javascript构造函数的prototype默认是不是Object.prototype呢？若不是，则构造函数的prototype什么对象呢？该怎么处理 _JavaScript

javascript构造函数的prototype默认是不是Object.prototype呢？若不是，则构造函数的prototype什么对象呢？
我是想通过下面的代码，找出javascript构造函数的prototype默认是哪个对象，可却发现不是不是Object,也不是Object.prototype，那是哪个对象呢？

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN " "http://www.w3.org/TR/html4/loose.dtd ">
<html>
<head>
<meta http-equiv= "Content-Type " content= "text/html; charset=GB18030 ">
<title> Insert title here </title>
</head>
<body>
<script type= "text/javascript ">
function circle(x,y,r)
{
this.x = x;
this.y = y;
this.r = r;
}

if(circle.prototype == null){
alert( "NULL ");
}else{
//结果输出NOT NULL
alert( "NOT NULL ");
alert(circle.prototype);
alert(typeof circle.prototype);

if(circle.prototype == Object){
alert( "SAME TO OBJECT ");
}else{
//结果输出DIRRERENT
alert( "DIFFERENT TO OBJECT ");
}

if(circle.prototype == Object.prototype){
alert( "SAME TO OBJECT 'S PROTOTYPE ");
}else{
//结果输出DIRRERENT
alert( "DIFFERENT TO OBJECT 'S PROTOTYPE ");
}

}

</script>
</body>
</html>

------解决方案--------------------

JScript code


<script   type= "text/javascript"> 
function circle(x, y, r) { 
    this.x = x;
    this.y = y;
    this.r = r;
}

if (circle.prototype == null){ 
    alert("NULL"); 
} else {
//结果输出NOT NULL
    alert("NOT NULL"); //circle是Function的实例，Function的实例是带prototype的
    if (circle.prototype == Object) {
        alert("SAME TO OBJECT");
    } else {
    //结果输出DIRRERENT 
    //prototype貌似继承了Object.prototype，不过继承不代表===吧？
    //证明继承的方法很简单delete circle.prototype.constructor;后在alert(circle.prototype.constructor)就会指向Object
        alert("DIFFERENT TO OBJECT");
    } 
    
    
    if(circle.prototype == Object.prototype){ 
        alert( "SAME   TO   OBJECT &apos;S   PROTOTYPE "); 
    }else{ 
     //结果输出DIRRERENT 
     //这里不解释了。。。看上面的解释就知道了。。。
        alert( "DIFFERENT   TO   OBJECT &apos;S   PROTOTYPE");
    }
} 


</script>