difference between const var and let javascript code example

Example 1: var vs let vs const

var: 
	- hoisted (always declared at top of scope, global if none)
    - function scope
let:
    - block scope
    - not redeclarable
const: 
    - block scope
    - not reassignable
    - not redeclarable
    
Note: Although it may seem like these hold only semantic meaning, using the
appropriate keywords helps the JS engines' compiler to decide on what to optimize.

Example 2: difference between var let and const in javascript with example

//functional scope 
 var a; // declaration
 a=10; // initialization; 
//global scope
// re-initialization possible
 let a;//only blocked scope & re-initialization possible
 a=10;
let a =20;
if(true){
  let b =30;
}
console.log(b); // b is not defined
const // const also blocked scope,Re-initialization and re-declaration not possible
const a; // throws error {when we declaring the value we should assign the value.
const a =20;
if(true){
  const b =30;
}
console.log(b); // b is not defined
console.log(a); // no output here because code execution break at leve b.

Example 3: Difference between let and var in javascript

let a = 'hello'; // globally scoped
var b = 'world'; // globally scoped
console.log(window.a); // undefined
console.log(window.b); // 'world'
var a = 'hello';
var a = 'world'; // No problem, 'hello' is replaced.
let b = 'hello';
let b = 'world'; // SyntaxError: Identifier 'b' has already been declared