Javascript is loosely typed, which means you don't have to explicitly specify a data type for variables etc. If you want to have a number, just set it:
var myNumber = 12;
Same with strings and others:
var myString = "twelve";
var myArray = [ 1, "two", 3, "four" ];
...
Read more here:
http://www.w3schools.com/js/js_datatypes.asp
https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Values,variables,and_literals