In JavaScript, variables are used to store and manipulate data. They act as containers that hold values of different types, such as numbers, strings, booleans, arrays, objects, and more. To use a variable, you must declare it and optionally assign a value to it. Here's how variables are declared and used in JavaScript:

Variable Declaration:

- Variables in JavaScript can be declared using the `var`, `let`, or `const` keywords.
- `var` was traditionally used in older versions of JavaScript but is still supported.
- `let` and `const` were introduced in ECMAScript 2015 (ES6) and provide block-scoped variables.
- Example:

javascript
var age; // Declaration without assignment
let name = "John"; // Declaration and assignment
const PI = 3.14; // Declaration and assignment of a constant

Variable Assignment:

- Variables can be assigned values using the assignment operator (`=`).
- Example:

javascript
age = 25; // Assigning a value to the 'age' variable
name = "Jane"; // Assigning a new value to the 'name' variable

Variable Initialization:

- Variables can be declared and assigned in a single statement, which is known as variable initialization.
- Example:

javascript
let x = 10; // Declaration and assignment in one statement

Variable Naming Rules:

- Variable names in JavaScript follow certain rules:
- They must start with a letter (a-z, A-Z) or an underscore (_) or a dollar sign ($).
- They can include letters, numbers, underscores, and dollar signs.
- They are case-sensitive.
- They should be meaningful and descriptive.
- Example:

javascript
let firstName = "John";
let myVariable = 10;

Variables are mutable, meaning their values can be changed during the execution of the program. However, the behavior varies depending on whether the variable is declared with `var`, `let`, or `const`. `var` and `let` variables can be reassigned new values, while `const` variables are read-only and cannot be reassigned once a value is assigned.

Understanding variables and their usage is fundamental in JavaScript programming, as they allow you to store and manipulate data throughout your code.

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