Exception mapping interceptor in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the necessary decorator for creating an interceptor.

NestJS

import { [1] } from '@nestjs/common';

Drag options to blanks, or click blank then click option'

AInjectable

BController

CModule

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller instead of @Injectable

Forgetting to import any decorator

Using @Module or @Service which are unrelated here

The @Injectable decorator is required to define an interceptor class in NestJS.

2fill in blank

medium

Complete the code to implement the intercept method signature in the interceptor class.

NestJS

intercept(context: ExecutionContext, next: [1]): Observable<any> {

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ACallHandler

BRequest

CResponse

DHttpException

Attempts:

3 left

💡 Hint

Common Mistakes

Using Request or Response instead of CallHandler

Confusing with exception classes

The intercept method receives a CallHandler as the second argument to handle the request flow.

3fill in blank

hard

Fix the error in the pipe method to catch exceptions and map them.

NestJS

return next.handle().pipe(catchError(error => throwError(() => new [1](error.message, 500))));

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AObservable

BError

CHttpException

DInterceptor

Attempts:

3 left

💡 Hint

Common Mistakes

Throwing generic Error instead of HttpException

Using Observable or Interceptor which are not error classes

To map exceptions properly, you throw a new HttpException with the error message.

4fill in blank

hard

Fill both blanks to create a custom exception mapping interceptor class.

NestJS

import { [1], NestInterceptor, ExecutionContext, CallHandler, HttpException } from '@nestjs/common';
import { Observable, throwError } from 'rxjs';
import { catchError } from 'rxjs/operators';

@[2]()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      catchError(error => {
        return throwError(() => new HttpException(error.message, error.status || 500));
      }),
    );
  }
}

Drag options to blanks, or click blank then click option'

AInjectable

BController

CModule

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Controller or @Module decorators

Forgetting to import Injectable

The class must be decorated with @Injectable and import it to be recognized as an interceptor.

5fill in blank

hard

Fill all three blanks to complete the interceptor that maps exceptions with status and message.

NestJS

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

@Injectable()
export class ExceptionMappingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    return next.handle().pipe(
      catchError(error => {
        const status = error?.[1] || 500;
        const message = error?.[2] || 'Internal Server Error';
        return throwError(() => new [3](message, status));
      }),
    );
  }
}

Drag options to blanks, or click blank then click option'

Astatus

Bmessage

CHttpException

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong property names like 'error' instead of 'status' or 'message'

Throwing generic Error instead of HttpException

The interceptor extracts status and message from the error object and throws a new HttpException with those values.

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