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What is the difference between useClass, useValue, useFactory, and useExisting?

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01

Understand the problem

Explain provider recipes.

diproviders
02

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03

Study the solution

Provider recipes tell DI how to create a dependency: useClass instantiates a class, useValue supplies a ready value, useFactory runs a function (with optional deps), and useExisting aliases another token. They enable mocking, configuration, and abstraction.

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04

Read the code

All four recipes
import { InjectionToken } from '@angular/core';

class Logger {}
class BetterLogger extends Logger {}
const API_URL = new InjectionToken<string>('apiUrl');

const providers = [
  { provide: Logger, useClass: BetterLogger },        // instantiate a class
  { provide: API_URL, useValue: 'https://api.example.com' }, // literal value
  { provide: 'CONFIG', useFactory: () => ({ debug: location.hostname === 'localhost' }) },
  { provide: 'AppLogger', useExisting: Logger },       // alias → same instance
];

export { providers };
05

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