Cache interceptor in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the CacheInterceptor from NestJS.

NestJS

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

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

AController

BCacheModule

CHttpException

DCacheInterceptor

Attempts:

3 left

💡 Hint

Common Mistakes

Importing CacheModule instead of CacheInterceptor

Using unrelated imports like Controller or HttpException

The CacheInterceptor is imported from @nestjs/common to enable caching behavior in controllers or routes.

2fill in blank

medium

Complete the code to apply the CacheInterceptor globally using app.useGlobalInterceptors.

NestJS

app.[1](new CacheInterceptor());

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

AuseGlobalGuards

BuseGlobalFilters

CuseGlobalInterceptors

DuseGlobalPipes

Attempts:

3 left

💡 Hint

Common Mistakes

Using useGlobalGuards or useGlobalFilters which are for other purposes

Forgetting to instantiate CacheInterceptor with new

To apply caching globally, you use useGlobalInterceptors with an instance of CacheInterceptor.

3fill in blank

hard

Fix the error in the code to enable caching on a controller method using the CacheInterceptor.

NestJS

@UseInterceptors([1])
@Get('data')
getData() {
  return { message: 'Hello' };
}

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

ALogger

BCacheInterceptor

CHttpException

DCacheModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using CacheModule instead of CacheInterceptor in the decorator

Using unrelated classes like Logger or HttpException

The @UseInterceptors decorator requires the CacheInterceptor to enable caching on the method.

4fill in blank

hard

Fill both blanks to configure the CacheModule with a TTL of 10 seconds.

NestJS

imports: [CacheModule.[1]({ ttl: [2] })]

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

Aregister

B10

C5

DforRoot

Attempts:

3 left

💡 Hint

Common Mistakes

Using forRoot instead of register for CacheModule

Setting TTL to 5 instead of 10

The CacheModule is configured with register method and a TTL (time to live) of 10 seconds.

5fill in blank

hard

Fill both blanks to create a custom cache interceptor class that extends CacheInterceptor and overrides trackBy method to cache only GET requests.

NestJS

export class CustomCacheInterceptor extends CacheInterceptor {
  trackBy(context: ExecutionContext): string | undefined {
    const request = context.switchToHttp().getRequest();
    if (request.method !== [1]) {
      return undefined;
    }
    return super.[2](context);
  }
}

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

A'POST'

BtrackBy

C'GET'

Dintercept

Attempts:

3 left

💡 Hint

Common Mistakes

Checking for 'POST' instead of 'GET'

Calling super.intercept instead of super.trackBy

The custom interceptor checks if the HTTP method is 'GET' to cache only GET requests. It overrides the trackBy method to customize cache key generation.

Practice

1. What is the main purpose of the CacheInterceptor in NestJS?

easy

A. To handle authentication and authorization

B. To store responses and speed up repeated requests

C. To log all incoming requests

D. To format response data before sending

Solution

1. Step 1: Understand the role of CacheInterceptor

   The CacheInterceptor caches HTTP responses to avoid recomputing them on repeated requests.

2. Step 2: Compare with other options

   Logging, authentication, and formatting are handled by other interceptors or middleware, not CacheInterceptor.

3. Final Answer:

   To store responses and speed up repeated requests -> Option B

4. Quick Check:

   CacheInterceptor caches responses = C [OK]

Hint: CacheInterceptor caches responses to speed up requests [OK]

Common Mistakes:

• Confusing CacheInterceptor with logging interceptors
• Thinking it handles security tasks
• Assuming it formats data

2. Which of the following is the correct way to apply the CacheInterceptor to a controller method in NestJS?

easy

A. Add CacheInterceptor in the constructor of the controller

B. @Cacheable() decorator on the method

C. @UseInterceptors(CacheInterceptor) above the method

D. Use @CacheInterceptor() as a method parameter

Solution

1. Step 1: Recall how to apply interceptors in NestJS

   Interceptors are applied using the @UseInterceptors() decorator above methods or controllers.

