The Heart of Reusability: Dependency Injection

Discover how Angular's core DI system allows you to build modular, efficient, and easily testable applications.

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Welcome! Angular apps are built of blocks. But how do these blocks share data? Let's look at Services.

// Imagine Components A and B need the same data...

The @Injectable Decorator

In Angular, a **Service** is just a class. To tell Angular that this class can be used in the Dependency Injection system, we must mark it with the `@Injectable()` decorator.

This metadata tells Angular that this class might have dependencies of its own, and that it can be injected into other classes (like Components).

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

@Injectable()
export class MyService { ... }

Injector Diagnostics

What is the primary purpose of the @Injectable decorator?

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Achievements

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Service Architect

Successfully define a service class with the correct decorator.

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Injection Master

Correctly wire up a component to receive a dependency.

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Singleton Pro

Demonstrate understanding of the root provider scope.

Mission: Architect a Singleton Service

Write an Angular service class named `LoggerService`. Ensure it is decorated properly to be a root singleton.

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Challenge: The Dependency Lifecycle

Arrange the steps required to get a working service in an Angular Component.

2. Register Provider (@Injectable)
3. Inject in Constructor
1. Define Service Class

Challenge: Complete the Singleton

Fill in the missing Angular keywords to create a valid root service.

@({ providedIn: '' })
export class DataService {}
// Component
constructor( service: DataService) {}

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Mastering Dependency Injection: The Heart of Angular

In modern web development, managing how different parts of your application communicate is critical. Angular solves this with a powerful **Dependency Injection (DI)** system. Unlike other frameworks where you might import modules directly or pass props down ten levels, Angular allows you to request dependencies in the constructor of your components, and it handles the rest.

The Power of the Singleton

When you use@Injectable({ providedIn: 'root' }), you are creating a **Singleton**. This means Angular creates exactly *one* instance of your service for the entire application.

This is incredibly powerful for **State Management**. If Component A writes data to a service, Component B (which injects the same service) sees that data instantly. No complex event bubbling required.

✔️ Best Practice

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

Tree-shakable and creates a single shared instance.

❌ Legacy / Bad Practice

// In app.module.ts
providers: [UserService]

Harder to tree-shake and can lead to accidental multiple instances in lazy modules.

Testing Made Easy

DI is not just about writing less code; it's about writing **testable** code. Because components ask for their dependencies, you can easily swap them out during testing.

Imagine a `BackendService` that makes real HTTP calls. In your unit tests, you don't want to hit the real server. With DI, you can simply inject a `MockBackendService` that returns fake data, ensuring your tests are fast and reliable.

Key Takeaway: Services are the place for *logic*. Components are the place for *views*. Dependency Injection is the bridge that connects them efficiently and loosely.

Angular DI Glossary

@Injectable()
A decorator that marks a class as a participant in the dependency injection system. It allows Angular to define how the class should be instantiated.
Dependency Injection (DI)
A design pattern where a class requests dependencies from external sources rather than creating them itself.
providedIn: 'root'
Configures the service to be provided in the root injector. This creates a singleton service available throughout the entire app.
Singleton
A design pattern where a class has only one instance. In Angular, root services are singletons.
Constructor Injection
The syntax used to ask for dependencies: `constructor(private myService: MyService)`.
Provider
An instruction to the DI system on how to obtain a value for a dependency (e.g., `useClass`, `useValue`).

About the Author

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TodoTutorial Team

Passionate developers and educators making programming accessible to everyone.

This article was written and reviewed by our team of Angular experts, who have years of experience building enterprise-scale applications.

Verification and Updates

Last reviewed: October 2025.

We strive to keep our content accurate and up-to-date. This tutorial is based on Angular 18+ specifications.

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