Lesson 04 — Interfaces shape automation contracts

Title: Structuring payloads, credentials, and Page contracts

Description: Interfaces describe allowed keys on objects — fixtures, REST bodies, POM boundaries — without caring about runtime class mechanics yet.

Why it matters for QA: Teams serialize JSON constantly; typed interfaces mirror backend schemas so mocks stay trustworthy.

1. Basic interface

interface User {
  email: string;
  password: string;
}

const user: User = {
  email: "test@mail.com",
  password: "123456",
};

2. Missing fields fail compile-time

// Uncomment to watch TypeScript complain — password missing.
// const brokenUser: User = {
//   email: "oops@mail.com",
// };

3. QA-centric interfaces

Login payloads:

interface LoginCredentials {
  email: string;
  password: string;
  rememberMe?: boolean;
}

API persona responses:

interface UserResponse {
  id: number;
  name: string;
  email: string;
}

4. Interfaces plus classes (implements)

Useful when multiple pages share lifecycle hooks:

interface OpensPage {
  open(): Promise<void>;
}

class LoginPage implements OpensPage {
  async open(): Promise<void> {
    console.log("navigate → /login");
  }
}

5. Practice

1. Extend UserResponse interface Add a new optional field:

• avatarUrl?: string
• it is a link to the user’s avatar image
• it is optional (may be missing)

2. Create BugTicket interface Create an interface with:

• title → bug name
• severity → bug level: "low", "medium", or "high"
• steps → list of steps to reproduce the bug (array of strings)

6. Stretch goal

Explore extends:

interface Admin extends UserResponse {
  permissions: string[];
}

This is about when to avoid repeating code and when it’s actually better to keep things separate.