VARIABLES IN JAVASCRIPT

What is a Variable?

A variable is like a container that holds data or values in a program. 

It allows you to store and manipulate information within your code.

Variable Declaration 

To create a variable in JavaScript, you use the var, let, or const keyword, followed by the variable name.

Example:
var name; 

let age; 

const pi = 3.14;

note:

    you cannot create blank const variables.

Variable Assignment

After declaring a variable, you can assign a value to it using the assignment operator =. 

You can also initialize a variable while declaring it.

Example:
var a=10; 

let myString = "Hello, world!";

 

Data Types

JavaScript variables can hold various types of data, including numbers, strings, booleans, arrays, objects, functions, and more.

Example:
var num = 42; 

var name = "John"; 

var status = true; 

var myArray = [1, 2, 3]; 

Variable Naming Rules

Variable names can consist of letters, digits, underscores, and dollar signs.

The first character must be a letter.

Variable names are case-sensitive.

They cannot be named as predefined keywords like var, let, const, if, else, etc.

Example:
var firstName; 

let user_age; 

const TAX_RATE = 0.08;

difference  between var, let, and const variables

1. var:

   a. Older way to declare variables.

   b. Variables declared with var have function scope.

   c. Can be re-declared and updated.

   - Example:

     var age = 30;

     document.write(age); // Output: 30

     

2. let:

   - Introduced in ES6 (ECMAScript 2015) to address issues with var.

   - Variables declared with let have block scope.

   - Can be updated but not re-declared in the same scope.

   - Preferred over var for variable declaration.

   - Example:

     let age = 30;

     document.write(age); // Output: 30

     

3. const:

   - Also introduced in ES6.

   - Used to declare constants, whose value cannot be changed once assigned.

   - Variables declared with const have block scope.

   - Must be assigned a value when declared and cannot be re-assigned.

   - Example:

     const PI = 3.14;

     PI = 3; // Error: Assignment to constant variable.

     const age = 30;

     age = 25; // Error: Assignment to constant variable.

     

In simple terms, 

var is the old way of declaring variables, 

let is the modern way for variables that can change, and 

const is for variables that should never change.