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How does Dependency Injection work in Angular?

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01

Understand the problem

Explain Angular's DI system.

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02

Attempt it yourself

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03

Study the solution

Angular has a hierarchical injector tree. You register providers (at root, module, or component level) and request dependencies via constructor parameters. The injector resolves and caches instances, enabling singletons (providedIn: 'root') and scoped instances. This decouples classes from how their dependenc

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04

Read the code

Inject a service via the constructor
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import { Component, Injectable } from '@angular/core';

@Injectable({ providedIn: 'root' })   // app-wide singleton, registered at root
export class Logger {
  log(msg: string) { return 'LOG: ' + msg; }
}

@Component({
  selector: 'app-root',
  template: '<p>{{ message }}</p>',
})
export class AppComponent {
  message: string;
  // Angular resolves the Logger token and injects the instance.
  constructor(private logger: Logger) {
    this.message = this.logger.log('DI works');
  }
}
05

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