JavaScript Variables

Variables store a value you can refer to later in the script. Variable names
can be nearly any valid identifi er. A JavaScript identifi er is a word that
contains only letters, numbers, $, and _, and that doesn’t start with a
number.
Variables are a great demonstration of how statements and expressions
can be combined. You use a variable declaration statement to create a
variable.

var myVariable;

If you already have a variable, you can use a variable assignment expression
to assign it a value.

myVariable = 42;
42
You can combine a variable declaration with an assignment expression.
var myVariable = 42;
You can also string together several variable declarations with commas.
Don’t forget the commas, though! Leaving off a comma can have the
unintended side effect of declaring a global variable when you don’t
mean to do that.var variable1 = 4,
variable2 = 8,
variable3 = 15;

JavaScript has a relatively small number of built-in data types, including
these common types:
var myNumber = 42;
var myString = "A string of text";
var myBoolean = true;
var myArray = [myNumber, myString, myBoolean];