Exception mapping interceptor in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Exception Mapping Interceptor in NestJS?

It is a special class that catches errors thrown during request handling and changes them into friendly responses before sending back to the client.

beginner

How does an Exception Mapping Interceptor improve user experience?

By converting technical errors into clear, simple messages that users can understand, it helps avoid confusion and makes the app feel more reliable.

intermediate

Which NestJS interface must an Exception Mapping Interceptor implement?

It must implement the NestInterceptor interface to properly intercept and handle exceptions.

intermediate

What method is used inside an Exception Mapping Interceptor to catch errors?

The intercept() method is used, which can catch errors using the RxJS catchError operator and then map them to custom responses.

intermediate

Why use an Exception Mapping Interceptor instead of try-catch blocks in controllers?

It centralizes error handling in one place, making code cleaner and easier to maintain, and ensures consistent error responses across the app.

What is the main purpose of an Exception Mapping Interceptor in NestJS?

A To catch and transform exceptions into user-friendly responses

B To log all incoming requests

C To validate user input data

D To manage database connections

Exception Mapping Interceptors catch errors and convert them into clear responses for clients.

Which RxJS operator is commonly used inside an Exception Mapping Interceptor to handle errors?

A catchError

B mergeMap

C filter

D map

catchError is used to catch errors in observable streams and handle them gracefully.

Where should you apply an Exception Mapping Interceptor in a NestJS app for global error handling?

A Only on specific controllers

B Globally in the main app module

C Inside service classes

D In database entities

Applying interceptors globally ensures all exceptions are caught and mapped consistently.

What interface must a class implement to be an interceptor in NestJS?

A CanActivate

B PipeTransform

C ExceptionFilter

D NestInterceptor

NestInterceptor interface defines the intercept() method required for interceptors.

Which of these is NOT a benefit of using Exception Mapping Interceptors?

A Centralized error handling

B Cleaner controller code

C Automatic database optimization

D Consistent error responses

Exception Mapping Interceptors do not affect database optimization.

Explain how an Exception Mapping Interceptor works in NestJS and why it is useful.

Think about how errors flow through the app and how interceptors can change responses.

Describe the steps to create and apply a global Exception Mapping Interceptor in a NestJS application.

Focus on the class structure, RxJS usage, and global application.

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