A variable is a named storage for data. We can use variables to store goodies, visitors, and other data.

Table of Contents

Naming Variables

Variable names must start with a letter, an underscore (_) or a dollar sign ($).
Variable names cannot contain spaces.
Variables cannot be the same as reserved keywords such as if or const.
By convention, JavaScript variable names are written in camelCase.
Variables should be given descriptive names that indicate their content and usage.
As JavaScript variables do not have set types, it can be useful to include an indication of the type in the name.

Ex:

// Valid variable
var $_$ = "money eyes";
var _Hello_ = "what a nice greeting";
var I_AM_HUNGRY = true;

//Invaild variable
var 2fast2catch = "bold claim";
var function = false;
var class = "easy";

// Valid variable

var $_$ = "money eyes";

var _Hello_ = "what a nice greeting";

var I_AM_HUNGRY = true;

//Invaild variable

var 2fast2catch = "bold claim";

var function = false;

var class = "easy";

Declaring Variable

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 4 ways:

Automatically
Using var
Using let
Using const

Automatically

Ex:

x = "Wrapline.pro";
y = 6;

console.log(x);
console.log(y);

x = "Wrapline.pro";

y = 6;

console.log(x);

console.log(y);

Output:

Wrapline.pro 
6

1 2	Wrapline.pro 6

Using var

var statement is used to declare variables in JavaScript that are function-scoped. The var statement is also used to declare global-scope variables.

Ex:

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

var a = "Hello Wrapline"

var b = 10;

var c = 12;

var d = b + c;

console.log(a);

console.log(b);

console.log(c);

console.log(d);

Output:

Hello Wrapline
10
12
22

Hello Wrapline

Using let

let is a keyword used to declare variables that are block-scoped. Variables defined with the let keyword cannot be redeclared and must be declared before use.

Ex:

{
    let num = 10;
    // calling the function inside block
    console.log(num)
}
// calling the function outside
// block throws a Error
console.log(num)

{

let num = 10;

// calling the function inside block

console.log(num)

}

// calling the function outside

// block throws a Error

console.log(num)

Output:

10
Uncaught ReferenceError: num is not defined

1 2	10 Uncaught ReferenceError: num is not defined

The num variable is block-scoped and it cannot be accessed outside the block. If we try to access the variable outside the block it throws a reference error.

Using const

Variables specified using const cannot be redeclared and have block-scope.

Ex:

const x = 12;
x = 13;
x += 1;

const x = 12;

x = 13;

x += 1;

Output:

Uncaught TypeError: Assignment to constant variable.

1	Uncaught TypeError: Assignment to constant variable.

Comparison of properties of let, var, and const

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