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.