In JavaScript, a variable is a named storage location for data. You can use variables to store all types of data, such as numbers, strings, and objects. There are several ways to declare a JavaScript variable:

1. Using the var keyword:

var x = 5;
var name = "John";

2. Using the let keyword:

let y = 10;
let surname = "Doe";

3. Using the const keyword:

const z = 15;
const address = "123 Main Street";

The var keyword is the traditional way to declare a JavaScript variable. However, the let and const keywords were introduced in ECMAScript 2015 and are recommended for use in most cases because they are more flexible and safer than var.

The let keyword is similar to var, but it allows you to declare variables with the same name in the same scope. This is called “shadowing” and can be useful in certain situations.

The const keyword is used to declare variables that cannot be reassigned. This means that once you have assigned a value to a const variable, you cannot change it.

Here are a few examples of declaring and using JavaScript variables:

// Declaring a variable with the var keyword
var x = 5;

// Declaring a variable with the let keyword
let y = 10;

// Declaring a variable with the const keyword
const z = 15;

// Assigning a new value to a variable declared with var
x = 10;

// Assigning a new value to a variable declared with let
y = 15;

// Attempting to reassign a value to a variable declared with const
// This will throw an error
z = 20;

// Declaring multiple variables with the same name using var
var a = 5;
var a = 10;

// Declaring multiple variables with the same name using let
let b = 5;
let b = 10;

// Declaring multiple variables with the same name using const
// This will throw an error
const c = 5;
const c = 10;

I hope this helps! Let me know if you have any other questions.