javascript 函数返回值的问题，请高手解答

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）