Nestling errors module

This module tries to make it easier to handle errors and send back meaningful information to the client.

It consist of a way to define your own messages for common errors and also intercept third party errors and normalize them into a common response.

Application level error messages

As a convention you can suffix your files with .errors.ts and keep the errors close to where they are used.

users.errors.ts

import { HttpStatus } from '@nestjs/common'
import { IErrorMessages } from '../errors/ErrorMessage'

// Optional: to typecheck interpolation vars
export interface UserEmailAlreadyExistError {
  email: string
}

export const userErrors: IErrorMessages = {
  type: 'user',
  errors: [
    {
      code: 'credentialsInvalid',
      statusCode: HttpStatus.BAD_REQUEST,
      message: 'Invalid Credentials.'
    },
    {
      code: 'emailAlreadyExist',
      statusCode: HttpStatus.BAD_REQUEST,
      message: 'Email {email} already exist.'
    }
    ...etc
  ]
}

In order to use the error messages they must be registered first:

ErrorMessage.addErrorMessages(userErrors)

The error messages can be used like this:

import { ErrorMessage } from '@nestling/errors'

throw new ErrorMessage('user:credentialsInvalid')

throw new ErrorMessage<UserEmailAlreadyExist>('user:emailAlreadyExists', {
  email: '[email protected]'
})

Setup

In able to filter for errors the exception filter must be added to the app's components:

import { HttpExceptionFilter } from '@nestling/errors'

@Module({
  components: [
    HttpExceptionFilter
  ]
})
export class ApplicationModule implements NestModule {
  configure (consumer: MiddlewaresConsumer): void {
  }
}

The filter will intercept all exceptions thrown, if an unknown exception is encountered a 500 error will be returned.

Adding extra error handlers

To register a new error handler use:

import ValidationError = require('ajv')
import { HttpStatus } from '@nestjs/common'
import { IErrorMessage } from '../../ErrorMessage'

export const validationErrorHandler = {
  type: ValidationError,
  handler: (exception): IErrorMessage => {
    return {
      code: `validation:${exception.errors[0].keyword}`,
      message: exception.errors[0].message,
      statusCode: HttpStatus.BAD_REQUEST
    }
  }
}

HttpExceptionFilter.addExceptionHandler(validationErrorHandler)

The type will be used to do an instance of check, the handler normalizes the error message into a common format.

Error Response Handler

To override the default response handler use:

HttpExceptionFilter.setErrorResponseHandler((error, response) => {
  response.setHeader(`x-error-code`, error.code)
  response.setHeader(`x-error-message`, error.message)
  response.setHeader(`x-error-statusCode`, error.statusCode)
  response
    .status(error.statusCode)
    .json(error)

  console.error(error)
})