Variable Identifiers

All JavaScript variable must be identified by unique name, this name call identifiers. Identifiers can be short name like ( x, y, …) or more descriptive name like (age, sum, …).

Rule of constructing name

Name can contain letter, digits, underscores and dollar sign

Names must begin with letter

Names can be begin with $ and _

Names are case sensitive (y and Y are different variables)

Reserved words (JavaScript keyword) can’t be used as name

JavaScript can holed many type of data like string, number

var pi = 3.4;
var person = "Ahmed"

Declaring variable

Creating variable in JavaScript call declaring variable

var CarName;

To assign a value to the variable use ( = )

carName = "Volvo"

Undefined

Variable declared without value

var carName;
//undefined

Let

The Let keyword was introduced in ES6(2015) Variables defined with let must be declared before use Variables defined with let cannot be redeclared That min we can’t do this

let x = "John Doe";
let x = 0;

Block scope

Before ES6(2015), JavaScript had only global scope and function scope in ES6(2015) introduced two important new JavaScript keyword: Let and Const this tow keyword provide block scope in JavaScript so the variables declare inside {} can’t be accessed from outside the block

{ let x = 2;}   
// x can NOT be used here

Variables declared with the var can't have Redeclaring variables using var can impose problem

var x = 10;
// Here x is 10
{
var x = 2;
// Here x is 2
}
// Here x is 2

Redeclaring

With keyword var is allowed anywhere in program With let you can’t redeclaring variable in same scope

var x = 2;    
// Allowed
let x = 3;    
// Not allowed 
{
let x = 2; 
// Allowed
let x = 3
// Not allowed
}
{
let x = 2;
// Allowed
var x = 3
// Not allowed
}

Let hoisting

Variable defined with var can be hoisted to the top and can be initialized at any time meaning you can use variable before it is declared Variable defined with let are also hoisted to the top but not initialized meaning you can’t use variable before it is declared (you must declare variable before used)

Const

The Const keyword was introduced in ES6(2015) Variables defined with Const cannot be redeclared Variables defined with Const can’t be Reassigned Variables defined with Const have block scope

Can’t be Reassigned

const PI = 3.141592653589793;
PI = 3.14;
// This will give an error
PI = PI + 10;   
// This will also give an error

You can’t reassigned PI variable because it is const

Must be assigned

const PI = 3.14159265359;

You can’t do this

const PI;
PI = 3.14159265359;

When to use const variable

Always use const variable when you declare

Array

Object

Function

RegExp

Constant object and array

The const keyword it dose not define constant value. It defines constant reference to the value Because of that you can’t

Reassign a constant value

Reassign a constant array

Reassign a constant object

But you can

Change a constant array

Change a constant object

Constant array

You can do

// You can create a constant array:
const cars = ["Saab", "Volvo", "BMW"];
// You can change an element:
cars[0] = "Toyota";
// You can add an element:
cars.push("Audi");

You can’t do

const cars = ["Saab", "Volvo", "BMW"];
cars = ["Toyota", "Volvo", "Audi"];    // ERROR

JavaScript variables