Day 3 of 100: Data Types and the null vs undefined Trap

Day 3 of 100 — 100 Days of Playwright with TypeScript Challenge. Phase: JavaScript Fundamentals. JavaScript (Days 1-33) → TypeScript (Days 34-63) → Playwright (Days 64-96) → AI Framework Capstone (Days 97-100).

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JavaScript has 7 primitive types. QA code touches 5 daily.

The Types You Use

const name = 'Playwright';        // string
const count = 42;                  // number
const passed = true;               // boolean
const missing = null;              // intentional empty
let notSet;                        // undefined

console.log(typeof name);    // 'string'
console.log(typeof count);   // 'number'
console.log(typeof missing); // 'object' (historic bug!)

null vs undefined — The QA Trap

// API returns null for empty field, undefined for missing field
const user = { name: 'Jane', middleName: null };
console.log(user.middleName);  // null — field exists, empty
console.log(user.nickname);    // undefined — field absent

// Test assertion difference:
// expect(body.middleName).toBeNull()      — explicit empty
// expect(body.nickname).toBeUndefined()   — not present

Type Coercion Gotchas

console.log('5' + 3);   // '53' — string concat!
console.log('5' - 3);   // 2 — numeric subtract
console.log(0 == '');   // true — loose equality lies
console.log(0 === '');  // false — always use ===

Tomorrow (Day 4): Operators and conditionals — the logic of every assertion.