Adding Enterprise Features to your NestJS App - Error Handling

Setting Up Error Handling in NestJS

To handle errors effectively, NestJS provides a powerful mechanism called Exception Filters. These filters allow you to manage exceptions and provide custom error responses.

Creating an Exception Filter

Exception Filters in NestJS allow you to catch unhandled exceptions in your application and modify the response that users receive.

// src/filters/http-exception.filter.ts
import { ArgumentsHost, Catch, ExceptionFilter, HttpException, HttpStatus } from '@nestjs/common';
import { Request, Response } from 'express';

@Catch(HttpException)
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();
    const request = ctx.getRequest<Request>();
    const status = exception.getStatus();

    const errorResponse = {
      path: request.url,
      statusCode: status,
      timestamp: new Date().toISOString(),
    };

    if (status === HttpStatus.INTERNAL_SERVER_ERROR) {
      response.status(status).json(errorResponse);
    } else if (
      status >= HttpStatus.BAD_REQUEST
      && status < HttpStatus.INTERNAL_SERVER_ERROR
    ) {
      response.status(status).json({
        ...errorResponse,
        message: exception.message
      })
    } else {
      // maybe add some other context...?
      response.status(status).json({
        ...errorResponse,
        message: exception.message
      }
    }
  }
}

`HttpExceptionFilter` is a simple example of an Exception Filter

Integrating Exception Filter in App

import { MiddlewareConsumer, Module, NestModule } from '@nestjs/common';
import { APP_FILTER } from '@nestjs/core';
import { MongooseModule } from '@nestjs/mongoose';

import { HttpExceptionFilter } from './filters/http-exception.filter';
import { TimerMiddleware } from './middlewares/timer.middleware';
import { ValidationMiddleware } from './middlewares/validation.middleware';
import { TodoModule } from './todo/todo.module';

@Module({
  imports: [
    MongooseModule.forRoot('mongodb://localhost/nestjs-todo'),
    TodoModule,
  ],
  providers: [
    {
      provide: APP_FILTER,
      useClass: HttpExceptionFilter,
    },
  ],
})
export class AppModule implements NestModule {
  configure(consumer: MiddlewareConsumer) {
    consumer.apply(TimerMiddleware).forRoutes('*')
  }
}

Integrating the `HttpExceptionFilter` into the main App module

Sample Code to Simulate Various Errors

// send_request.ts
import axios from 'axios';

async function createTodo() {
  try {
    const response = await axios.post('http://localhost:3000/todos', {
      title: 'Learn NestJS',
      description: 'Explore the basics of NestJS',
    });
    return response.data;
  } catch (error) {
    if (error.response) {
      console.log(`HTTP ${error.response.status}: `, error.response.data);
    }
  }
}

async function createInvalidTodo() {
  try {
    await axios.post('http://localhost:3000/todos', {
      title: '' // Invalid title
    });
  } catch (error) {
    if (error.response) {
      console.log(`HTTP ${error.response.status}: `, error.response.data);
    }
  }
}

async function simulateInternalServerError() {
  try {
    await axios.get('http://localhost:3000/error');
  } catch (error) {
    if (error.response) {
      console.log(`HTTP ${error.response.status}: `, error.response.data);
    }
  }
}

async function getTodos() {
  const response = await axios.get('http://localhost:3000/todos');
  console.log('Todos: ', response.data);
}

async function run() {
  try {
    console.log('Running database migration');
    execSync('npm run typeorm migration:run');

    console.log('Creating a valid todo');
    await createTodo();

    console.log('Creating an invalid todo');
    await createInvalidTodo();

    console.log('Simulating internal server error');
    await simulateInternalServerError();

    console.log('Getting all todos');
    await getTodos();
  } catch (error) {
    console.error('Error during run: ', error.message);
  }
}

run();

Made with