To understand shadowing in JavaScript, we need to be clear with the scope first.
Refer the below link :
<a target="_blank" href="https://hashnode.com/post/cltpqjm3z000108lad1yw445r">SCOPE</a>
Shadowing is nothing but a variable replacing the value of another variable inside a block if declared with same name.
<pre><code class="lang-javascript">var a = 10 ;
 {
 var a = 100;
 console.log(a); // displays 10
 }
 console.log(a)// displays 10
</code></pre>
The variable inside block shadows the variable in global space and the value gets replaced. Same applies with let and const as well.
Now, while shadowing a variable, it should not cross the boundary of the scope, i.e. we can shadow var variable by let variable but cannot do the <a href="http://opposite.It" class="autolinkedURL autolinkedURL-url" target="_blank">opposite.It</a> is called as illegal shadowing.
For example :
<pre><code class="lang-javascript">let a = 10;
 {
 var a = 100;
 console.log(a);// throws syntax error since a has already been declared with let in its local scope , since it won't be there in global scope it throws error 
 }
</code></pre>

To understand shadowing in JavaScript, we need to be clear with the scope first.

Refer the below link :

[SCOPE](https://hashnode.com/post/cltpqjm3z000108lad1yw445r)

Shadowing is nothing but a variable replacing the value of another variable inside a block if declared with same name.

```javascript
var a = 10 ;
            {
               var a = 100;
                console.log(a); // displays 10
             }
          console.log(a)// displays 10 
```

The variable inside block shadows the variable in global space and the value gets replaced. Same applies with let and const as well.

Now, while shadowing a variable, it should not cross the boundary of the scope, i.e. we can shadow *var* variable by *let* variable but cannot do the opposite.It is called as illegal shadowing.

For example :

```javascript
let a = 10;
                  {
                     var a = 100;
                    console.log(a);//  throws syntax error since a has already been declared with let in its local scope , since it won't be there in global scope it throws error 
                  }
```

Block scope and variable shadowing in JS