understanding $.extend

understanding $.extend

Hey guys , i have a question about the extend function , check out my fiddle . 


now i hav ethe following code : 

  1. var a = {
  2.          'lala' : 'one',
  3.          'baba' : 'two', 
  4.          'dada'  : 'three',
  5.          'kaka'  : 'four',
  6.          }

  8.          var b = {
  9.          'lala' : 'one',
  10.          'baba' : 'two', 
  11.          'dada'  : 'three',
  12.          'kaka'  : 'four',
  13.          }

  15.          var _str = $.extend({} , b , a);

  17.          console.log(_str);

now the doc's says that when u add an empty {} object literal as the 1st param , the merged objects are preserved . but i don't see a difference doing either , 

  1. var _str = $.extend({} , b , a);
or 

  1. var _str = $.extend( b , a);
the result is 

Object { lala: "one", baba: "two", dada: "three", kaka: "four" }

also what is the 4th param in $.extend for ?. 

Thanks .