Constructor injection in NestJS - Performance & Optimization

Performance: Constructor injection

MEDIUM IMPACT

Constructor injection affects the initialization speed of services and components during app startup, impacting the time to first meaningful paint.

Injecting dependencies into a service or controller

NestJS

import { Injectable } from '@nestjs/common';
import { SomeDependency } from './some-dependency';

@Injectable()
export class MyService {
  constructor(private readonly dependency: SomeDependency) {}
}

Dependencies are injected automatically at construction, enabling faster and synchronous initialization.

📈 Performance GainReduces runtime lookup overhead; improves startup speed and lowers LCP.

Injecting dependencies into a service or controller

NestJS

import { Injectable, Injector } from '@nestjs/common';
import { SomeDependency } from './some-dependency';

@Injectable()
export class MyService {
  private dependency: SomeDependency;
  constructor(private injector: Injector) {}

  async init() {
    this.dependency = this.injector.get(SomeDependency);
  }
}

Using the Injector service to get dependencies at runtime delays initialization and adds overhead.

📉 Performance CostBlocks app startup until dependencies are resolved; triggers multiple runtime lookups increasing initialization time.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Manual Injector.get() calls	0 (no DOM impact)	0	Blocks script execution delaying paint	[X] Bad
Constructor injection	0 (no DOM impact)	0	Non-blocking synchronous init, faster paint	[OK] Good

Rendering Pipeline

Constructor injection happens during the app's bootstrap phase before rendering. It ensures all dependencies are ready synchronously, so the app can render main content faster.

→App Initialization

→Script Execution

→First Contentful Paint

⚠️ BottleneckRuntime dependency resolution when using manual injector calls

Core Web Vital Affected

LCP

Constructor injection affects the initialization speed of services and components during app startup, impacting the time to first meaningful paint.

Optimization Tips

1Always prefer constructor injection over manual Injector.get() calls for dependencies.

2Constructor injection reduces runtime overhead and speeds up app initialization.

3Faster initialization improves Largest Contentful Paint (LCP) metric.

Performance Quiz - 3 Questions

What is the main performance benefit of constructor injection in NestJS?

A It allows lazy loading of dependencies to improve initial load time.

B It delays dependency resolution until first use, saving memory.

C Dependencies are resolved synchronously at startup, reducing runtime overhead.

D It reduces the size of the final JavaScript bundle.

DevTools: Performance

How to check: Record a startup profile and look for long scripting tasks during app bootstrap.

What to look for: Long blocking tasks caused by runtime dependency resolution indicate bad pattern; shorter startup scripting time indicates good constructor injection.

Practice

1. What is the main purpose of constructor injection in NestJS?

easy

A. To provide required services to a class when it is created

B. To manually create instances of services inside methods

C. To define routes for HTTP requests

D. To write database queries inside the constructor

Solution

1. Step 1: Understand constructor injection concept

   Constructor injection is used to pass dependencies (services) into a class automatically when it is created.

2. Step 2: Identify the purpose in NestJS context

   In NestJS, constructor injection helps keep code clean by providing needed services without manual instantiation.

3. Final Answer:

   To provide required services to a class when it is created -> Option A

4. Quick Check:

   Constructor injection = provide services automatically [OK]

Hint: Constructor injection means passing services via constructor [OK]

Common Mistakes:

• Thinking constructor injection creates routes
• Manually creating service instances inside methods
• Confusing injection with database queries

2. Which of the following is the correct syntax for constructor injection of a service named UsersService in a NestJS class?

easy

A. constructor(private readonly usersService: UsersService) {}

B. constructor(usersService: UsersService) {}

C. constructor(private usersService) {}

D. constructor(public readonly usersService) {}

Solution

1. Step 1: Check for correct access modifier and typing

   In NestJS, constructor injection requires private readonly with the service type for proper injection and immutability.

2. Step 2: Compare options

   constructor(private readonly usersService: UsersService) {} (A) is correct. B misses access modifier, C misses type, D misses type annotation.

3. Final Answer:

   constructor(private readonly usersService: UsersService) {} -> Option A

4. Quick Check:

   Use private readonly with type for injection [OK]

Hint: Use 'private readonly serviceName: ServiceType' in constructor [OK]

Common Mistakes:

• Omitting access modifier like private or public
• Not specifying the service type
• Using public readonly without specifying the service type

3. Given the following code, what will be the output when appController.getHello() is called?

import { Injectable, Controller } from '@nestjs/common';

@Injectable()
class HelloService {
  getMessage() {
    return 'Hello, NestJS!';
  }
}

@Controller()
class AppController {
  constructor(private readonly helloService: HelloService) {}

  getHello() {
    return this.helloService.getMessage();
  }
}

medium

A. Error: helloService is not defined

B. undefined

C. 'Hello, NestJS!'

D. 'Hello, World!'

Solution

1. Step 1: Understand constructor injection usage

   The AppController receives HelloService via constructor injection and stores it as helloService.

2. Step 2: Trace method call

   Calling getHello() calls helloService.getMessage() which returns 'Hello, NestJS!'.

3. Final Answer:

   'Hello, NestJS!' -> Option C

4. Quick Check:

   Injected service method returns 'Hello, NestJS!' [OK]

Hint: Injected service methods return expected values [OK]

Common Mistakes:

• Assuming service is undefined without injection
• Confusing method return values
• Expecting default or unrelated strings

4. Identify the error in the following NestJS service injection code:

@Injectable()
class CatsService {}

@Controller('cats')
class CatsController {
  constructor(catsService: CatsService) {}

  findAll() {
    return 'All cats';
  }
}

medium

A. CatsService is not decorated with @Injectable()

B. Missing access modifier (private/public) in constructor parameter

C. Controller decorator is missing path

D. findAll method should be async

Solution

1. Step 1: Check constructor injection syntax

   The constructor parameter catsService: CatsService lacks an access modifier like private or public, so it won't be assigned as a class property.

2. Step 2: Verify other parts

   @Injectable() is present on service, @Controller('cats') has path, and findAll can be sync. So no other errors.

3. Final Answer:

   Missing access modifier (private/public) in constructor parameter -> Option B

4. Quick Check:

   Constructor injection needs access modifier [OK]

Hint: Always add private/public to constructor params for injection [OK]

Common Mistakes:

• Omitting private/public in constructor injection
• Thinking @Injectable is missing when it is present
• Assuming controller path is mandatory for injection

5. You want to inject two services, AuthService and UsersService, into a NestJS controller. Which constructor correctly injects both services following best practices?

hard

A. constructor(public authService: AuthService, public usersService: UsersService) {}

B. constructor(authService: AuthService, usersService: UsersService) {}

C. constructor(private authService: AuthService, usersService: UsersService) {}

D. constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {}

Solution

1. Step 1: Check for access modifiers and readonly usage

   Best practice is to use private readonly for injected services to keep them immutable and private.

2. Step 2: Verify both services are injected with correct syntax

   constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {} injects both services with private readonly, while others miss modifiers or use public.

3. Final Answer:

   constructor(private readonly authService: AuthService, private readonly usersService: UsersService) {} -> Option D

4. Quick Check:

   Use private readonly for multiple injected services [OK]

Hint: Use private readonly for all injected services in constructor [OK]

Common Mistakes:

• Omitting access modifiers on one or both services
• Using public instead of private readonly
• Mixing access modifiers inconsistently