01_variable_constant.js

var a = 10;
let b = 20;
const c = 30;
console.log(a, b, c);

a = 15;
b = 25;
// c = 35 it shows an error bcz we do not inilise the constant value again 

// 2. Block scope example 
if (true) {
    let x = 100;
    const y = 200;
    var z = 300;
}
console.log(z); // var scope is global 
// console.log(x);
// console.log(y); // these are not assesible .

// 3. changing of const  const array/object
const person = { name: "Aman", age: 21 };
person.age = 22;
console.log(person);

// 4. hosting ka difference 
console.log(x1);
var x1 = 5;

// console.log(x2)  let  can not be use before thatr defination 
let x2 = 10;

// 5. temporal dead zone 
{
    //console.log(typeof (x3));  let and var shows the TDZ 
    let x3 = " hellow ";
}

console.log(typeof x4); // var shows only the undefined it no show the error 
var x4 = " world ";

// ---------------- NEW EXAMPLES ----------------

// 6. var can be re-declared, let/const cannot (same scope)
var a = 50;   // no error, bas overwrite ho jayega
console.log(a);
// let b = 60; // ❌ Error: Identifier 'b' has already been declared

// 7. function scope vs block scope (var ignores block, let/const respect it)
function testScope() {
    if (true) {
        var funcScoped = "main function ke andar accessible";
        let blockScoped = "sirf is block ke andar accessible";
    }
    console.log(funcScoped);   // ✅ works
    // console.log(blockScoped); // ❌ ReferenceError
}
testScope();

// 8. classic interview question: var vs let in loop with setTimeout
console.log("---loop example---");
for (var i = 0; i < 3; i++) {
    setTimeout(() => console.log("var i:", i), 100);
}
// output: var i: 3, var i: 3, var i: 3
// kyunki var function/global scoped hai, saare callbacks ek hi 'i' ko share karte hain

for (let j = 0; j < 3; j++) {
    setTimeout(() => console.log("let j:", j), 200);
}
// output: let j: 0, let j: 1, let j: 2
// kyunki let block scoped hai, har iteration ka apna naya 'j' banta hai

// 9. const with array — array khud modify ho sakta hai (push/pop allowed)
const arr = [1, 2, 3];
arr.push(4);       // ✅ allowed, reference same hai
console.log(arr);  // [1,2,3,4]
// arr = [5,6];     // ❌ Error, naya reference assign nahi kar sakte

// 10. accessing undeclared variable vs var before declaration
console.log(typeof undeclaredVar); // "undefined" - error nahi aayega
// console.log(undeclaredVar);     // ❌ ReferenceError - typeof ke bina error dega

// 11. global object attachment (var attaches to window/global, let-const nahi)
var globalVar = "main window pe hoon";
let globalLet = "main window pe nahi hoon";
console.log(window.globalVar);  // "main window pe hoon" (browser mein)
console.log(window.globalLet);  // undefined