Service creation in NestJS - Performance & Optimization

Performance: Service creation

MEDIUM IMPACT

Service creation affects initial application startup time and memory usage, impacting how fast the app becomes responsive.

Creating services eagerly at app startup

NestJS

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

@Injectable()
export class HeavyService implements OnModuleInit {
  private initialized = false;

  async onModuleInit() {
    // defer heavy logic asynchronously
    await this.lazyInit();
  }

  private async lazyInit() {
    if (!this.initialized) {
      // heavy initialization logic
      this.initialized = true;
    }
  }
}

// In AppModule providers: [HeavyService]

Defers heavy initialization asynchronously after startup, improving responsiveness.

📈 Performance Gainreduces startup blocking time by 80-90%

Creating services eagerly at app startup

NestJS

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

@Injectable()
export class HeavyService {
  constructor() {
    // heavy initialization logic
  }
}

// In AppModule providers: [HeavyService]

Service initializes heavy resources immediately, delaying app startup.

📉 Performance Costblocks app startup for 100+ ms depending on logic

Performance Comparison

Pattern	Startup Delay	Memory Usage	Responsiveness	Verdict
Eager heavy service creation	High (100+ ms)	High	Slow initial response	[X] Bad
Lazy asynchronous service init	Low (10-20 ms)	Moderate	Fast initial response	[OK] Good

Rendering Pipeline

Service creation happens during app bootstrap before rendering. Heavy synchronous service initialization delays the app becoming interactive.

→App Initialization

→Module Loading

⚠️ BottleneckSynchronous heavy logic in service constructors or lifecycle hooks

Optimization Tips

1Avoid heavy synchronous logic in service constructors.

2Use async lifecycle hooks for deferred initialization.

3Lazy load services if possible to reduce startup delay.

Performance Quiz - 3 Questions

What is the main performance risk of creating heavy services eagerly in NestJS?

A It blocks app startup and delays responsiveness

B It increases network requests

C It causes layout shifts in the browser

D It reduces CSS selector efficiency

DevTools: Performance

How to check: Record a startup profile in DevTools or Node.js profiler, look for long blocking tasks during app bootstrap.

What to look for: Long synchronous tasks in service constructors or OnModuleInit hooks indicate blocking service creation.

Practice

1.

What is the main purpose of creating a service in a NestJS application?

easy

A. To handle HTTP requests directly

B. To style the user interface

C. To define database schemas

D. To hold the business logic separate from controllers

Solution

1. Step 1: Understand the role of services in NestJS

   Services are designed to contain business logic, keeping controllers clean and focused on handling requests.

2. Step 2: Differentiate services from controllers and other parts

   Controllers handle HTTP requests, schemas define data structure, and styling is unrelated to services.

3. Final Answer:

   To hold the business logic separate from controllers -> Option D

4. Quick Check:

   Service purpose = business logic separation [OK]

Hint: Services = business logic holders, not request handlers [OK]

Common Mistakes:

• Confusing services with controllers
• Thinking services handle HTTP requests
• Assuming services define database schemas

2.

Which decorator is used to make a class injectable as a service in NestJS?

easy

A. @Injectable()

B. @Module()

C. @Controller()

D. @Service()

Solution

1. Step 1: Identify the decorator for service injection

   In NestJS, @Injectable() marks a class as a provider that can be injected.

2. Step 2: Differentiate from other decorators

   @Controller() is for controllers, @Module() for modules, and @Service() does not exist in NestJS.

3. Final Answer:

   @Injectable() -> Option A

4. Quick Check:

   Service decorator = @Injectable() [OK]

Hint: Remember: Injectable means ready to inject [OK]

Common Mistakes:

• Using @Controller() instead of @Injectable()
• Confusing @Module() with service decorator
• Assuming @Service() exists in NestJS

3.

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

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

@Injectable()
export class HelloService {
  getHello(): string {
    return 'Hello, NestJS!';
  }
}

medium

A. "Hello, NestJS!"

B. undefined

C. Error: Method not found

D. "Hello World!"

Solution

1. Step 1: Analyze the getHello method

   The method returns the string 'Hello, NestJS!'.

2. Step 2: Determine the output of calling getHello()

   Calling getHello() returns exactly the string defined, no errors or undefined.

3. Final Answer:

   "Hello, NestJS!" -> Option A

4. Quick Check:

   Method return = "Hello, NestJS!" [OK]

Hint: Read method return value carefully [OK]

Common Mistakes:

• Assuming method returns undefined
• Confusing output with error messages
• Mixing up string content

4.

Identify the error in this service code snippet:

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

@Injectable()
export class MathService {
  add(a: number, b: number) {
    return a + b
  }
}

medium

A. Missing parentheses in method declaration

B. Missing return type annotation for add method

C. No error, code is correct

D. Missing semicolon after return statement

Solution

1. Step 1: Check syntax correctness

   The method add correctly takes two numbers and returns their sum. Semicolons are optional in TypeScript.

2. Step 2: Verify method declaration and return type

   Return type annotation is optional; TypeScript infers it. Parentheses are present and correct.

3. Final Answer:

   No error, code is correct -> Option C

4. Quick Check:

   Code syntax = correct [OK]

Hint: Semicolons optional; return types inferred [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Expecting explicit return type always
• Misreading method syntax

5.

You want to create a service that provides a list of users and inject it into a controller. Which of the following is the correct way to create and inject the service?

1. Create service with @Injectable()
2. Define a method getUsers() returning an array
3. Inject service in controller constructor
4. Use service method inside controller method

hard

A. Create service without @Injectable(), inject with @Inject() decorator

B. Create service with @Injectable(), inject via constructor parameter

C. Create service with @Controller(), inject via property decorator

D. Create service with @Module(), inject manually inside methods

Solution

1. Step 1: Create service with @Injectable()

   This decorator makes the service injectable and reusable.

2. Step 2: Inject service via constructor parameter in controller

   Constructor injection is the NestJS standard for dependency injection.

3. Step 3: Use service methods inside controller methods

   This keeps code modular and clean.

4. Final Answer:

   Create service with @Injectable(), inject via constructor parameter -> Option B

5. Quick Check:

   Service injection = @Injectable() + constructor [OK]

Hint: Use @Injectable() and constructor injection [OK]

Common Mistakes:

• Omitting @Injectable() on service
• Using @Controller() on service
• Injecting service incorrectly with property decorators