Interceptor interface in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Interceptor interface from NestJS.

NestJS

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

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

AModule

BController

CNestInterceptor

DInjectable

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Controller instead of NestInterceptor

Using Injectable instead of NestInterceptor

Forgetting to import NestInterceptor

The NestInterceptor interface is imported from @nestjs/common to create interceptors.

2fill in blank

medium

Complete the code to implement the Interceptor interface in a class.

NestJS

export class LoggingInterceptor implements [1] { }

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APipeTransform

BNestInterceptor

CExceptionFilter

DGuard

Attempts:

3 left

💡 Hint

Common Mistakes

Implementing PipeTransform instead of NestInterceptor

Implementing Guard or ExceptionFilter by mistake

To create an interceptor, the class must implement the NestInterceptor interface.

3fill in blank

hard

Fix the error in the intercept method signature by completing the parameter type.

NestJS

intercept(context: [1], next: CallHandler): Observable<any> { return next.handle(); }

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AResponse

BHttpContext

CRequest

DExecutionContext

Attempts:

3 left

💡 Hint

Common Mistakes

Using Request or Response instead of ExecutionContext

Using a non-existent HttpContext type

The intercept method receives an ExecutionContext parameter to access details about the current request.

4fill in blank

hard

Fill both blanks to correctly import and use CallHandler in the interceptor.

NestJS

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

intercept(context: ExecutionContext, next: [2]): Observable<any> {
  return next.handle();
}

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AExecutionContext

BCallHandler

CObservable

DNestInterceptor

Attempts:

3 left

💡 Hint

Common Mistakes

Importing wrong types or missing CallHandler

Using wrong parameter types in intercept method

ExecutionContext and CallHandler are both imported from @nestjs/common and used in the intercept method.

5fill in blank

hard

Fill all three blanks to create a basic interceptor that logs before and after the request.

NestJS

import { Injectable, [1], ExecutionContext, CallHandler } from '@nestjs/common';
import { [2] } from 'rxjs';
import { tap } from 'rxjs/operators';

@Injectable()
export class LoggingInterceptor implements [3] {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before...');
    const now = Date.now();
    return next.handle().pipe(
      tap(() => console.log(`After... ${Date.now() - now}ms`))
    );
  }
}

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AInjectable

Btap

CNestInterceptor

DObservable

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing tap operator with Observable type

Forgetting to add @Injectable decorator

Not implementing NestInterceptor interface

The interceptor class is decorated with @Injectable(), implements NestInterceptor, and uses Observable from rxjs.

Practice

1. What is the primary purpose of an interceptor in NestJS?

easy

A. To run code before and after a request is handled

B. To define database schemas

C. To handle HTTP routing

D. To manage user authentication

Solution

1. Step 1: Understand the role of interceptors

   Interceptors in NestJS are designed to execute code before and after the main request handler runs.

2. Step 2: Compare with other options

   Database schemas, routing, and authentication are handled by other parts of NestJS, not interceptors.

3. Final Answer:

   To run code before and after a request is handled -> Option A

4. Quick Check:

   Interceptor purpose = run code before/after request [OK]

Hint: Interceptors wrap request handling with extra code [OK]

Common Mistakes:

• Confusing interceptors with middleware
• Thinking interceptors handle routing
• Assuming interceptors manage authentication

2. Which method must be implemented when creating a NestJS interceptor?

easy

A. handle()

B. intercept()

C. execute()

D. process()

Solution

1. Step 1: Recall the interceptor interface

   The NestJS interceptor interface requires implementing the intercept() method.

2. Step 2: Verify other options

   Methods like handle(), execute(), or process() are not part of the interceptor interface.

3. Final Answer:

   intercept() -> Option B

4. Quick Check:

   Interceptor method = intercept() [OK]

Hint: Remember: intercept() runs your interceptor logic [OK]

Common Mistakes:

• Using handle() instead of intercept()
• Confusing with middleware method names
• Forgetting to implement intercept()

3. Given this interceptor code snippet, what will be logged?

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

@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before handler');
    return next.handle().pipe(
      tap(() => console.log('After handler'))
    );
  }
}

medium

A. Only 'Before handler' is logged

B. Only 'After handler' is logged

C. 'Before handler' then 'After handler' are logged

D. No logs are produced

Solution

1. Step 1: Analyze the intercept method

   The interceptor logs 'Before handler' immediately, then calls next.handle() which returns an Observable.

2. Step 2: Understand the tap operator

   The tap operator logs 'After handler' when the Observable emits after the request handler finishes.

3. Final Answer:

   'Before handler' then 'After handler' are logged -> Option C

4. Quick Check:

   Logs before and after handler = 'Before handler' then 'After handler' are logged [OK]

Hint: tap() logs after the handler completes [OK]

Common Mistakes:

• Thinking only one log appears
• Ignoring the Observable pipe behavior
• Assuming synchronous logging only

4. What is wrong with this interceptor implementation?

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

@Injectable()
export class ErrorInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    try {
      return next.handle();
    } catch (error) {
      console.error('Error caught:', error);
      throw error;
    }
  }
}

medium

A. The interceptor should not throw errors

B. The intercept method must return void, not Observable

C. Missing call to next.handle() inside try block

D. The try-catch block won't catch async errors from next.handle()

Solution

1. Step 1: Understand async error handling in interceptors

   next.handle() returns an Observable, so errors happen asynchronously and are not caught by try-catch.

2. Step 2: Identify the problem with the try-catch

   The try-catch only catches synchronous errors, so async errors must be handled with RxJS operators like catchError.

3. Final Answer:

   The try-catch block won't catch async errors from next.handle() -> Option D

4. Quick Check:

   Async errors need RxJS catchError, not try-catch [OK]

Hint: Use RxJS catchError for async errors, not try-catch [OK]

Common Mistakes:

• Using try-catch for async Observable errors
• Assuming intercept returns void
• Not handling errors inside pipe

5. You want to create an interceptor that measures how long a request takes and adds this duration to the response object as response.duration. Which approach correctly implements this?

hard

A. Record start time, call next.handle(), then use tap() to calculate duration and modify response before returning

B. Record start time, use try-catch around next.handle(), then set duration after catch

C. Modify response before calling next.handle(), then record duration after

D. Use a middleware instead of interceptor to measure time

Solution

1. Step 1: Understand timing with interceptors

   Start time must be recorded before calling next.handle(), which returns an Observable.

2. Step 2: Use tap() to run code after handler finishes

   tap() lets you calculate duration after the response is ready and modify it safely.

3. Step 3: Evaluate options

   Record start time, call next.handle(), then use tap() to calculate duration and modify response before returning correctly uses tap() after next.handle() to set response.duration. Record start time, use try-catch around next.handle(), then set duration after catch uses try-catch incorrectly for async. Modify response before calling next.handle(), then record duration after modifies response too early. Use a middleware instead of interceptor to measure time suggests middleware, which is not the question focus.

4. Final Answer:

   Record start time, call next.handle(), then use tap() to calculate duration and modify response before returning -> Option A

5. Quick Check:

   Use tap() after next.handle() to measure time [OK]

Hint: Use tap() after next.handle() to add timing info [OK]

Common Mistakes:

• Trying to catch async errors with try-catch
• Modifying response before handler runs
• Confusing middleware with interceptor roles