Webmaster in a Nutshell, Second Edition

Variables

Variables are declared, and optionally initialized, with the var statement:

var i;
var j = 1+2+3;
var k,l,m,n;
var x = 3, message = 'hello world';

Variable declarations in top-level JavaScript code may be omitted, but they are required to declare local variables within the body of a function.

JavaScript variables are untyped: they can contain values of any data type.

Global variables in JavaScript are implemented as properties of a special global object. Local variables within functions are implemented as properties of the Argument object for that function.