.message.lastIndexOf('_1'))
      output = fieldName.charAt(0).toUpperCase() + fieldName.slice(1) +   
      ' already exists'
  } catch (ex) {
      output = 'Unique field already exists'
  }
  return output
}

By using the getErrorMessage function exported from this helper file, we will add meaningful error messages when handling errors thrown by Mongoose operations performed for user CRUD.