JavaScript模拟 超类 子类的问题

Question

function Rectangle(w,h) {
this.width = w;
this.height = h;
this.long = 10;
}
Rectangle.prototype.volume = 20;
Rectangle.prototype.area = function() {return this.width * this.height;};
Rectangle.prototype.area2 = function() {return this.long * this.volume;};
function PositionedRectangle(x, y, w, h) {
    Rectangle.call(this, w, h);
    this.x = x;
    this.y = y;
this.long = 100;
}
PositionedRectangle.prototype = new Rectangle();
delete PositionedRectangle.prototype.width;
delete PositionedRectangle.prototype.height;
delete PositionedRectangle.prototype.long;
delete PositionedRectangle.prototype.volume;
PositionedRectangle.prototype.constructor = PositionedRectangle;
PositionedRectangle.prototype.contains = function(x,y) {
    return (x > this.x && x < this.x + this.width && y > this.y && y < this.y + this.height);
}var r2 = new PositionedRectangle(2,2,2,2);
document.write(r2.contains(3,3) + "<br />");
document.write(r2.height+ "<br />");
document.write(r2.area()+ "<br />");
document.write(r2.area2()+ "<br />");
document.write(r2.x + "," + r2.y + "," + r2.width + "," + r2.height + "<br />");
document.write(r2 instanceof PositionedRectangle && r2 instanceof Rectangle && r2 instanceof Object);delete没有做到删除超类 Rectangle的任何属性，后面r2.heigh，r2.area(），r2.area2()…… 都有结果?求助delete到底做了什么？为什么没有达到删除属性的效果

Answer 1

<script>
function t()
{
}
t.prototype.f=function(){};
var o=new t();
alert(o.f);
delete o.constructor.prototype.f;
alert(o.f);
</script>现在要考虑的是delete t.prototype 反而是不可以的 实例的o.f调用的是 o.constructor.prototype.f 而不是 t.prototype楼主的疑惑应该可以部分解答了,这个还是跟原型链的概念有关

Answer 2

<script>
function Rectangle(w,h) {
    this.width = w;
    this.height = h;
    this.long = 10;
}
Rectangle.prototype.volume = 20;
Rectangle.prototype.area = function() {return this.width * this.height;};
Rectangle.prototype.area2 = function() {return this.long * this.volume;};
function PositionedRectangle(x, y, w, h) {
    Rectangle.call(this, w, h);
    this.x = x;
    this.y = y;
    this.long = 100;
}
PositionedRectangle.prototype = new Rectangle();
delete PositionedRectangle.prototype.width;
delete PositionedRectangle.prototype.height;
delete PositionedRectangle.prototype.long;
delete PositionedRectangle.prototype.constructor.prototype.volume;
PositionedRectangle.prototype.constructor = PositionedRectangle;
PositionedRectangle.prototype.contains = function(x,y) {
    return (x > this.x && x < this.x + this.width && y > this.y && y < this.y + this.height);
}var r2 = new PositionedRectangle(2,2,2,2);
document.write(r2.contains(3,3) + "<br />");
document.write(r2.height+ "<br />");
document.write(r2.area()+ "<br />");
document.write(r2.area2()+ "<br />");
document.write(r2.x + "," + r2.y + "," + r2.width + "," + r2.height + "<br />");
document.write(r2 instanceof PositionedRectangle && r2 instanceof Rectangle && r2 instanceof Object);alert(r2.volume);</script>实验结果验证了我的想法

Answer 3

你只删除了 原型对象里的属性，构造函数里的属性无法这样删除，那怎么删除上面代码中的
this.width = w;
    this.height = h;
    this.long = 10;
呢？这些代码引用 JavaScript权威指南 第五版

