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Felipe Leao
Felipe Leao

Posted on • Updated on

ExpressJS: How to throw custom errors

The problem

  • Throwing readable custom errors is one of the most critical steps in the development of web applications. The communication between services must be clear and straight to the point.

The tool

  • Express middlewares when building NodeJS applications

The Solution:

  • Build an express middleware to throw custom responses error statusCode and messages

  • Our example will utilize a simple API built with ExpressJS and Prisma as an ORM

// app.ts

import express, { Request, Response, NextFunction } from 'express'
import 'express-async-errors'
import { router } from './routes'
import cors from 'cors'

const app = express()

app.use((err: {err: Error, statusCode: number, message: string}, _req: Request, res: Response, _next: NextFunction) => {
  if (err.err && err.err instanceof Error) {
    return res.status(err.statusCode).json({
      message: err.message,
      status: 'error'
  return res.status(500).json({
    status: 'error',
    message: 'Internal server error'

export { app }
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  • In addition, we can also define how our error response will be displayed to our clients. Here you can choose to show whatever you want and whatever you feel is pertinent for the client to see.

// lib/error

const builder = ({ statusCode, message }: {statusCode: number, message: string}) => {
  return {
    err: new Error(),

export const error = { builder }

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  • The object error should be thrown in the following manner

// src/services/userService.ts

import { error } from '../../lib/error'
import prismaClient from '../database/client'
import { IUser } from '../types/IUser'

interface ICreateUserRequest {
  name: string
  email: string
  password: string
const create = async ({ name, email, password }: ICreateUserRequest): Promise<IUser> => {
  if (!email) {
    throw error.builder({ statusCode: 422, message: 'Email not provided' })

  if (await prismaClient.user.findFirst({ where: { email } })) {
    throw error.builder({ statusCode: 412, message: 'Email already taken' })

  const user = await prismaClient.user.create({
    data: { name, email, password }

  return user

export const userService = {create}

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Top comments (6)

crowdozer profile image
crowdozer • Edited

That's a lot like how I do it 😁

I like to create a generic restful error class instead of throwing via function. Easier for me to identify what's going on

// errors.ts
export default class HttpError extends Error {
    constructor(message = '', code = 500) {
        this.code = code

    code: number
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// error handler
import { Request, Response, NextFunction } from 'express'
import HttpError from 'errors'

export default function ErrorHandler(err: any, req: Request, res: Response, next: NextFunction) {
    const message = err.message || 'An unknown error has occurred'
    const status = err instanceof HttpError ? err.code : 500

    return res.status(status).json({
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throw new HttpError('You did something bad!', 400)

// bubble through the application & turns into
  "message": "You did something bad!",
  "status": 400
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Then you can go crazy creating semantically relevant errors... 😼

export class ValidationError extends HttpError {
    constructor(reason: string) {
        super('Validation error: ' + reason, 400)

throw new ValidationError("aaaaah")
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felipeleao18 profile image
Felipe Leao

I appreciate your contribution to the post, which is very useful. Both methods can be used, and they are all extremely effective

brunosoares99 profile image
Bruno Soares

nice!!! It's so good!!!

felipeleao18 profile image
Felipe Leao

Thank you!!!!!

pedrolcio_soaresvaz_53 profile image
Pedro LĂșcio Soares Vaz

This is what i was looking for, nice post

felipeleao18 profile image
Felipe Leao

I'm glad to hear that!