Let_VS_var_VS_const.js

// VAR

function show(){
    // var e ; // Hoisting
    console.log("E is ", e); // undefined
    if(10>2){
        var e = 10; // e = 10;
    }
    console.log(e);
}

show();
E is  undefined
10
undefined

function show(){
    console.log("E is ", e); // undefined
    var e = 20;
    console.log("E is ", e); // 20
    if(10>2){
        var e = 10; // e = 10;
         console.log("E is ", e); // 10
    }
    var e  = 30;
    console.log(e); //30;
}

show();
E is  undefined
E is  20
E is  10
30


// LET

function show(){
     console.log("E is ", e); // Error
   
    if(10>2){
        let e = 10; // e = 10;               // here let has block scope, so can't able to do hoisting outside function
         console.log("E is ", e); // 10
    }
    
    console.log(e); //Error;
}

show();
VM325:2 Uncaught ReferenceError: e is not defined         
    at show (<anonymous>:2:27)
    at <anonymous>:1:1
show @ VM325:2
(anonymous) @ VM346:1


// ok so now try to acces it in function level.

function show(){
    if(10>2){          // here, Hoisting is done inside the if statement but it wasn't initialized.
        console.log("E is ", e);        
        let e = 10; // e = 10;
         console.log("E is ", e); // 10
    }
    console.log(e); //Error;
}

//  still this will give Error, bcz let jha par declare hota h hmesha uske niche se hi execution start krta hai.
//  anything that is declared in the function scope above the let , will be treated as TDZ (Temporal Dead zone).
//  variable declare to hota hai lekin initialize ni hota.. Undefined se bhi initialize ni hota. isliye memory m ni hota

show();
VM356:5 Uncaught ReferenceError: Cannot access 'e' before initialization  
    at show (<anonymous>:5:30)       
    at <anonymous>:1:1                  
show @ VM356:5
(anonymous) @ VM378:1


function show(){
    let e = 20;
    console.log('E is ', e);
    if(10>2){
        let e = 10; // e = 10;
         console.log("E is ", e); // 10
    }
    console.log(e);   // 20
}

show();
E is  20
E is  10
20


function show(){
    let e = 20;
    console.log('E is ', e);   // 20
    if(10>2){
        let e = 10; // e = 10;
         console.log("E is ", e); // 10
    }
    let e = 100;
    console.log(e);   //Error, bcz of redeclaration, mutliple declaration is only allowed in different scope
}
VM469:10 Uncaught SyntaxError: Identifier 'e' has already been declared    // re-declaration wasn't allowed.


let rr = 100;

window.rr;    // we can't access let on window object because let jha par declare hota h hmesha uske niche se hi execution start krta hai.
undefined        // and window is on the top.


// CONST 

const MAX  = 100;

MAX++;     
VM586:1 Uncaught TypeError: Assignment to constant variable.           // we can't increament constant values.
    at <anonymous>:1:4
(anonymous) @ VM586:1
MAX = 200;       
VM607:1 Uncaught TypeError: Assignment to constant variable.      // we can't re-declare constant values.
    at <anonymous>:1:5
(anonymous) @ VM607:1


const obj = {id:1001, name:'Ram'};       // this is shallow constant, bcz it has reference 
obj.id++;      // This is possible because in obj reference is stored not the values. and we are only changing values instead of addresses.
1001
obj;
{id: 1002, name: 'Ram'}             // no concept of deep constant

obj = {};                // This is not possible because we are trying to change the address, that is constant.
VM734:1 Uncaught TypeError: Assignment to constant variable.
    at <anonymous>:1:5
(anonymous) @ VM734:1


            // JS doesn't support deep constant.


const arr = [10,20,30];
arr[0] = 1111;   // This is possible because in obj reference is stored not the values. and we are only changing values instead of addresses.
1111
arr;
(3) [1111, 20, 30]

arr = [100,200,300];  // This is not possible because we are trying to change the constant address.
VM911:1 Uncaught TypeError: Assignment to constant variable.
    at <anonymous>:1:5
(anonymous) @ VM911:1