//父類
 function Person(name){
    this.name = name;
};

 // 子類
 function Student(sex){
  Person.apply(this,arguments); //繼承父類的構造函數
  this.sex=sex;
 };

Student.prototype = Person.prototype; //執行完此句時，Student.prototype.constructor 指向的是Person,為什么了？因為Person.prototype.constructor指向Person,對象的賦值實質上是引用賦值，所以Student.prototype.constructor也指向Person
Student.prototype.constructor = Student;  //  將Student.prototype.constructor 指回Person

Student.prototype.add=function(){alert("add")};
Person.prototype.add();//彈出add

Student.prototype = new Person();  //如果此處不傳遞參數，可以不寫（);即直接寫成 new Person;
2 Student.prototype.constructor = Student;

var  F = function(){};
F.prototype = Person.prototype;
Student.prototype  = new F();
Student.prototype.constructor  = Student;