I found three ways to cast a variable to String in JavaScript.
I searched for those three options in the jQuery source code, and they are all in use
There are differences, but they are probably not relevant to your question. For example, the toString prototype does not exist on undefined variables, but you can cast undefined to a string using the other two methods:
var foo;
var myString1 = String(foo); // "undefined" as a string
var myString2 = foo + ''; // "undefined" as a string
var myString3 = foo.toString(); // throws an exception
http://jsfiddle.net/f8YwA/