hardLow-Level Design

Explain 'dependency injection' and how it can be used in Node.js.

Q: Explain 'dependency injection' and how it can be used in Node.js.

**TL;DR.** **Dependency injection (DI)** means a module receives its **collaborators from outside** (constructor/parameters) instead of creating them itself. This **decouples** code, makes it **testable** (inject mocks/fakes), and centralizes wiring. In Node you often do DI **manually** (pass deps in) or with a container (Awilix, NestJS, tsyringe). db logger mailer <path c

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This question assesses knowledge of a fundamental software design pattern for modularity and testability.

design patternsdependency injectiontestingmodularity

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Manual constructor injection

class UserService {
  constructor({ repo, mailer, logger }) {  // deps injected
    this.repo = repo; this.mailer = mailer; this.logger = logger;
  }
  async register(data) {
    const u = await this.repo.save(data);
    await this.mailer.send(u.email, 'welcome');
    return u;
  }
}
// composition root wires real deps; tests pass fakes:
const svc = new UserService({ repo: new PgUserRepo(), mailer, logger });

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