Day 44 of 100: Access Modifiers and Abstract Classes

Day 44 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: TypeScript. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

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public, private, protected, abstract — visibility control for framework base classes.

The Three Modifiers

class ApiClient {
  public baseUrl: string;            // default — everywhere
  protected token: string;           // this class + subclasses
  private cache = new Map();         // this class only

  constructor(baseUrl: string, token: string) {
    this.baseUrl = baseUrl;
    this.token = token;
  }
}

class AdminApiClient extends ApiClient {
  useToken() { return this.token; }       // OK — protected
  // peek() { return this.cache; }         // error — private
}

Abstract Classes

abstract class BaseApiTest {
  abstract endpoint: string;                       // subclass MUST define
  abstract validate(body: unknown): boolean;       // subclass MUST implement

  async run(): Promise {                  // shared logic
    const res = await fetch(this.endpoint);
    return this.validate(await res.json());
  }
}

class UsersApiTest extends BaseApiTest {
  endpoint = '/api/users';
  validate(body: unknown) { return Array.isArray(body); }
}
// new BaseApiTest()  — error: cannot instantiate abstract

Caution

• Abstract base + one level of subclass: fine for API test families
• Deep abstract chains: the Selenium-era trap — prefer fixtures (Day 76)
• Most Playwright POMs need zero inheritance

Tomorrow (Day 45): Utility types — Partial, Pick, Omit for test variants.