逆推继承看原型

function F1(age) {
this.age = age;
}
function F2(age) {
this.age = age;
}
F2.prototype = new F1(10);
function F3(age) {
this.age = age;
}
F3.prototype = new F2(20);
var f3 = new F3(30);
console.log(f3.age);//