0Pricing
NestJS Enterprise Backend APIs · Lesson

Error Handling and Interceptors

Implement custom exception filters for graceful error handling and use interceptors to transform responses or log requests.

Error Handling and Interceptors is a free NestJS Enterprise Backend APIs lesson on CoddyKit — lesson 3 of 3. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the NestJS Enterprise Backend APIs learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Graceful API Error Handling

When building APIs, it's crucial to handle errors gracefully. Instead of crashing or returning vague messages, your API should provide clear, consistent feedback to clients.

This makes your API user-friendly and helps in debugging both client-side and server-side issues.

NestJS Built-in Exceptions

NestJS provides a set of standard HTTP exceptions, extending from HttpException. These exceptions map directly to common HTTP status codes.

  • BadRequestException (400)
  • UnauthorizedException (401)
  • NotFoundException (404)
  • InternalServerErrorException (500)

Using these ensures your API speaks the standard HTTP language.

Throwing Built-in Exceptions

You can throw these exceptions directly from your controllers or services. NestJS's built-in exception layer will catch them and automatically format a standardized JSON error response.

Try running this example and access /greet/World (success) and /greet/error (error) to see the difference:

import { NestFactory } from '@nestjs/core';
import { Controller, Get, Param, NotFoundException, Module } from '@nestjs/common';

@Controller('greet')
class GreetController {
  @Get(':name')
  hello(@Param('name') name: string) {
    if (name === 'error') {
      throw new NotFoundException('Name "error" is not allowed!');
    }
    return `Hello, ${name}!`;
  }
}

@Module({
  controllers: [GreetController],
})
class AppModule {}

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
  console.log('App running on http://localhost:3000');
  console.log('Try http://localhost:3000/greet/World');
  console.log('Try http://localhost:3000/greet/error');
}
bootstrap();

Custom Exception Filters

While built-in exceptions are useful, you might need more control over the error response format. This is where custom exception filters come in.

A custom filter lets you catch specific exceptions (or all exceptions) and define exactly what response is sent back to the client, including custom logging or integration with external services.

Building a Custom Filter

To create a custom filter, you implement the ExceptionFilter interface and decorate it with @Catch(), specifying the exception type(s) it should handle. Here's a basic example:

import { ExceptionFilter, Catch, ArgumentsHost, HttpException, HttpStatus } from '@nestjs/common';
import { Request, Response } from 'express';

@Catch(HttpException)
export class CustomHttpExceptionFilter 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();

    response
      .status(status)
      .json({
        statusCode: status,
        timestamp: new Date().toISOString(),
        path: request.url,
        customMessage: `An error occurred: ${exception.message}`,
      });
  }
}

Applying Custom Filters

Once created, a custom filter needs to be applied. You can apply filters:

  • Globally: Using app.useGlobalFilters(new MyFilter()) in main.ts.
  • Controller-scoped: Using @UseFilters(MyFilter) on a controller class.
  • Method-scoped: Using @UseFilters(MyFilter) on a specific route handler method.

Global filters catch exceptions across your entire application, providing a consistent error structure.

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module'; // Assume AppModule exists
import { CustomHttpExceptionFilter } from './custom-http-exception.filter'; // Assume filter exists

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalFilters(new CustomHttpExceptionFilter());
  await app.listen(3000);
}

Introducing Interceptors

Interceptors are another powerful feature in NestJS, inspired by Aspect-Oriented Programming (AOP). They allow you to 'intercept' requests and responses, adding extra logic before or after a route handler executes.

Think of them as hooks into the request-response lifecycle, giving you fine-grained control.

Common Interceptor Uses

Interceptors are incredibly versatile and can be used for various purposes:

  • Logging: Log request details, execution time, and responses.
  • Transforming responses: Modify the outgoing data structure (e.g., wrap in a data object).
  • Caching: Implement response caching for performance.
  • Binding extra logic: Execute code before/after a method.

Logging Interceptor Example

Here's a simple interceptor that logs the time taken for a request to complete. It uses RxJS operators like tap and finalize to interact with the observable stream.

import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';
import { tap, finalize } from 'rxjs/operators';

@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before request...');

    const now = Date.now();
    return next.handle().pipe(
      tap(() => console.log(`After request... ${Date.now() - now}ms`)),
      finalize(() => console.log('Request/response cycle finalized.'))
    );
  }
}

Transforming Responses

Interceptors can also transform the data returned by your route handlers. This is common for standardizing API responses, for example, by wrapping all successful data in a data property to ensure consistency.

import { Injectable, NestInterceptor, ExecutionContext, CallHandler } from '@nestjs/common';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

interface Response<T> {
  data: T;
}

@Injectable()
export class TransformInterceptor<T> implements NestInterceptor<T, Response<T>> {
  intercept(context: ExecutionContext, next: CallHandler): Observable<Response<T>> {
    return next.handle().pipe(map(data => ({ data })));
  }
}

Quick Check: Error vs Intercept

You've learned about both Exception Filters and Interceptors. Let's test your understanding of their primary roles.

Recap & Next Steps

In this lesson, we covered essential techniques for building robust NestJS APIs:

  • Graceful Error Handling using built-in HTTP exceptions.
  • Custom Exception Filters to customize error responses and handle specific exceptions.
  • Interceptors for logging, transforming responses, and adding cross-cutting concerns.

Mastering these concepts will help you build professional, maintainable, and user-friendly APIs.

Frequently asked questions

Is the “Error Handling and Interceptors” lesson free?

Yes — the full text of “Error Handling and Interceptors” is free to read here on the web, and the NestJS Enterprise Backend APIs course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the NestJS Enterprise Backend APIs course, upgrade to CoddyKit PRO.

What will I learn in “Error Handling and Interceptors”?

Implement custom exception filters for graceful error handling and use interceptors to transform responses or log requests. You practise NestJS Enterprise Backend APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start NestJS Enterprise Backend APIs?

No prior experience is required. NestJS Enterprise Backend APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “Error Handling and Interceptors” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this NestJS Enterprise Backend APIs lesson?

Yes. Every NestJS Enterprise Backend APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Routes and Request Handling
  2. CRUD Operations with TypeORM
  3. Error Handling and Interceptors
← Back to NestJS Enterprise Backend APIs