it is possible to convert a string to an object property? - JavaScript

Question

can convert a string into a property? I'm not very good with javascript objects, so I'd like your help ...

This is jsfiddle: http://jsfiddle.net/972fqz39/

var person = {    
  lastName : "Doe"   
};

var stringa= "lastName";
alert(person.stringa);

in this case it return undefined, i know that maybe i can to use person.lastName, but in my big code i can't use it.

So is possible to convert stringa in a property ??

Thanks!