Variables & Types de Données

        
          // 1. Variable Declarations - Modern approach
let userName = "Alice";        // Block-scoped, can be reassigned
const PI = 3.14159;           // Block-scoped, cannot be reassigned
const CONFIG = {              // Object properties can be modified
  apiUrl: "https://api.example.com",
  timeout: 5000
};
CONFIG.timeout = 10000;        // ✅ This works

// 2. Data Types
let string = "Hello World";    // String
let number = 42;               // Number (all numbers are floats)
let boolean = true;            // Boolean
let array = [1, 2, 3, 4];      // Array
let object = {                 // Object
  name: "John",
  age: 30,
  isActive: true
};

// 3. Special Values
let nothing = null;            // Intentional absence of value
let notDefined;                // undefined (default value)
let bigNumber = 1234567890123456789012345678901234567890n; // BigInt
let uniqueId = Symbol('user_id'); // Unique identifier

// 4. Type Checking
console.log(typeof userName);   // "string"
console.log(typeof number);     // "number"
console.log(typeof boolean);    // "boolean"
console.log(typeof array);      // "object" (arrays are objects)
console.log(typeof nothing);    // "object" (historical bug)
console.log(typeof notDefined); // "undefined"
console.log(typeof bigNumber);  // "bigint"
console.log(typeof uniqueId);   // "symbol"

// 5. Type Conversion
let num = 42;
let str = String(num);         // "42"
let backToNum = Number(str);   // 42
let boolStr = Boolean(1);      // true
let emptyStr = Boolean("");    // false

// 6. Common Pitfalls
// ❌ Don't do this:
if (userName = "Bob") {        // Assignment instead of comparison
  console.log("This always runs!");
}

// ✅ Do this instead:
if (userName === "Bob") {
  console.log("Name matches!");
}