JS函数return结果undefined

Question

var k = (function fun(x) { 

if(x>0) {

    x--;

    console.log(x);

} else{

    console.log("lala");

   return "lala";

}

    fun(x);  

})(3)

console.trace(k);

打印的结果是

2

1

0

lala

undefined

(anonymous) @ test.html:47

为什么是undefined呢！？？？