const PI = 3.14;
PI = 3.14159; // ❌ TypeError: Assignment to constant variable

const user = { name: "Alice" };
user.name = "Bob"; // ✅ This is ALLOWED!
user = {};         // ❌ TypeError — can't reassign

flowchart TD
    A["const x = 5"] --> B["x = 10 ❌ Error"]
    A --> C["x = 'hello' ❌ Error"]

    D["const obj = { name: 'Alice' }"] --> E["obj.name = 'Bob' ✅ Allowed"]
    D --> F["obj = {} ❌ Error"]

    style A fill:#10b981,color:#fff
    style D fill:#f59e0b,color:#fff
    style B fill:#ef4444,color:#fff
    style C fill:#ef4444,color:#fff
    style F fill:#ef4444,color:#fff
    style E fill:#10b981,color:#fff

// 1. Must initialize
// const x; // ❌ SyntaxError: Missing initializer

// 2. When to use const
const API_URL = "https://api.example.com";
const MAX_RETRIES = 3;
const USER_ROLES = { admin: "Admin", user: "User" };

// 3. const with objects — can mutate properties
const person = { name: "Alice", age: 30 };
person.age = 31;      // ✅ Allowed
person.city = "NYC";  // ✅ Allowed
// person = {};       // ❌ Error

// 4. const with arrays — can mutate contents
const numbers = [1, 2, 3];
numbers.push(4);      // ✅ Allowed
numbers[0] = 10;      // ✅ Allowed
// numbers = [];     // ❌ Error

// 5. const in loops — works with for...of
const items = [1, 2, 3];
for (const item of items) {
  console.log(item); // ✅ Allowed — new binding per iteration
}

const config = Object.freeze({
  apiUrl: "https://api.example.com",
  timeout: 5000,
});
config.timeout = 10000; // ❌ Fails silently (or TypeError in strict mode)