代码是随手写的，只提供思路。

这个原理很简单，看代码就懂，不多说了。

(function (){
var h = 0;
handle = function (){return h++};
var f = function (){};
extend = function (a, b){
f.prototype = a;
var ret = new f;
if (typeof b == 'function') {
b.call(ret);
} else if (typeof b == 'object') {
for (var key in b) {
ret[key] = b[key];
}
}
return ret;
};
})();

(function (){
ClassA = function (){
this.hello = 'world';
};
ClassA.virtualmethod = handle();
ClassA.prototype = extend({}, function (){
this.virtualmethod = function (){
var impl = this[ClassA.virtualmethod];
if (impl) {
impl.apply(this, arguments);
} else {
alert('ClassA.virtualmethod not yet impl.');
}
};
});
})();

(function (){ // ClassB extend ClassA
ClassB = function (){
ClassA.apply(this, arguments);
};

// 继承性
ClassB.prototype = extend(ClassA.prototype, function (){
this[ClassA.virtualmethod] = function (){
alert('this is ClassA.virtualmethod, impl in ClassB.nwill show this.hello.');
alert('this.hello = ' + this.hello);
}

// 封装性
this.test = this.virtualmethod;
this.virtualmethod = undefined;
});
})();

(function (){ // ClassC extend ClassB
ClassC = function (){
ClassB.apply(this, arguments);
};

ClassC.prototype = extend(ClassB.prototype, function (){
// 多态性
var impl = this[ClassA.virtualmethod];
this[ClassA.virtualmethod] = function (){
alert('this is ClassA.virtualmethod, impl in ClassC. nwill run ClassB's impl.');
impl.apply(this, arguments);
};
});
})();

// test case
var a = new ClassA;
a.virtualmethod(); // not yet impl

var b = new ClassB;
b.test();
alert('b.virtualmethod is: '+b.virtualmethod);

var c = new ClassC;
c.test();