srsandy/JavaScript Notes.md

## JavaScript Notes.md

      
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              JavaScript Notes.md
            
          
    Javascript Notes

var, let & const


var  provides functional scoping but not block scoping. var allow to log the variable without declaring it though it shows undefined.
let  provides functional scoping as well as block scoping.
const provies functional scoping as well as block scoping but you cannot reassign value to a const type variable.

let and const will not allow you the log the variable without declaring it.


Keyword
Scope
Hoisting
Can Be Reassigned
Can Be Redeclared


var
Function scope
Yes
Yes
Yes


let
Block scope
No
Yes
No


const
Block scope
No
No
No


Hositing

If we try to use a variable before it has been declared and initialized, it will return undefined.
  console.log(x);
  var x = 5;
Output: undefined.
However, if we try to remove var keyword, so now we are no longer declaring the variable, only initializing it. It will return a ReferenceError and stop the execution of the script.
  console.log(x);
  x = 5;
Output: ReferenceError: x is not defined.
This is because of Hoisting. In Javascript, variable and function declarations are moved to the top of their scope.
// The code we wrote
console.log(x);
var x = 10;

// How JavaScript interpreted it
var x;
console.log(x);
x = 10;
Since only the declaration is hoisted, not the initialization, the value returned is undefined.
Javascript saved x to memory before the execution of the script. Since it was called before it was defined, the result is undefined and not 10.
Arrow function & this
Keyword	Scope	Hoisting	Can Be Reassigned	Can Be Redeclared
var	Function scope	Yes	Yes	Yes
let	Block scope	No	Yes	No
const	Block scope	No	No	No