Answer 4

<script>
function Rectangle(w,h) {
  this.width = w;
  this.height = h;
  this.long = 10;
}
Rectangle.prototype.volume = 20;
Rectangle.prototype.area = function() {return this.width * this.height;};
Rectangle.prototype.area2 = function() {return this.long * this.volume;};
function PositionedRectangle(x, y, w, h) {
 // Rectangle.call(this, w, h);
  this.x = x;
  this.y = y;
  this.long = 100;
}
PositionedRectangle.prototype = new Rectangle();
delete PositionedRectangle.prototype.width;
delete PositionedRectangle.prototype.height;
delete PositionedRectangle.prototype.long;
delete PositionedRectangle.prototype.constructor.prototype.width;
PositionedRectangle.prototype.constructor = PositionedRectangle;
PositionedRectangle.prototype.contains = function(x,y) {
  return (x > this.x && x < this.x + this.width && y > this.y && y < this.y + this.height);
}var r2 = new PositionedRectangle(2,2,2,2);
document.write(r2.contains(3,3) + "<br />");
document.write(r2.height+ "<br />");
document.write(r2.area()+ "<br />");
document.write(r2.area2()+ "<br />");
document.write(r2.x + "," + r2.y + "," + r2.width + "," + r2.height + "<br />");
document.write(r2 instanceof PositionedRectangle && r2 instanceof Rectangle && r2 instanceof Object);alert(r2.width);</script>

Answer 5

知道了 你删除了从子类函数调用超类构造函数这一步，我还以为这一步是构造函数链必须的
还有，根据这段代码的例子，如果你删除了 Rectangle.call(this, w, h);这一步，width,height属性无法被子类函数调用，最后显示的结果就是undefined了，，，这看起来好矛盾，难道JavaScript权威指南例子就是存在这个矛盾的问题……

Answer 6

function Rectangle(w,h) {
this.width = w;
this.height = h;
this.long =10;
}
Rectangle.prototype.volume = 20;
Rectangle.prototype.area = function() {return this.width * this.height;};
Rectangle.prototype.area2 = function() {return this.long * this.volume;};
function PositionedRectangle(x, y, w, h) {
    Rectangle.call(this, w, h);
    this.x = x;
    this.y = y;
this.long = 100;
}
PositionedRectangle.prototype = new Rectangle();
PositionedRectangle.prototype.volume = 200;
//PositionedRectangle.prototype.constructor = PositionedRectangle;
delete PositionedRectangle.prototype.width;
delete PositionedRectangle.prototype.height;
delete PositionedRectangle.prototype.volume;
delete PositionedRectangle.prototype.long;
delete PositionedRectangle.long;
delete PositionedRectangle.prototype.constructor.prototype.width;
delete PositionedRectangle.prototype.constructor.prototype.volume;
delete PositionedRectangle.prototype.constructor.prototype.long;
PositionedRectangle.prototype.constructor = PositionedRectangle;
PositionedRectangle.prototype.contains = function(x,y) {
    return (x > this.x && x < this.x + this.width && y > this.y && y < this.y + this.height);
}var r2 = new PositionedRectangle(2,2,2,2);
document.write(Rectangle.prototype);//toString()返回
document.write(r2.contains(3,3) + "<br />");
document.write("width:" + r2.width+ "<br />" + "volume:" + r2.volume+ "<br />" + "long:" + r2.long );
document.write(r2.area()+ "<br />");
document.write("long*volume:" + r2.area2()+ "<br />");
document.write(r2.x + "," + r2.y + "," + r2.width + "," + r2.height + "<br />");
document.write(r2 instanceof PositionedRectangle && r2 instanceof Rectangle && r2 instanceof Object);最终这样测试了，Rectangle.call(this, w, h);这一步的参数w,h调用暂时不去管，上面测试结果感觉构造函数里this.Attribute  在构造函数里创建的属性delete无法删除
delete PositionedRectangle.prototype.xxx 删除的是PositionedRectangle.prototype原型创建的属性
delete PositionedRectangle.prototype.constructor.prototype.xxx 删除的是Rectangle原型Rectangle.prototype创建的属性
按照权威指南里源代码注释说：
// We create this prototype object for inheritance purposes, but we
// don't actually want to inherit the width and height properties that
// each Rectangle object has, so delete them from the prototype.
delete PositionedRectangle.prototype.width;
delete PositionedRectangle.prototype.height;感觉并没有起到删除从Rectangle继承的属性，不理解注释的意义，PositionedRectangle.prototype里也没有width和height