Module decorator and metadata in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this NestJS module output when imported?

Consider this NestJS module definition. What will be the value of exports metadata after compilation?

NestJS

import { Module } from '@nestjs/common';
import { ServiceA } from './serviceA';

@Module({
  providers: [ServiceA],
  exports: [ServiceA],
})
export class MyModule {}

A [ServiceA]

B []

C [MyModule]

D [ServiceA, MyModule]

💡 Hint

Look at what is explicitly listed in the exports array inside the @Module decorator.

The exports metadata array contains only what is explicitly exported. Here, only ServiceA is exported, so the exports array is [ServiceA].

📝 Syntax

intermediate

Which option correctly defines a NestJS module with imports and controllers?

Select the code snippet that correctly uses the @Module decorator with imports and controllers arrays.

A

import { Module } from '@nestjs/common';
@Module({
  imports: OtherModule,
  controllers: AppController,
})
export class AppModule {}

B

import { Module } from '@nestjs/common';
@Module({
  imports: [OtherModule],
  controllers: [AppController],
})
export class AppModule {}

C

import { Module } from '@nestjs/common';
@Module({
  imports: {OtherModule},
  controllers: {AppController},
})
export class AppModule {}

D

import { Module } from '@nestjs/common';
@Module({
  imports: 'OtherModule',
  controllers: 'AppController',
})
export class AppModule {}

💡 Hint

Remember that imports and controllers expect arrays, not single values or objects.

The @Module decorator expects arrays for imports and controllers. Option B correctly uses arrays. Options B, C, and D use invalid types causing syntax or runtime errors.

🔧 Debug

advanced

Why does this NestJS module fail to provide ServiceB to other modules?

Given this module, why can't other modules inject ServiceB?

NestJS

import { Module } from '@nestjs/common';
import { ServiceB } from './serviceB';

@Module({
  providers: [ServiceB],
})
export class FeatureModule {}

A ServiceB must be declared in controllers, not providers.

B ServiceB is not listed in the imports array, so it can't be injected.

C The module is missing the @Injectable decorator on ServiceB.

D ServiceB is not exported in the module's exports array, so it's not available outside FeatureModule.

💡 Hint

Think about how NestJS shares providers between modules.

Providers must be exported explicitly to be available for injection in other modules. Here, ServiceB is only provided but not exported, so other modules cannot inject it.

🧠 Conceptual

advanced

What is the role of the 'imports' array in the @Module decorator?

Choose the best description of what the 'imports' array does in a NestJS module.

A It declares the controllers that belong to this module.

B It defines the components that should be bootstrapped at application start.

C It lists other modules whose exported providers are required by this module.

D It specifies the services that this module provides to others.

💡 Hint

Think about how modules share providers in NestJS.

The imports array allows a module to use providers exported by other modules. Controllers and providers are declared separately.

❓ state_output

expert

What is the value of 'exports' metadata after this module is compiled?

Analyze this NestJS module and determine the final 'exports' array value.

NestJS

import { Module } from '@nestjs/common';
import { ServiceX } from './serviceX';
import { ServiceY } from './serviceY';

@Module({
  providers: [ServiceX, ServiceY],
  exports: [ServiceX],
})
export class SharedModule {}

A [ServiceX]

B [ServiceY]

C []

D [ServiceX, ServiceY]

💡 Hint

Only what is listed in exports is shared outside the module.

Even though both services are providers, only ServiceX is exported and thus available to other modules.