Introduction to JavaScript Variables

Variables store data values. Each variable has a name, called an identifier. A variable's name is used to identify it's stored value.

Naming Variables

Identifiers must start with a letter and may contain:

They are case sensitive, so "myVariable" would point to a different value than "myvariable". JavaScript keywords can not be used as variable names.

Data Types

Each variable has a data type. This refers to the type of data the variable stores. Here is a list of JavaScript datatypes:


A string variable stores characters. The value is placed in quotation marks. See the example below.

var myStringvariable = "Hi, I am a string!";

var username = "cylon6";


Number variables can store whole nubers or decimals.

var salesTax = .06;

var count = 36;


Boolean variables have 2 possible values: true or false.

permission1 = true;

permission2 = false;

Reviewing propositional logic will help you understand boolean variables.


An array stores a set of data. You will find more information on arrays in JavaScript Datasets

var games = ["Dungeons & Dragons", "Spades", "Twister"];

var grades = [96, 88, 75, 63, 58];


An Object also stores sets of data. This data includes variables called properties and functions called methods. Learn more about objects in JavaScript Datasets and Objects and Constructors.

var user = {username: "cylon6", password:"jsakhd&8hj*hHG5", email: ""};


A null variable does not yet have a value. You can declare a variable as null then later give it a value.

var average = null;


An undefined variable has a value that is unkown or does not exist. If you reference a variable that isn't initalize or an object property that does not exist you will get "undefined". If you reference a variable before it has been assigned a value you will also get "undefined".

Loose Typing

JavaScript is a loosely typed language. When you declare a JavaScript variable, you do not have to specify its data type. A JavaScript variable receives its type from the value you store inside it. So a variable's type can change during the execution of a JavaScript application.