Injectable decorator in NestJS - Performance & Optimization

Performance: Injectable decorator

MEDIUM IMPACT

This affects the dependency injection system's initialization and service instantiation speed, impacting app startup and runtime responsiveness.

Managing service dependencies in a NestJS application

NestJS

@Injectable()
class UserRepository {}

@Injectable()
class UserService {
  constructor(private readonly repo: UserRepository) {}
}

NestJS injects and reuses singleton instances, reducing redundant object creation and improving startup speed.

📈 Performance GainSingle instantiation per service; faster startup and lower memory use.

Managing service dependencies in a NestJS application

NestJS

class UserService {
  constructor() {
    this.repo = new UserRepository();
  }
}

class UserRepository {}

Manually creating dependencies causes tight coupling and repeated instantiation, increasing startup time and memory use.

📉 Performance CostBlocks app startup longer due to manual object creation; no reuse of instances.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Manual new calls for dependencies	N/A	N/A	N/A	[X] Bad
Using @Injectable with DI container	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

The @Injectable decorator marks classes for NestJS's dependency injection container, which resolves and instantiates dependencies before app runtime.

→Initialization

→Dependency Resolution

→Service Instantiation

⚠️ BottleneckDependency Resolution stage can slow startup if many manual instantiations occur.

Core Web Vital Affected

INP

This affects the dependency injection system's initialization and service instantiation speed, impacting app startup and runtime responsiveness.

Optimization Tips

1Always use @Injectable to let NestJS manage service lifecycles efficiently.

2Avoid manual new calls for dependencies to reduce startup blocking.

3Reuse singleton instances to save memory and improve runtime speed.

Performance Quiz - 3 Questions

How does using @Injectable improve NestJS app performance?

A It enables efficient reuse of service instances, reducing startup time.

B It delays service instantiation until first use, increasing startup time.

C It forces manual creation of dependencies, increasing memory use.

D It disables dependency injection, improving runtime speed.

DevTools: Performance

How to check: Record app startup and look for long scripting tasks related to service instantiation.

What to look for: Long blocking times during initialization indicate manual instantiation overhead.