Lesson 02 — TypeScript basics for QA data

Title: Variables, arrays, and object literals with types

Description: Attach explicit types to the values you move through tests: credentials, IDs, flags. TypeScript catches mistakes before runtime.

Why it matters for QA: Fixtures and API mocks mirror production payloads. Typed shapes stop "undefinded" typos and wrong property names from slipping into suites.

1. Primitive variables

let name: string = "QA Engineer";
let age: number = 25;
let isActive: boolean = true;

Prefer const when you do not reassign:

const environment = "staging";

2. Arrays

let users: string[] = ["Anna", "John"];
let ids: Array<number> = [1, 2, 3];

Mixed arrays without unions need explicit annotation:

type Cell = string | number;
const row: Cell[] = ["login", 200];

3. Objects with inline shapes

let user: { name: string; age: number } = {
  name: "Anna",
  age: 30,
};

If you forget age, the checker fails immediately — exactly what you want before committing fixtures.

4. Practice: credential sketch

Implement typed variables for:

let email: string;
let password: string;
let role: string;

email = "learner@example.com";
password = "CorrectHorseBatteryStaple!";
role = "tester";

console.log({ email, role });

Extend with readonly intent mentally: credentials often flow one direction into helpers — immutability habits start here.

5. Think like automation data

When modeling UI state before interfaces:

const checkoutTotals = {
  subtotal: 49.99,
  taxRate: 0.07,
};

let cartEmpty: boolean = false;

6. Stretch goal

Define two objects representing bug reproduction inputs (steps, expected, actual) with inline types. Log them using JSON.stringify.