JavaScript is a dynamically-typed language, which means that you don't have to specify the data type of a variable when you declare it. The data type of a variable is determined by the value that is assigned to it. For example:

var x = 10; // x is a number
var y = "hello"; // y is a string
var z = [1, 2, 3]; // z is an array

Variable Naming Rules

There are a few rules that you need to follow when naming variables in JavaScript:

• Variable names can only contain letters, digits, underscores, and dollar signs.
• Variable names cannot start with a digit.
• Variable names are case-sensitive.

It is also a good practice to use descriptive and meaningful names for your variables, as this makes your code easier to read and understand.

Using Variables

Once you have declared a variable, you can use it to store and retrieve data in your program. For example:

var x = 10;
console.log(x); // prints 10
 
x = "hello";
console.log(x); // prints "hello"

You can also perform various operations on variables, such as mathematical calculations, string concatenation, and more. For example:

var x = 10;
var y = 20;
var z = x + y; // z is 30
 
var str1 = "hello";
var str2 = "world";
var str3 = str1 + " " + str2; // str3 is "hello world"