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210lalaundefined(anonymous) @ test.html:47为什么是undefined呢！？？？