02.Variables.md

Variables In Javascript

What is a Variable?

A variable is a container for a value, like a number we might use in a sum, or a string that we might use as part of a sentence.
Let's look at a simple example:

var value = "Hello World"; 
var x = 10;  

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• The value stored in a variable can be changed during program execution.
• A variable is only a name given to a memory location, all the operations done on the variable effects that memory location.
• In JavaScript, all the variables must be declared before they can be used.

Before ES2015 (ES6), JavaScript variables were solely declared using the var keyword followed by the name of the variable and semi-colon.

After ES2015 (ES6), we now have two new variable containers: let and const.

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Declaring a Variable

There are 3 ways to Declare a JavaScript Variable in ES6 :

• Using var
• Using let
• Using const

Using var

var name; // undefined
var name = "Souradeep";
name = "< Whatever Your Name is >";

var num = 10;

var sum = 5 + 10 + 1;

Using let

let x; // undefined
let name = 'Souradeep';

let age = 22;
age = 41; //Works same as var.

Using const

const name = 'Souradeep';
name = 'Mikhail'; // will give Assignment to constant variable error.

Difference Between Let and Var

let	var
let is block-scoped.	var is function scoped.
let does not allow to redeclare variables.	var allows to redeclare variables.
Hoisting does not occur in let.	Hoisting occurs in var.

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There are two types of variables in JavaScript : local variable and global variable.

JavaScript local variable

A JavaScript local variable is declared inside block or function. It is accessible within the function or block only. For example:

function abc() {  
    var x = 10; // local variable  
}

If (10 < 13) {  
 var y = 20; // Also a local variable  
}  

JavaScript global variable

A JavaScript global variable is accessible from any function. A variable i.e. declared outside the function or declared with window object is known as global variable. For example:

var data=200; // global variable  

function first(){
    var localData = 400;

    console.log(data);
    console.log(localData);
}  
function second(){  
    console.log(data);  
    console.log(localData); // Will give error saying 'localData not defined'
}  

first(); //calling JavaScript function  

second();  

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