javascript 函数返回的问题，请教各位高手。

Question

function aFun(){function _(arg){this.arg = arg;}_.prototype.testFun = function(){console.log('this is prototype testFun')};_.testFun2 = function(){console.log('this is testFunc2');}return _;};var afun = new aFun('123');console.log(afun instanceof aFun)afun.testFun2(); //可以调用afun.testFun(); //不是在原型链上么？，为啥不可以调用？？？，var aFun2 = (function (){function _(arg){this.arg = arg;}_.prototype.testFun = function(){console.log('this is prototype testFun')};_.testFun2 = function(){console.log('this is testFunc2');}return _;})();var afun2 = new aFun2('ddd');console.log(afun2 instanceof aFun2)afun2.testFun2(); //不可以调用，为啥？afun2.testFun(); //可以调用，为啥可以？（我要疯了）