Exception mapping interceptor in NestJS - Mini Project: Build & Apply

Exception Mapping Interceptor in NestJS

📖 Scenario: You are building a NestJS backend API that needs to handle errors gracefully. Instead of sending raw error messages to clients, you want to map exceptions to friendly HTTP responses.

🎯 Goal: Create a NestJS interceptor called ExceptionMappingInterceptor that catches exceptions thrown by route handlers and maps them to custom error responses.

📋 What You'll Learn

Create an interceptor class named ExceptionMappingInterceptor implementing NestInterceptor.

Use RxJS catchError operator to catch exceptions.

Map exceptions to a custom error object with statusCode and message.

Throw an HttpException with the mapped error.

Apply the interceptor to a sample controller method.

💡 Why This Matters

🌍 Real World

Backend APIs often need to handle errors gracefully and send clear messages to clients. This interceptor helps centralize error handling.

💼 Career

Understanding interceptors and error handling in NestJS is essential for backend developers building robust and maintainable APIs.

1

Create the ExceptionMappingInterceptor class

Create a class called ExceptionMappingInterceptor that implements NestInterceptor. Import Injectable from @nestjs/common. Add the @Injectable() decorator above the class.

NestJS

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  // Your code here
}

Need a hint?

Remember to import Injectable, NestInterceptor, ExecutionContext, and CallHandler from @nestjs/common.

2

Add RxJS catchError to intercept method

Inside the intercept method, return next.handle() piped with catchError from rxjs/operators. Import catchError from rxjs/operators and throwError from rxjs.

NestJS

import { catchError } from 'rxjs/operators';
import { throwError } from 'rxjs';

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler) {
    return next.handle().pipe(
      // Your code here
    );
  }
}

Need a hint?

Use next.handle().pipe(catchError(...)) to catch errors from the route handler.

3

Map exceptions to custom error and throw HttpException

Inside the catchError callback, create a variable mappedError with statusCode set to 500 and message set to 'Internal server error'. Import HttpException from @nestjs/common. Throw a new HttpException with mappedError and mappedError.statusCode using throwError(() => new HttpException(...)).

NestJS

import { HttpException } from '@nestjs/common';

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler) {
    return next.handle().pipe(
      catchError(error => {
        // Your code here
      })
    );
  }
}

Need a hint?

Use throwError(() => new HttpException(...)) to rethrow the mapped error.

4

Apply the ExceptionMappingInterceptor to a controller method

Create a controller class called AppController with a method getHello that throws an error with message 'Something went wrong'. Import Controller, Get, and UseInterceptors from @nestjs/common. Apply the ExceptionMappingInterceptor to the getHello method using @UseInterceptors(ExceptionMappingInterceptor).

NestJS

import { Controller, Get, UseInterceptors } from '@nestjs/common';

@Controller()
export class AppController {
  // Your code here
}

Need a hint?

Use @UseInterceptors(ExceptionMappingInterceptor) above the method to apply the interceptor.