var x=9; //Declared 1st time
var x=10; //Declared 2nd time: Allowed here

let v=9; //Declared 1st time
let v=2; //Declared 2nd time: Not allowed as both in same scope (comment down this line and execute in browser console).

const PI; //Declared 1st time without value assign: Not allowed as const needs value assigned at the time of declaration (comment down this line and execute in browser console).
const PI=3.14; //Declared 2nd time: Allwed as it is proper declaration.

PI=3.14159; //Reassign const value: Not allowed (comment down this line and execute in browser console).


function BlockScope(){

console.log('----------block space start-----------');

var x=2; //Declared 3rd time: Allowed here
console.log(x); //x will be 2 over here

let v=3; // Declared 3rd time: Allowed as scope is changed here.
console.log(v); //v will be 3 over here 

const PI=3.1415; //Declared 3rd time: allowed as it follow Scope Principle.

console.log(PI); //PI will be 3.1415

console.log('----------block space end-----------');
}

BlockScope();

console.log(x); //x will be 2 which do not follow block scope (means var do not follow scope principle).

console.log(v); // v will be 9 here (means let follow scope principle).

console.log(PI); // PI will be 3.14 and not 3.1415 as it follow scope principle.

v=12; //allowed and value will be chagned to 12 when you run(3rd point)