类与 OOP

TypeScript 的类在 JavaScript class 语法上增加了 访问修饰符、抽象类、implements、readonly 等类型能力。本课系统讲解 OOP 建模，为 nestjs-essentials 的 Controller/Service 分层与 nextjs-essentials 中少量 class-based 工具（如 Error 子类）打基础，并衔接 typescript-essentials 的接口知识。

学习目标

• 使用 public/private/protected/readonly 控制成员可见性
• 用 abstract class 定义可扩展基类
• 理解 implements 与 extends 的分工

基本类与修饰符

class TrackService {
  readonly apiBase: string;

  constructor(apiBase: string) {
    this.apiBase = apiBase;
  }

  public async list(): Promise<Track[]> {
    return this.fetchAll();
  }

  private async fetchAll(): Promise<Track[]> {
    const res = await fetch(`${this.apiBase}/tracks`);
    return res.json();
  }

  protected log(message: string): void {
    console.log(`[TrackService] ${message}`);
  }
}

interface Track {
  slug: string;
  title: string;
}

继承与 super

class CachedTrackService extends TrackService {
  private cache = new Map<string, Track>();

  override async list(): Promise<Track[]> {
    if (this.cache.size > 0) {
      return [...this.cache.values()];
    }
    const tracks = await super.list();
    tracks.forEach((t) => this.cache.set(t.slug, t));
    return tracks;
  }
}

抽象类

abstract class Repository<T extends { id: number }> {
  abstract findById(id: number): Promise<T | null>;
  abstract save(entity: T): Promise<T>;

  async exists(id: number): Promise<boolean> {
    const found = await this.findById(id);
    return found !== null;
  }
}

class LessonRepository extends Repository<Lesson> {
  async findById(id: number): Promise<Lesson | null> {
    return null;
  }
  async save(entity: Lesson): Promise<Lesson> {
    return entity;
  }
}

interface Lesson {
  id: number;
  title: string;
}

implements 接口

interface Logger {
  info(message: string): void;
  error(message: string, err?: unknown): void;
}

class ConsoleLogger implements Logger {
  info(message: string): void {
    console.log(message);
  }
  error(message: string, err?: unknown): void {
    console.error(message, err);
  }
}