Logging interceptor in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the necessary NestJS module for interceptors.

NestJS

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

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

AInjectable

BNestInterceptor

CController

DModule

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Controller instead of NestInterceptor

Using Injectable instead of NestInterceptor

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

2fill in blank

medium

Complete the code to implement the NestJS interceptor interface.

NestJS

export class LoggingInterceptor implements [1] {

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ANestInterceptor

BCanActivate

CExceptionFilter

DPipeTransform

Attempts:

3 left

💡 Hint

Common Mistakes

Implementing PipeTransform instead of NestInterceptor

Implementing ExceptionFilter instead of NestInterceptor

The NestInterceptor interface must be implemented to create an interceptor.

3fill in blank

hard

Fix the error in the method signature to correctly intercept the request.

NestJS

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

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

Acall

Bhandler

Cnext

Drequest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'handler' or 'call' instead of 'next'

Using 'request' which is not the correct parameter here

The second parameter is conventionally named next and is of type CallHandler.

4fill in blank

hard

Fill both blanks to log the request method and URL inside the interceptor.

NestJS

const request = context.switchToHttp().getRequest();
console.log(`Request: ${request.[1] ${request.[2]`);

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Amethod

Burl

Cbody

Dheaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'body' or 'headers' instead of 'method' or 'url'

Mixing the order of method and url

The method and url properties of the request object give the HTTP method and URL.

5fill in blank

hard

Fill all three blanks to complete the interceptor's return statement that logs after the response is sent.

NestJS

return next.handle().pipe(tap(() => {
  console.log(`Response sent for ${context.switchToHttp().getRequest().[1] ${context.switchToHttp().getRequest().[2]`);
  console.log(`Execution time: ${Date.now() - [3]ms`);
}));

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

Amethod

Burl

CstartTime

Dheaders

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'headers' instead of 'method' or 'url'

Using 'Date.now()' directly instead of a stored start time

The method and url log the request details, and startTime is used to calculate execution duration.

Practice

1. What is the main purpose of a LoggingInterceptor in a NestJS application?

easy

A. To manage user authentication and authorization

B. To handle database connections automatically

C. To log details about incoming requests and outgoing responses

D. To serve static files like images and CSS

Solution

1. Step 1: Understand the role of interceptors

   Interceptors in NestJS run code before and after request handlers, often used for logging or modifying requests/responses.

2. Step 2: Identify the purpose of LoggingInterceptor

   A LoggingInterceptor specifically tracks and logs request and response details for debugging and monitoring.

3. Final Answer:

   To log details about incoming requests and outgoing responses -> Option C

4. Quick Check:

   LoggingInterceptor = logs requests/responses [OK]

Hint: LoggingInterceptor logs request and response info [OK]

Common Mistakes:

• Confusing interceptors with middleware
• Thinking interceptors handle authentication
• Assuming interceptors serve static files

2. Which of the following is the correct way to implement the intercept method in a NestJS LoggingInterceptor?

easy

A. intercept(context, next) { return next.handle(); }

B. intercept(context, next) { next.handle(); }

C. intercept(context) { return next.handle(); }

D. intercept(next) { return context.handle(); }

Solution

1. Step 1: Recall the intercept method signature

   The intercept method receives two parameters: context and next. It must return next.handle() to continue the request lifecycle.

2. Step 2: Check each option for correct syntax

   intercept(context, next) { return next.handle(); } correctly uses both parameters and returns next.handle(). Options A, C, and D either miss parameters or do not return the observable.

3. Final Answer:

   intercept(context, next) { return next.handle(); } -> Option A

4. Quick Check:

   intercept(context, next) returns next.handle() [OK]

Hint: intercept always returns next.handle() with both parameters [OK]

Common Mistakes:

• Omitting return statement
• Missing one of the parameters
• Calling handle on wrong object

3. Given this LoggingInterceptor code snippet, what will be logged when a request is handled?

intercept(context, next) {
  const now = Date.now();
  return next.handle().pipe(
    tap(() => console.log(`Request took ${Date.now() - now}ms`))
  );
}

medium

A. Logs the time taken to process the request in milliseconds

B. Logs the request URL only

C. Logs the response body content

D. Logs an error message if the request fails

Solution

1. Step 1: Analyze the timing logic

   The code records the current time before the request is handled, then logs the difference after the response is sent.

2. Step 2: Understand the logged message

   The console.log outputs "Request took Xms" where X is the elapsed time in milliseconds.

3. Final Answer:

   Logs the time taken to process the request in milliseconds -> Option A

4. Quick Check:

   LoggingInterceptor measures request duration [OK]

Hint: Look for Date.now() difference to find elapsed time [OK]

Common Mistakes:

• Assuming it logs request URL or body
• Thinking it logs errors only
• Ignoring the timing calculation

4. Identify the error in this LoggingInterceptor code:

intercept(context, next) {
  const now = Date.now();
  next.handle().pipe(
    tap(() => console.log(`Request took ${Date.now() - now}ms`))
  );
}

medium

A. tap operator is not imported

B. Incorrect use of Date.now()

C. Missing context parameter

D. Missing return statement before next.handle()

Solution

1. Step 1: Check the intercept method return

   The intercept method must return the observable from next.handle() to continue the request lifecycle.

2. Step 2: Identify missing return

   The code calls next.handle().pipe(...) but does not return it, so the request will not proceed properly.

3. Final Answer:

   Missing return statement before next.handle() -> Option D

4. Quick Check:

   intercept must return next.handle() observable [OK]

Hint: Always return next.handle() in intercept method [OK]

Common Mistakes:

• Forgetting to return the observable
• Misusing Date.now() for timing
• Ignoring required parameters

5. You want to create a LoggingInterceptor that logs the request method, URL, and the time taken for each request. Which code snippet correctly implements this behavior?

hard

A. intercept(context, next) {
const req = context.getRequest();
const now = Date.now();
next.handle().pipe(
tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

B. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

C. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
map(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

D. intercept(context, next) {
const req = context.switchToHttp().getRequest();
const now = Date.now();
return next.handle().pipe(
tap(() => console.error(`${req.method} ${req.url} - ${Date.now() - now}ms`))
);
}

Solution

1. Step 1: Access request object correctly

   Use context.switchToHttp().getRequest() to get the HTTP request object in NestJS interceptors.

2. Step 2: Use tap operator for side effects

   The tap operator is used to perform logging without altering the response stream. Using map or console.error is incorrect here.

3. Final Answer:

   intercept(context, next) { const req = context.switchToHttp().getRequest(); const now = Date.now(); return next.handle().pipe( tap(() => console.log(`${req.method} ${req.url} - ${Date.now() - now}ms`)) ); } -> Option B

4. Quick Check:

   Use switchToHttp(), tap, and return next.handle() [OK]

Hint: Use switchToHttp().getRequest() and tap for logging [OK]

Common Mistakes:

• Not returning next.handle()
• Using map instead of tap
• Using console.error instead of console.log
• Incorrect request object access