How dependency injection works in Angular - Performance Optimization Steps

Performance: How dependency injection works in Angular

MEDIUM IMPACT

Dependency injection affects initial load time and runtime responsiveness by managing service creation and reuse efficiently.

Injecting services in Angular components

Angular

import { Injectable, Component } from '@angular/core';

@Injectable({ providedIn: 'root' })
export class DataService {
  constructor() { }
}

@Component({
  selector: 'app-example',
  template: '<p>Example</p>'
})
export class ExampleComponent {
  constructor(private dataService: DataService) { }
}

Providing the service at root level shares a single instance app-wide, reducing memory use and improving load and interaction speed.

📈 Performance GainSingle service instance reused, reducing memory and CPU overhead during component creation.

Injecting services in Angular components

Angular

import { Injectable, Component } from '@angular/core';

@Injectable({ providedIn: 'root' })
export class DataService {
  constructor() { }
}

@Component({
  selector: 'app-example',
  template: '<p>Example</p>',
  providers: [DataService]
})
export class ExampleComponent {
  constructor(private dataService: DataService) { }
}

Providing the service in the component creates a new instance each time the component is created, causing redundant service instances and extra memory use.

📉 Performance CostTriggers multiple service instantiations, increasing memory and CPU usage during component creation.

Performance Comparison

Pattern	Service Instances	Memory Use	Initialization Time	Verdict
Service provided in component	Multiple per component	High	Longer due to repeated instantiation	[X] Bad
Service provided in root	Single shared instance	Low	Short due to reuse	[OK] Good

Rendering Pipeline

Angular's dependency injection resolves service instances before component rendering, affecting the setup phase before the browser paints the UI.

→Script Execution

→Component Initialization

→Layout

⚠️ BottleneckComponent Initialization due to repeated service instantiation if DI is misused

Core Web Vital Affected

INP

Dependency injection affects initial load time and runtime responsiveness by managing service creation and reuse efficiently.

Optimization Tips

1Provide services at root or module level to share instances and reduce memory use.

2Avoid providing services in components unless a new instance per component is required.

3Use Angular's DI system to manage service lifecycles efficiently for better interaction performance.

Performance Quiz - 3 Questions

What is the main performance benefit of providing a service at the root level in Angular?

A It delays service creation until the component is visible.

B It creates a single shared instance, reducing memory and CPU use.

C It forces a new instance for each component, improving isolation.

D It removes the need for constructor injection.

DevTools: Performance

How to check: Record a performance profile while navigating to a component. Look for repeated constructor calls of the service in the flame chart.

What to look for: Multiple service instantiations indicate inefficient DI usage; a single instance shows good reuse.

Practice

1. What is the main purpose of dependency injection in Angular?

easy

A. To write all code inside a single file

B. To manually create instances of services inside components

C. To provide components with the services they need automatically

D. To prevent components from using any services

Solution

1. Step 1: Understand dependency injection concept

   Dependency injection automatically provides required services to components without manual creation.

2. Step 2: Compare options with this concept

   Only To provide components with the services they need automatically describes automatic provision of services, which is the core of dependency injection.

3. Final Answer:

   To provide components with the services they need automatically -> Option C

4. Quick Check:

   Dependency injection = automatic service provision [OK]

Hint: Dependency injection means Angular gives services automatically [OK]

Common Mistakes:

• Thinking you must create service instances manually
• Believing dependency injection restricts service use
• Confusing dependency injection with code organization

2. Which decorator makes a class injectable in Angular?

easy

A. @Component

B. @Input

C. @NgModule

D. @Injectable

Solution

1. Step 1: Identify Angular decorators for injection

   The decorator @Injectable marks a class as available for dependency injection.

2. Step 2: Eliminate other decorators

   @Component is for components, @NgModule for modules, and @Input for input properties, none make a class injectable.

3. Final Answer:

   @Injectable -> Option D

4. Quick Check:

   @Injectable = makes class injectable [OK]

Hint: Use @Injectable to mark services for injection [OK]

Common Mistakes:

• Confusing @Component with @Injectable
• Using @NgModule to make services injectable
• Forgetting to add @Injectable to services

3. Given this Angular service and component code, what will be logged?

import { Injectable } from '@angular/core';

@Injectable({ providedIn: 'root' })
export class LoggerService {
  log(message: string) {
    console.log('Log:', message);
  }
}

import { Component } from '@angular/core';
import { LoggerService } from './logger.service';

@Component({ selector: 'app-root', template: '' })
export class AppComponent {
  constructor(private logger: LoggerService) {
    this.logger.log('Hello');
  }
}

medium

A. Log: Hello

B. Error: LoggerService not found

C. No output

D. Log: undefined

Solution

1. Step 1: Understand service injection in constructor

   The LoggerService is injected into AppComponent via constructor and used to log a message.

2. Step 2: Analyze the log call

   The log method prints 'Log:' plus the message 'Hello', so console shows 'Log: Hello'.

3. Final Answer:

   Log: Hello -> Option A

4. Quick Check:

   Injected service logs message correctly [OK]

Hint: Injected services work via constructor parameters [OK]

Common Mistakes:

• Expecting error if service is providedIn root
• Thinking log method won't run without template
• Confusing console output with return value

4. What is wrong with this Angular service injection code?

import { Injectable } from '@angular/core';

@Injectable()
export class DataService {
  getData() { return [1, 2, 3]; }
}

import { Component } from '@angular/core';

@Component({ selector: 'app-data', template: '' })
export class DataComponent {
  constructor(private dataService: DataService) {}
}

medium

A. DataComponent constructor should not have parameters

B. DataService is not provided in any injector

C. DataService class is missing @Component decorator

D. getData method must be static

Solution

1. Step 1: Check service provider configuration

   The service has @Injectable() but no providedIn or provider registration, so Angular doesn't know where to provide it.

2. Step 2: Understand injection failure cause

   Without being provided, Angular throws an error when injecting DataService into DataComponent.

3. Final Answer:

   DataService is not provided in any injector -> Option B

4. Quick Check:

   Missing provider causes injection error [OK]

Hint: Always provide services via providedIn or providers array [OK]

Common Mistakes:

• Assuming @Injectable alone provides the service
• Adding @Component to services incorrectly
• Thinking constructor parameters are optional for injection

5. You want to inject two different services of the same class but with different configurations in Angular. Which approach correctly achieves this?

hard

A. Use Angular injection tokens to provide different configurations and inject by token

B. Create two different classes extending the service and inject each separately

C. Use @Injectable({ providedIn: 'root' }) on the class and inject twice in constructor

D. Inject the service once and manually change its configuration before each use

Solution

1. Step 1: Understand multiple instances need

   Injecting the same class twice with different configs requires distinguishing them in Angular's injector.

2. Step 2: Use injection tokens for different configs

   Angular injection tokens allow providing multiple versions of the same class with different configurations and injecting them separately.

3. Step 3: Evaluate other options

   Use @Injectable({ providedIn: 'root' }) on the class and inject twice in constructor injects the same instance twice, B duplicates classes unnecessarily, D is error-prone and not recommended.

4. Final Answer:

   Use Angular injection tokens to provide different configurations and inject by token -> Option A

5. Quick Check:

   Injection tokens enable multiple configured instances [OK]

Hint: Use injection tokens for multiple differently configured services [OK]

Common Mistakes:

• Injecting same service twice without tokens
• Creating duplicate service classes unnecessarily
• Changing service config manually at runtime