What is Scope in JavaScript?

Before ES6 (2015), JavaScript had only Global Scope and Function Scope.

ES6 introduced two important new JavaScript keywords: let and const.

These two keywords provide Block Scope in JavaScript.

Variables declared inside a { } block cannot be accessed from outside the block:

Block is defined by a ' { } '. It is also known as a compound statement. It combines multiple java script statements into one. We use a block to give multiple statements to places where we expect a single statement.

Example:

// single statement 
if(true) true 

// multiple statement with block 
if(true) {
	// statement 1 
	// statement 2 
	// statement 3
}


{
var a = 10;
let b  = 100;
const c  = 1000;
}

Now if we observer this program as soon as the program runs the b and c gets a undefined value in block scope and a gets undefined in a global scope unlike let and const.
Once javascript encounters the last statement and ending of the scope the let and const are no longer accessible and hence they are known as block-scoped variables.

Shadowing in JavaScript

var a = 100;
{
var a = 10;
let b  = 100;
const c  = 1000;
console.log(a);

}
console.log(a);

// output 
10
100

This is shadowing the a is 100 as the code hits the last line of the scope it destroys the a value as 10 in the global scope and it went back to the 100 and this is how shadowing happens.
The a inside the block shadows the global a value that is 100 and instead will print 10 if we do console.log(a) inside a block
Here it will give a syntax error as it is a illegal shadowing we cannot shadow a let with a var but we can do with a let itsef