2. Step 2: Check each option

   Only @UseInterceptors(CacheInterceptor) above the method uses the correct decorator syntax. Options B, C, and D are invalid or do not exist in NestJS.

3. Final Answer:

   @UseInterceptors(CacheInterceptor) above the method -> Option C

4. Quick Check:

   UseInterceptors decorator = A [OK]

Hint: Use @UseInterceptors(CacheInterceptor) to apply caching [OK]

Common Mistakes:

• Using non-existent @Cacheable decorator
• Trying to inject interceptor in constructor
• Passing interceptor as method parameter

3. Given this code snippet, what will be the output on the second request to /hello?

import { Controller, Get, UseInterceptors } from '@nestjs/common';
import { CacheInterceptor } from '@nestjs/cache-manager';

@Controller()
export class HelloController {
  private count = 0;

  @UseInterceptors(CacheInterceptor)
  @Get('hello')
  getHello() {
    this.count++;
    return `Hello count: ${this.count}`;
  }
}

medium

A. "Hello count: 1" first request, "Hello count: 2" second request

B. Error because count is private

C. "Hello count: 0" on both requests

D. "Hello count: 1" on first and second request

Solution

1. Step 1: Understand CacheInterceptor behavior

   CacheInterceptor caches the response of the first request and returns it for subsequent requests without calling the method again.

2. Step 2: Analyze the count variable increment

   On the first request, count increments to 1 and returns "Hello count: 1". On the second request, the cached response is returned, so count does not increment again.

3. Final Answer:

   "Hello count: 1" on first and second request -> Option D

4. Quick Check:

   Cache returns cached response = B [OK]

Hint: CacheInterceptor returns cached response, method not re-executed [OK]

Common Mistakes:

• Assuming count increments on every request
• Thinking private variables cause errors here
• Ignoring caching effect on method execution

4. Identify the error in this NestJS CacheInterceptor usage:

import { Controller, Get, UseInterceptors } from '@nestjs/common';
import { CacheInterceptor } from '@nestjs/cache-manager';

@Controller()
export class TestController {
  @UseInterceptors(CacheInterceptor())
  @Get('test')
  test() {
    return 'Test response';
  }
}

medium

A. CacheInterceptor should not be called as a function

B. Missing CacheModule import in the controller

C. Method test must be async to use CacheInterceptor

D. CacheInterceptor must be applied globally, not per method

Solution

1. Step 1: Check how CacheInterceptor is applied

   CacheInterceptor is a class, not a function, so it should be passed without parentheses to @UseInterceptors().

2. Step 2: Verify other options

   CacheModule import is required but not shown here (not an error in this snippet). Method does not need to be async. CacheInterceptor can be applied per method.

3. Final Answer:

   CacheInterceptor should not be called as a function -> Option A

4. Quick Check:

   Use class name without () = A [OK]

Hint: Pass CacheInterceptor class, not call it as function [OK]

Common Mistakes:

• Calling CacheInterceptor with parentheses
• Assuming method must be async
• Thinking CacheModule import error here

5. You want to customize the cache key for a route in NestJS using CacheInterceptor. Which approach correctly achieves this?

hard

A. Extend CacheInterceptor and override the trackBy method to return a custom key

B. Pass a custom key string directly to @UseInterceptors(CacheInterceptor)

C. Set a custom key in the controller constructor

D. Use @CacheKey('customKey') decorator on the method

Solution

1. Step 1: Understand how to customize cache keys

   CacheInterceptor allows customization by extending it and overriding the trackBy method to define how keys are generated.

2. Step 2: Evaluate other options

   Passing a string to @UseInterceptors() is invalid. Setting keys in constructor or using @CacheKey decorator does not exist in NestJS.

3. Final Answer:

   Extend CacheInterceptor and override the trackBy method to return a custom key -> Option A

4. Quick Check:

   Override trackBy method = D [OK]

Hint: Override trackBy method in extended CacheInterceptor [OK]

Common Mistakes:

• Trying to pass key directly to decorator
• Using non-existent @CacheKey decorator
• Setting keys in constructor instead of method