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**Dependency Injection (DI)** is a mechanism Angular uses to **automatically supply the needed dependencies** (for example, services) to components, directives, or other classes.

### In simple terms:

Instead of creating objects manually with `new`, Angular **creates and supplies them itself** when they are needed.

---

### Example:

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

@Component({
  selector: 'app-profile',
  template: `{{ userService.userName }}`
})
export class ProfileComponent {
  constructor(public userService: UserService) {}
}
```

Here `UserService` is automatically created by Angular and passed into the component's constructor.
You do not need to write `new UserService()` - Angular already knows how to do it.

---

### Why DI is needed:

1. **Less coupling.** Components do not know *how* services are created, only *that they exist*.
2. **Easier to test.** Dependencies can be swapped for fakes (mocks).
3. **Reusability.** One service can be used in multiple places.
4. **Lifetime management.** Angular decides when to create and destroy dependencies.

---

**Summary:**
Dependency Injection is a way for Angular to take over creating and supplying the needed objects, so the code stays cleaner, more flexible, and easier to maintain.

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