var x = 1,

         y = z = 0;

 function add(n) {

         return n = n + 1;

 }

 y = add(x);

 function add(n) {

         return n = n + 3;

 }

 z = add(x);

 console.log(x);//x值未变1

 console.log(y);//4

 console.log(z);//4