如下代码，为什么b.sayFrom()会是undefined？？

class Person{
    constructor(){
        this.country = '中国';
        this.hairColor = 'black';
    }
    sayFrom(){
        console.log(this.country);
    }
}

class H extends Person{
    constructor(){
        super();
        this.name = 'honey';
        this.country = 'China';
    }
    sayFrom(){
        console.log(this.country);
    }
}
var a = new Person();
var b = new H();
a.sayFrom();//中国
b.sayFrom(); //undefined