JavaScript Variables

Variables are the building blocks of any programming language. In JavaScript, a variable can be used to store reusable values. These values can be anything from Number, Boolean, String, and even a value returned from a function. The values of the variables are allocated using the assignment operator(“=”). 

There are some basic rules to declare a variable in JavaScript:

• These are case-sensitive
• Can only begin with a letter, underscore(“_”) or “$” symbol
• It can contain letters, numbers, underscore, or “$” symbol
• A variable name cannot be a reserved keyword.

JavaScript is a dynamically typed language so the type of variables is decided at runtime. Therefore there is no need to explicitly define the type of a variable. We can declare variables in JavaScript in three ways:

• JavaScript var keyword
• JavaScript let keyword
• JavaScript const keyword

Syntax:

var geek = "Hello Geek"        // declaration using var
let $ = "Welcome"                 // declaration using let
const _example = "Gfg"        // declaration using const   

All three keywords do the basic task of declaring a variable but with some differences Initially, all the variables in JavaScript were written using the var keyword but in ES6 the keywords let and const were introduced.

Example 1: In this example, we will declare variables using var

Javascript

var a = "Hello Geeks"
var b = 10;
var c = 12;
var d = b + c;
console.log(a);
console.log(b);
console.log(c);
console.log(d);

Output:

Hello Geeks
10
12
22

Example 2: In this example, we will declare variables using let 

Javascript

let a = "Hello learners"
let b = "joining";
let c = " 12";
let d = b + c;

console.log(a);
console.log(b);
console.log(c);
console.log(d);

Output

Hello learners
joining
 12
joining 12

To learn more about JavaScript let check this article JavaScript Let

Example 3: In this example, we will declare the variable using the const keyword

Javascript

const a = "Hello learners"
console.log(a);

const b = 400;
console.log(b);

const c = "12";
console.log(c);

c = "new"
console.log(c)

Output:

Explanation: const keyword is used when we assign a value permanently to a variable. So when we try to change the value of a variable declared with the const keyword it will throw an error. The variables declared with var and let are mutable that is their value can be changed but variables declared using const are immutable. 

To learn more about JavaScript const check this article JavaScript Const

Note: The newly introduced keywords let and const are block scoped whereas var is function scoped. 

Let us see an example to understand the difference:

Example:

Javascript

{
    let a = 2;
    var b = 4
    const c = "Hello"
    console.log(a)
    console.log(b)
    console.log(c)
}

console.log(b)
console.log(c)
console.log(a

Output:

Explanation: Since variables “a” and “c” are block scoped so we were not able to access them outside their block.

To learn more about the scope of variables refer to this article Understanding variable scopes in JavaScript

Comparison of properties of let, var, and const keywords in JavaScript:

Property	var	let	const
Scope	Function scoped	Block scoped	Block scoped
Updation	Mutable	Mutable	Immutable
Redeclaration	Can be redeclared	Cannot be redeclared	Cannot be redeclared
Hoisting	Hoisted at top	Hoisted at top	Hoisted at top
Origins	Pre ES2015	ES2015(ES6)	ES2015(ES6)
Support	Supported in the old version of Browser	Not supported in the old version of the Browser	Not supported in the old version of the Browser