Shared modules in NestJS - Performance & Optimization

Performance: Shared modules

MEDIUM IMPACT

This affects the initial load time and memory usage by controlling how code is reused across the app.

Reusing common services or components across multiple modules

NestJS

import { CommonService } from './common.service';
import { Module } from '@nestjs/common';

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

@Module({
  imports: [SharedModule],
})
export class FeatureModule {}

@Module({
  imports: [SharedModule],
})
export class AnotherFeatureModule {}

CommonService is provided once in SharedModule and reused, reducing duplication and memory use.

📈 Performance Gainsingle instance shared, reduces memory and speeds up app load

Reusing common services or components across multiple modules

NestJS

import { CommonService } from './common.service';
import { Module } from '@nestjs/common';

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

@Module({
  imports: [FeatureModule],
  providers: [CommonService], // duplicated provider
})
export class AnotherFeatureModule {}

CommonService is provided multiple times in different modules causing duplicated instances and larger bundle size.

📉 Performance Costincreases memory usage and slows startup due to duplicated code and initialization

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicated providers in multiple modules	N/A	N/A	N/A	[X] Bad
Single shared module providing common services	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Shared modules reduce duplicated code loading and initialization during app bootstrap, improving the critical rendering path.

→Bundle Loading

→Module Initialization

→Memory Allocation

⚠️ BottleneckModule Initialization when duplicated providers cause multiple instances

Core Web Vital Affected

LCP

This affects the initial load time and memory usage by controlling how code is reused across the app.

Optimization Tips

1Provide common services once in a shared module to avoid duplication.

2Import shared modules in feature modules instead of re-providing services.

3Avoid multiple instances of the same provider to reduce memory and startup cost.

Performance Quiz - 3 Questions

What is the main performance benefit of using a shared module in NestJS?

A It increases the number of providers for better scalability.

B It avoids duplicated service instances and reduces memory usage.

C It automatically caches HTTP requests.

D It delays module loading until needed.

DevTools: Network and Performance

How to check: Open DevTools, go to Network tab to check bundle sizes and requests. Use Performance tab to record app startup and check module initialization time.

What to look for: Look for duplicated code in bundles and longer initialization times indicating duplicated providers.

Practice

1. What is the main purpose of a Shared Module in NestJS?

easy

A. To define database entities

B. To create a new controller for handling requests

C. To share providers and services across multiple modules

D. To configure middleware for the application

Solution

1. Step 1: Understand the role of Shared Modules

   Shared modules are designed to hold providers and services that can be reused in different parts of the app.

2. Step 2: Compare with other module roles

   Controllers handle requests, entities define data models, and middleware configures request handling, which are not the main purpose of shared modules.

3. Final Answer:

   To share providers and services across multiple modules -> Option C

4. Quick Check:

   Shared modules = share providers [OK]

Hint: Shared modules share providers, not controllers or entities [OK]

Common Mistakes:

• Confusing shared modules with controllers
• Thinking shared modules define database entities
• Assuming shared modules configure middleware

2. Which of the following is the correct way to export a provider from a shared module in NestJS?

easy

A. Add the provider to the exports array in the module decorator

B. Add the provider to the controllers array

C. Add the provider to the imports array

D. Add the provider to the bootstrap array

Solution

1. Step 1: Recall module decorator properties

   Providers are shared by listing them in the exports array of the module decorator.

2. Step 2: Understand other arrays

   controllers is for controllers, imports is for importing other modules, and bootstrap is not used in modules.

3. Final Answer:

   Add the provider to the exports array in the module decorator -> Option A

4. Quick Check:

   Export providers via exports array [OK]

Hint: Use exports array to share providers [OK]

Common Mistakes:

• Putting providers in controllers array
• Confusing imports with exports
• Using bootstrap array which doesn't exist in modules

3. Given this shared module code, what will be the output when AppModule injects CommonService and calls getMessage()?

import { Module } from '@nestjs/common';
import { CommonService } from './common.service';

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

// CommonService
export class CommonService {
  getMessage() {
    return 'Hello from SharedModule';
  }
}

// AppModule
import { Module } from '@nestjs/common';
import { SharedModule } from './shared.module';

@Module({
  imports: [SharedModule],
})
export class AppModule {
  constructor(private commonService: CommonService) {}

  printMessage() {
    console.log(this.commonService.getMessage());
  }
}

medium

A. Error: CommonService not found

B. Undefined

C. Hello from SharedModule

D. Hello from AppModule

Solution

1. Step 1: Check provider export and import

   SharedModule exports CommonService, and AppModule imports SharedModule, so CommonService is available in AppModule.

2. Step 2: Understand service method call

   Calling getMessage() returns the string 'Hello from SharedModule'.

3. Final Answer:

   Error: CommonService not found -> Option A

4. Quick Check:

   Service must be injected via constructor and registered as provider [OK]

Hint: Import shared module to access exported services [OK]

Common Mistakes:

• Forgetting to export the provider
• Not importing the shared module in AppModule
• Assuming service returns undefined

4. Identify the error in this shared module setup:

import { Module } from '@nestjs/common';
import { LoggerService } from './logger.service';

@Module({
  providers: [LoggerService],
  imports: [LoggerService],
  exports: [LoggerService],
})
export class LoggerModule {}

medium

A. LoggerService must be declared as a global provider

B. LoggerService should not be in the imports array

C. LoggerService should not be exported

D. LoggerService must be in controllers array

Solution

1. Step 1: Review module decorator arrays

   The imports array is for importing other modules, not providers or services.

2. Step 2: Identify correct placement

   LoggerService should be in providers and exports, but not in imports.

3. Final Answer:

   LoggerService should not be in the imports array -> Option B

4. Quick Check:

   Imports array = modules only [OK]

Hint: Imports array only accepts modules, not providers [OK]

Common Mistakes:

• Putting providers in imports array
• Confusing controllers with providers
• Thinking exports are optional for sharing

5. You have two modules, FeatureModule and SharedModule. SharedModule exports UtilityService. You want FeatureModule to use UtilityService and also export it for other modules. How should you configure FeatureModule to achieve this?

hard

A. Import SharedModule and add UtilityService to FeatureModule controllers

B. Only import SharedModule in FeatureModule, no exports needed

C. Declare UtilityService again in FeatureModule providers and exports

D. Import SharedModule and export UtilityService in FeatureModule's exports array

Solution

1. Step 1: Import SharedModule in FeatureModule

   To use UtilityService, FeatureModule must import SharedModule which exports it.

2. Step 2: Export UtilityService from FeatureModule

   To share UtilityService further, FeatureModule must export it in its exports array.

3. Final Answer:

   Import SharedModule and export UtilityService in FeatureModule's exports array -> Option D

4. Quick Check:

   Import then export to re-share service [OK]

Hint: Import then export shared service to re-share it [OK]

Common Mistakes:

• Not exporting UtilityService from FeatureModule
• Re-declaring UtilityService providers unnecessarily
• Adding services to controllers array