Repository pattern in NestJS - Performance & Optimization

Performance: Repository pattern

MEDIUM IMPACT

This pattern affects how data fetching and state management impact page load and interaction responsiveness.

Fetching data multiple times in different components

NestJS

class UserRepository {
  private userCache = null;
  constructor(private http: HttpClient) {}
  getUser() {
    if (this.userCache) return of(this.userCache);
    return this.http.get('/api/user').pipe(tap(data => this.userCache = data));
  }
}

@Component({})
class ProfileComponent {
  constructor(private userRepo: UserRepository) {}
  ngOnInit() {
    this.userRepo.getUser().subscribe();
  }
}

@Component({})
class DashboardComponent {
  constructor(private userRepo: UserRepository) {}
  ngOnInit() {
    this.userRepo.getUser().subscribe();
  }
}

Centralizes data fetching and caches results to avoid duplicate network calls, improving responsiveness.

📈 Performance GainSingle network request shared across components, reducing load and improving INP.

Fetching data multiple times in different components

NestJS

class UserService {
  constructor(private http: HttpClient) {}
  getUser() {
    return this.http.get('/api/user');
  }
}

@Component({})
class ProfileComponent {
  constructor(private userService: UserService) {}
  ngOnInit() {
    this.userService.getUser().subscribe();
  }
}

@Component({})
class DashboardComponent {
  constructor(private userService: UserService) {}
  ngOnInit() {
    this.userService.getUser().subscribe();
  }
}

Each component fetches user data separately causing duplicate HTTP requests and slower interaction.

📉 Performance CostTriggers multiple network requests, increasing load time and blocking UI responsiveness.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Multiple fetches in components	Low	Low	High due to blocking	[X] Bad
Repository with caching	Low	Low	Low, non-blocking	[OK] Good

Rendering Pipeline

The repository pattern reduces redundant data fetching, lowering network blocking and improving interaction responsiveness during rendering.

→Network

→JavaScript Execution

→Paint

⚠️ BottleneckNetwork requests causing blocking during interaction

Core Web Vital Affected

INP

This pattern affects how data fetching and state management impact page load and interaction responsiveness.

Optimization Tips

1Cache data in the repository to avoid duplicate network calls.

2Centralize data access to improve interaction responsiveness.

3Avoid fetching the same data multiple times in different components.

Performance Quiz - 3 Questions

How does using a repository pattern with caching affect network requests?

A It has no effect on network requests.

B It increases network requests by adding extra layers.

C It reduces duplicate network requests by sharing cached data.

D It delays network requests until user interaction.

DevTools: Network

How to check: Open DevTools, go to Network tab, reload page, and observe number of API calls for the same data.

What to look for: Multiple identical requests indicate poor repository usage; a single request reused is optimal.

Practice

1. What is the main purpose of the Repository pattern in NestJS?

easy

A. To separate data access logic from business logic

B. To handle HTTP requests directly

C. To manage application configuration settings

D. To create UI components

Solution

1. Step 1: Understand the role of Repository pattern

   The Repository pattern is designed to isolate data access logic from the rest of the application.

2. Step 2: Identify what Repository does in NestJS

   In NestJS, repositories handle database operations, keeping business logic clean and testable.

3. Final Answer:

   To separate data access logic from business logic -> Option A

4. Quick Check:

   Repository pattern = separate data access [OK]

Hint: Repository = data access layer, not UI or config [OK]

Common Mistakes:

• Confusing repository with controller responsibilities
• Thinking repository manages UI or HTTP directly
• Mixing repository with configuration management

2. Which of the following is the correct way to inject a repository in a NestJS service using TypeORM?

easy

A. @Inject(UserRepository) private userRepository: Repository<User>;

B. @InjectableRepository(User) private userRepository: Repository<User>;

C. @RepositoryInject(User) private userRepository: Repository<User>;

D. @InjectRepository(User) private userRepository: Repository<User>;

Solution

1. Step 1: Recall the correct decorator for repository injection

   In NestJS with TypeORM, @InjectRepository() is used to inject repositories.

2. Step 2: Match the syntax with the correct option

   @InjectRepository(User) private userRepository: Repository<User>; uses @InjectRepository(User) which is the official and correct syntax.

3. Final Answer:

   @InjectRepository(User) private userRepository: Repository<User>; -> Option D

4. Quick Check:

   @InjectRepository = correct injection [OK]

Hint: Use @InjectRepository to inject TypeORM repositories [OK]

Common Mistakes:

• Using wrong decorator names like @InjectableRepository
• Confusing @Inject with @InjectRepository
• Omitting the entity inside @InjectRepository()

3. Given the following NestJS service code, what will findAll() return?

import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './user.entity';

@Injectable()
export class UserService {
  constructor(@InjectRepository(User) private userRepo: Repository<User>) {}

  async findAll() {
    return await this.userRepo.find();
  }
}

medium

A. Undefined, because find() is not a valid method

B. A single User entity

C. An array of all User entities from the database

D. A Promise that resolves to null

Solution

1. Step 1: Understand what this.userRepo.find() does

   The find() method of TypeORM repository returns all records of the entity as an array.

2. Step 2: Analyze the return type of findAll()

   Since find() returns a Promise resolving to an array of User entities, findAll() returns that array.

3. Final Answer:

   An array of all User entities from the database -> Option C

4. Quick Check:

   Repository.find() returns array [OK]

Hint: Repository.find() returns all records as an array [OK]

Common Mistakes:

• Thinking find() returns a single entity
• Assuming find() is not a valid method
• Confusing Promise with null return

4. Identify the error in this NestJS repository injection code snippet:

@Injectable()
export class ProductService {
  constructor(private productRepository: Repository<Product>) {}
}

medium

A. Missing @InjectRepository decorator for productRepository

B. Repository should be injected using @InjectableRepository

C. Constructor should not have parameters

D. Repository cannot be injected in services

Solution

1. Step 1: Check how repositories are injected in NestJS

   Repositories must be injected with @InjectRepository(Entity) decorator to work properly.

2. Step 2: Identify missing decorator in the code

   The code misses @InjectRepository(Product) before the repository parameter.

3. Final Answer:

   Missing @InjectRepository decorator for productRepository -> Option A

4. Quick Check:

   Repository injection needs @InjectRepository [OK]

Hint: Always use @InjectRepository for repositories in constructor [OK]

Common Mistakes:

• Forgetting @InjectRepository decorator
• Using wrong decorator names
• Assuming repositories inject automatically without decorators

5. You want to create a custom method findActiveUsers() in your UserRepository that returns only users with isActive = true. Which approach correctly applies the Repository pattern in NestJS?

hard

A. Add findActiveUsers() method directly in the UserService without modifying repository

B. Extend the Repository class and add findActiveUsers() method, then inject this custom repository

C. Use a global function outside repository and service to filter active users

D. Modify the User entity to include a static method findActiveUsers()

Solution

1. Step 1: Understand extending repository for custom methods

   To add custom queries, extend the Repository class and add methods like findActiveUsers().

2. Step 2: Confirm proper injection of custom repository

   Inject the custom repository in services to keep data access logic encapsulated.

3. Final Answer:

   Extend the Repository class and add findActiveUsers() method, then inject this custom repository -> Option B

4. Quick Check:

   Custom repo methods = extend Repository [OK]

Hint: Add custom queries by extending Repository class [OK]

Common Mistakes:

• Putting query logic in service instead of repository
• Using global functions outside repository pattern
• Adding static methods to entity instead of repository