Exception mapping interceptor in NestJS

Introduction

An exception mapping interceptor helps catch errors in your NestJS app and change them into friendly responses.

You want to send custom error messages to users instead of default ones.

You need to log errors and still return a clean response.

You want to transform different error types into a standard format.

You want to handle errors globally in one place.

You want to add extra info to error responses like timestamps.

Syntax

NestJS

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

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      catchError(error => {
        // Map or transform the error here
        const mappedError = new Error('Custom error message');
        return throwError(() => mappedError);
      }),
    );
  }
}

The interceptor uses RxJS catchError to catch errors from the request stream.

You must return an observable error using throwError(() => error) in RxJS 7+.

Examples

Simple error mapping to a new error with a custom message.

NestJS

catchError(error => {
  const mappedError = new Error('Something went wrong');
  return throwError(() => mappedError);
})

Maps only 404 errors to a custom message, passes others unchanged.

NestJS

catchError(error => {
  if (error.status === 404) {
    return throwError(() => new Error('Resource not found'));
  }
  return throwError(() => error);
})

Logs the original error and returns a generic error message.

NestJS

catchError(error => {
  console.error('Logging error:', error.message);
  return throwError(() => new Error('Internal server error'));
})

Sample Program

This interceptor catches any error thrown in the controller and changes it to a 400 Bad Request with a custom message.

When you call GET /test, instead of the original error, you get a clean custom error response.

NestJS

import { Injectable, NestInterceptor, ExecutionContext, CallHandler, HttpException, HttpStatus } from '@nestjs/common';
import { Observable, catchError, throwError } from 'rxjs';

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      catchError(error => {
        // Map any error to HttpException with status 400 and custom message
        const customError = new HttpException('Custom bad request error', HttpStatus.BAD_REQUEST);
        return throwError(() => customError);
      }),
    );
  }
}

// Usage example in a controller
import { Controller, Get, UseInterceptors } from '@nestjs/common';

@Controller('test')
@UseInterceptors(ExceptionMappingInterceptor)
export class TestController {
  @Get()
  getTest() {
    throw new Error('Original error');
  }
}

Important Notes

Interceptors run before and after the request handler, so they can catch errors thrown anywhere in the flow.

Always return a new error inside throwError(() => error) to keep RxJS happy.

You can use this pattern to unify error responses across your app.

Summary

An exception mapping interceptor catches errors and changes them to friendly responses.

Use RxJS catchError inside the interceptor to handle errors.

This helps keep your API responses consistent and user-friendly.

Practice

1. What is the main purpose of an ExceptionMappingInterceptor in NestJS?

easy

A. To catch exceptions and transform them into user-friendly responses

B. To log all incoming HTTP requests

C. To validate incoming request data automatically

D. To manage database connections

Solution

1. Step 1: Understand the role of interceptors

   Interceptors in NestJS can modify requests or responses and handle errors.

2. Step 2: Identify the function of exception mapping

   An ExceptionMappingInterceptor specifically catches exceptions and converts them into consistent, user-friendly API responses.

3. Final Answer:

   To catch exceptions and transform them into user-friendly responses -> Option A

4. Quick Check:

   Exception mapping = user-friendly error response [OK]

Hint: Exception interceptors catch errors and format responses [OK]

Common Mistakes:

• Confusing interceptors with middleware for logging
• Thinking interceptors validate data automatically
• Assuming interceptors manage database connections

2. Which RxJS operator is commonly used inside an ExceptionMappingInterceptor to handle errors?

easy

A. map

B. filter

C. tap

D. catchError

Solution

1. Step 1: Recall RxJS operators for error handling

   The catchError operator is designed to catch errors in an observable stream.

2. Step 2: Match operator to interceptor usage

   ExceptionMappingInterceptor uses catchError to catch exceptions and transform them.

3. Final Answer:

   catchError -> Option D

4. Quick Check:

   RxJS error handling = catchError [OK]

Hint: Use catchError to catch and transform errors in observables [OK]

Common Mistakes:

• Using map which transforms data but doesn't catch errors
• Confusing filter with error handling
• Using tap which is for side effects, not error catching

3. Given this ExceptionMappingInterceptor code snippet, what will be the output if the handler throws an error?

intercept(context, next) {
  return next.handle().pipe(
    catchError(err => {
      return throwError(() => new BadRequestException('Mapped error'));
    }),
  );
}

medium

A. The original error is sent to the client

B. No error is sent; the request succeeds silently

C. A BadRequestException with message 'Mapped error' is sent to the client

D. A syntax error occurs and the app crashes

Solution

1. Step 1: Analyze the catchError block

   When an error occurs, catchError catches it and returns a new BadRequestException with a custom message.

2. Step 2: Understand the effect on the client response

   The client receives the new BadRequestException with message 'Mapped error' instead of the original error.

3. Final Answer:

   A BadRequestException with message 'Mapped error' is sent to the client -> Option C

4. Quick Check:

   Error replaced by BadRequestException [OK]

Hint: catchError replaces original error with mapped exception [OK]

Common Mistakes:

• Thinking original error passes through unchanged
• Assuming no error is sent and request succeeds
• Confusing syntax error with runtime error

4. Identify the error in this ExceptionMappingInterceptor code:

intercept(context, next) {
  return next.handle().pipe(
    catchError(err => {
      throw new HttpException('Error occurred', 400);
    }),
  );
}

medium

A. No error; code is correct

B. Using throw inside catchError instead of returning an observable error

C. Incorrect status code 400 instead of 500

D. Missing import for HttpException

Solution

1. Step 1: Understand catchError usage

   Inside catchError, you must return an observable error, not throw an error directly.

2. Step 2: Identify correct error handling

   The code should return throwError(() => new HttpException(...)) instead of throwing.

3. Final Answer:

   Using throw inside catchError instead of returning an observable error -> Option B

4. Quick Check:

   catchError must return observable error, not throw [OK]

Hint: Return throwError inside catchError, don't throw directly [OK]

Common Mistakes:

• Throwing error directly inside catchError
• Ignoring observable error return requirement
• Assuming status code 400 is wrong here

5. You want to create an ExceptionMappingInterceptor that converts all QueryFailedError exceptions into BadRequestException with a custom message, but lets other exceptions pass unchanged. Which code snippet correctly implements this behavior?

hard

A. return next.handle().pipe(catchError(err => {
if (err instanceof QueryFailedError) {
return throwError(() => new BadRequestException('Invalid query'));
}
return throwError(() => err);
}));

B. return next.handle().pipe(catchError(err => {
if (err instanceof QueryFailedError) {
throw new BadRequestException('Invalid query');
}
throw err;
}));

C. return next.handle().pipe(catchError(err => {
if (err.name === 'QueryFailedError') {
return new BadRequestException('Invalid query');
}
return err;
}));

D. return next.handle().pipe(catchError(err => {
if (err instanceof BadRequestException) {
return throwError(() => new QueryFailedError());
}
return throwError(() => err);
}));

Solution

1. Step 1: Check error type and return observable error

   The interceptor must check if the error is a QueryFailedError instance and return a new BadRequestException wrapped in throwError.

2. Step 2: Pass other errors unchanged as observable errors

   Other errors must be re-thrown as observable errors using throwError to keep the stream consistent.

3. Final Answer:

   Option A correctly checks error type and returns observable errors properly -> Option A

4. Quick Check:

   Use instanceof + throwError for error mapping [OK]

Hint: Use instanceof and return throwError for mapped and original errors [OK]

Common Mistakes:

• Throwing errors directly inside catchError
• Returning error objects instead of observable errors
• Checking error name string instead of instanceof
• Swapping error types incorrectly