js对象函数如何不劫持callback函数的this指向？

function Afun(){
    this.bfun = new Bfun(this.callBack);
}

Afun.prototype.callBack = function(){
    console.log(this);//这个函数被`Bfun`调用时，this指的是`Bfun`，如何让它指为`Afun`？
}

function Bfun(callBack){
    callBack();
}

new Afun();//输出`Bfun`，但是我希望是`Afun`

---

function Afun() {
    this.bfun = new Bfun( this.callBack.bind(Afun) );
}

---

this.bfun = new Bfun(this.callBack.bind(this));

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Glob...