原型未按预期工作(Prototype is not working as expected)

 我使用下面的代码来学习JS原型以及如何使用对象，（在我在网上阅读它之后我刚接触到JS ...）我想创建一个其他类可以使用和扩展的根对象确定如果我这样做...目前我在声音上有错误（无法设置未定义的属性'声音'，任何想法我做错了或者有更好的方法来写它，（对我来说这是第三个）草案...：） ） 
 
var obj = function(name){

    this.name  = name;
};

obj.prototype.getName = function(){
    return this.name;
}

var person = new obj("Bill");

console.log(person.getName());


var animal = new obj();

animal.prototype.sound = function(sound){
    console.log(sound)
}


var dog = new animal();


dog.sound("wof");

Im using the following code in order to learn JS prototype and how to works with objects,(after I read about it on the net Im new to JS...)I want to create a root object that other class can use and extend not sure if I do that right...currently I've error at the sound (Cannot set property 'sound' of undefined),any idea what Im doing wrong or maybe there is better way to write it,(for me it's the third draft...:) )
 
var obj = function(name){

    this.name  = name;
};

obj.prototype.getName = function(){
    return this.name;
}

var person = new obj("Bill");

console.log(person.getName());


var animal = new obj();

animal.prototype.sound = function(sound){
    console.log(sound)
}


var dog = new animal();


dog.sound("wof");

原文：https://stackoverflow.com/questions/27173578