Dependency Injection

Dependency Injection (DI) is one of Angular's core mechanisms.

Instead of components creating the services they need, Angular creates and manages those services and hands them over. The result: less coupling, easier testing, and code that's simpler to maintain.

• What Is Dependency Injection
• Services and @Injectable
• Injecting a Service
• providedIn Options
• Providing Services in bootstrapApplication
• Component-Level Services

What Is Dependency Injection

Say a component needs a UserService to fetch user data.

Without DI:

export class UserListComponent {
  private userService = new UserService(); // created manually
}

This works, but it has problems:

• The component is tightly coupled to UserService
• You can't swap it out for a mock in tests
• Each component creates its own instance — no shared state

With DI, Angular handles the creation and provides the instance:

export class UserListComponent {
  constructor(private userService: UserService) {} // Angular injects it
}

The component just declares what it needs. Angular figures out how to provide it.

Services and @Injectable

A service is a class that handles business logic, data fetching, or shared state.

To make a class injectable, add the @Injectable decorator:

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

@Injectable({
  providedIn: 'root',
})
export class UserService {
  getUsers() {
    return [
      { id: 1, name: 'Charmy' },
      { id: 2, name: 'Alice' },
    ];
  }
}

@Injectable tells Angular this class can be injected. providedIn: 'root' means the service is a singleton — one instance shared across the entire application.

Injecting a Service

Constructor Injection (traditional)

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

@Component({
  standalone: true,
  selector: 'app-user-list',
  template: `
    <ul>
      <li *ngFor="let user of users">{{ user.name }}</li>
    </ul>
  `,
})
export class UserListComponent {
  users = [];

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

The inject Function (Angular 14+)

Angular 14 introduced the inject function, which lets you inject dependencies without a constructor:

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

@Component({
  standalone: true,
  selector: 'app-user-list',
  template: `...`,
})
export class UserListComponent {
  private userService = inject(UserService);
  users = this.userService.getUsers();
}

inject can be used during property initialization, which keeps the code concise. It's the recommended approach in modern standalone Angular.

providedIn Options

The providedIn property on @Injectable determines the scope of the service.

providedIn: 'root'

The most common option. One instance shared across the entire application:

@Injectable({
  providedIn: 'root',
})
export class UserService {}

Use this for globally shared services — API calls, auth state, user preferences.

providedIn: 'platform'

Shared across multiple Angular applications running on the same page. Rarely needed.

providedIn: 'any'

Each lazy-loaded module gets its own instance. Also rarely used.

Providing Services in bootstrapApplication

In a standalone app, global services and configuration go in the providers array of bootstrapApplication:

// main.ts
import { bootstrapApplication } from '@angular/platform-browser';
import { provideHttpClient } from '@angular/common/http';
import { AppComponent } from './app/app.component';

bootstrapApplication(AppComponent, {
  providers: [
    provideHttpClient(),
  ],
});

You can also provide custom services or values:

bootstrapApplication(AppComponent, {
  providers: [
    { provide: UserService, useClass: UserService },
    { provide: 'API_URL', useValue: 'https://api.example.com' },
  ],
});

To inject a string token, use @Inject:

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

@Component({ ... })
export class AppComponent {
  constructor(@Inject('API_URL') private apiUrl: string) {
    console.log(apiUrl); // "https://api.example.com"
  }
}

Component-Level Services

Services can be scoped to a specific component and its children by declaring them in the component's providers:

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

@Component({
  standalone: true,
  selector: 'app-user-section',
  providers: [UserService], // separate instance, scoped to this component
  template: `...`,
})
export class UserSectionComponent {}

This creates a new UserService instance that's isolated from the root-level one. Any child components will share this instance, but nothing outside UserSectionComponent can access it.

Useful when you need isolated state — form state, local data management, or keeping two parts of the UI independent.

Conclusion

Angular DI separates the creation and management of services from the components that use them:

• @Injectable marks a class as injectable
• Use constructor injection or the inject function to consume services
• providedIn: 'root' creates a global singleton
• bootstrapApplication providers handles app-wide configuration
• Component-level providers create isolated instances scoped to that component tree

Once you're comfortable with DI, the natural next topics are:

• Angular service design pattern
• HttpClient and API requests
• Mocking services in tests