function defineSubclass(superclass,  //父类的构造函数                        constructor,  //新的子类的构造函数                        methods,      //实例方法：复制至原型中                        statics)      //类属性：复制至构造函数中{  //建立子类的原型对象  constructor.prototype = inherit(superclass.prototype);  constructor.prototype.constructor = constructor;  //像对常规类一样复制方法和类属性  if(methods) extend(constructor.prototype,methods);  if(statics) extend(constructor,statics);  return constructor;}function inherit(p){  if(!p){
   throw TypeError();}  if(Object.create){Object.create(p);}  var t = typeof p;  if(t !== "object" && t !== "function")  {    throw TypeError();  }  function f(){};  f.prototype = p;  return new f();}function extend(proto,methods){  proto[methods] = methods();  return proto;}function Person(name,age){  this.name = name;  this.age = age;}Person.prototype.eat = function(){alert("eat!");}function Student(){}defineSubclass(Person,Student,function learn(){alert("learn!");})

 

//通过父类构造函数的方法来做到这一点

Function.prototype.extend = function(constructor,methods,statics){  return defineSubclass(this,constructor,methods,statics);}