Dependency Injection

What is Dependency Injection?

Dependency Injection (DI) is a design pattern where a class receives its dependencies from external sources rather than creating them itself.

Without DI (❌ Bad)

export class UserComponent {
  private userService = new UserService(); // Creates dependency internally

  loadUsers() {
    this.userService.getUsers().subscribe(users => {
      console.log(users);
    });
  }
}

Problems:

• Hard to test (can’t mock UserService)
• Tightly coupled to UserService
• If UserService requires dependencies, UserComponent must know about them

With DI (✅ Good)

export class UserComponent {
  constructor(private userService: UserService) {} // Dependency injected

  loadUsers() {
    this.userService.getUsers().subscribe(users => {
      console.log(users);
    });
  }
}

Benefits:

• Easy to test (can inject mock service)
• Loosely coupled
• Angular manages the service lifecycle

Constructor Injection vs inject() Function

Angular provides two ways to inject dependencies:

Constructor Injection (Traditional)

Single Dependency

import { Component } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user',
  template: '<div>{{ users | json }}</div>'
})
export class UserComponent {
  users: User[] = [];

  constructor(private userService: UserService) {
    this.userService.getUsers().subscribe(users => {
      this.users = users;
    });
  }
}

Multiple Dependencies

export class UserComponent {
  constructor(
    private userService: UserService,
    private notificationService: NotificationService,
    private router: Router,
    private route: ActivatedRoute
  ) {}
}

inject() Function (Modern - Angular 14+)

Single Dependency

import { Component, inject } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user',
  template: '<div>{{ users | json }}</div>'
})
export class UserComponent {
  users: User[] = [];
  private userService = inject(UserService);

  ngOnInit() {
    this.userService.getUsers().subscribe(users => {
      this.users = users;
    });
  }
}

Multiple Dependencies

export class UserComponent {
  private userService = inject(UserService);
  private notificationService = inject(NotificationService);
  private router = inject(Router);
  private route = inject(ActivatedRoute);
}

Comparison

Aspect	Constructor	inject()
Angular version	All versions	14+
Syntax	Less boilerplate	More explicit
Readability	Parameter list can get long	Clear and organized
Optional injection	❌ Requires Optional()	✅ Built-in support
Flexibility	Less flexible	More flexible

Tip

Modern recommendation: Use inject() in new code (Angular 14+). It’s more readable and flexible.

Providing Dependencies

Method 1: providedIn: ‘root’ (Recommended)

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

@Injectable({ providedIn: 'root' })
export class UserService {
  // This service is a singleton available app-wide
}

Benefits:

• Singleton instance across the entire app
• Tree-shakeable (unused services removed in production build)
• Simplest approach
• Best for services needed throughout the app

Method 2: providedIn: ‘platform’

@Injectable({ providedIn: 'platform' })
export class LoggingService {
  // Shared across all app instances
}

Use when: Service must be shared across multiple Angular applications on the same page

Method 3: Feature Module Providers

@NgModule({
  providers: [UserService]
})
export class UserModule {}

Use when: Service is only needed within a specific feature module

Method 4: Component Providers

@Component({
  selector: 'app-user',
  providers: [UserService]
})
export class UserComponent {
  // Each component instance gets its own UserService
}

Use when: Service must be unique per component instance

Optional Dependencies

With Constructor Injection

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

export class UserComponent {
  constructor(@Optional() private userService?: UserService) {
    if (this.userService) {
      // Use service
    }
  }
}

With inject()

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

export class UserComponent {
  private userService = inject(UserService, { optional: true });

  ngOnInit() {
    if (this.userService) {
      // Use service
    }
  }
}

Practical Examples

User Service with API

import { Injectable, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class UserService {
  private http = inject(HttpClient);
  private apiUrl = '/api/users';

  getUsers(): Observable<User[]> {
    return this.http.get<User[]>(this.apiUrl);
  }

  getUserById(id: string): Observable<User> {
    return this.http.get<User>(`${this.apiUrl}/${id}`);
  }
}

Component Using Injected Service

import { Component, inject, OnInit } from '@angular/core';
import { UserService } from './user.service';

@Component({
  selector: 'app-user-list',
  template: `
    <div *ngFor="let user of users">
      <h3>{{ user.name }}</h3>
      <p>{{ user.email }}</p>
    </div>
  `
})
export class UserListComponent implements OnInit {
  users: User[] = [];
  private userService = inject(UserService);

  ngOnInit() {
    this.userService.getUsers().subscribe(users => {
      this.users = users;
    });
  }
}

Multiple Injected Dependencies

import { Component, inject } from '@angular/core';
import { UserService } from './user.service';
import { NotificationService } from './notification.service';
import { Router } from '@angular/router';

@Component({
  selector: 'app-user-manager'
})
export class UserManagerComponent {
  private userService = inject(UserService);
  private notificationService = inject(NotificationService);
  private router = inject(Router);

  deleteUser(userId: string) {
    this.userService.deleteUser(userId).subscribe({
      next: () => {
        this.notificationService.showSuccess('User deleted');
        this.router.navigate(['/users']);
      },
      error: (err) => {
        this.notificationService.showError('Failed to delete user');
      }
    });
  }
}

Dependency Injection Checklist

• Services use @Injectable({ providedIn: 'root' })
• Components inject services via inject() or constructor
• No manual service instantiation with new
• Optional dependencies marked with @Optional() or { optional: true }
• Services are singletons where appropriate
• No circular dependencies between services
• Service dependencies documented in JSDoc

Best Practices

✅ Always use DI for services and dependencies ✅ Use providedIn: ‘root’ for app-wide services ✅ Keep services focused on a single responsibility ✅ Test with mock services injected via DI ✅ Use inject() in new code (Angular 14+) ✅ Provide in the narrowest scope needed ✅ Document required dependencies in service JSDoc

---

References

• inject() Function