Professor.prototype.tSkill = 'JAVA';
function Professor(){}
 
var professor = new Professor();
 
// 原型链继承
Teacher.prototype = professor;
function Teacher(){
  this.mSkill = 'JS/JQ';
}
var teacher = new Teacher();
 
Student.prototype = teacher;
function Student(){
  this.pSkill = 'HTML/CSS';
}
 
var student = new Student();
console.log(student);
student.mSkill;	// JS/JQ
student.tSkill;	// JAVA
// 我没有的属性，可以到我的原型上找，我的原型上没有，再去上一层原型找，这种继承关系叫原型链

const instance = new Number(3)
Number.prototype.constructor === Number
Number.prototype.__proto__ === Object.prototype
Number.__proto__ === Function.prototype

/*
 * Object是实例，实例没有constructor，会从原型上寻找，即Function.prototype.constructor
 * Object 是new Function()构建出来的
 */
Object.constructor === Function === Function.prototype.constructor;
Object.__proto__ === Function.prototype
 
/*
 * Function 是实例对象，同时 Function 也是构造函数
 */
Function.__proto__ === Function.prototype
 
Function.prototype.__proto__ === Object.prototype //Function.prototype是对象

Professor.prototype.tSkill = 'JAVA';
function Professor(){}
 
var professor = new Professor();
 
Teacher.prototype = professor;
function Teacher(){
  this.mSkill = 'JS/JQ';
  this.students = 500;
}
var teacher = new Teacher();
 
Student.prototype = teacher;
function Student(){
  this.pSkill = 'HTML/CSS';
}
 
var student = new Student();
student.students++;
console.log(student.students);
console.log(teacher, student);
student.students; // 501
teacher.students; // 500
答：可以修改成功，但是不能修改上一级原型上的值

function Car(){}
Car.prototype.name = 'Mazda';
var car = new Car();
// 给属性赋值
Car.prototype.name = 'Benz';
console.log(car.name); // Benz

function Car(){}
Car.prototype.name = 'Mazda';
var car = new Car();
console.log(car.name); // Mazda
// 重写prototype
Car.prototype = {
  name: 'Benz'
};
console.log(car.name); // Mazda

function XX() {};
XX.prototype.num = 1;
var obj1 = Object.create(XX.prototype);
var obj2 = new XX();
console.log(obj1);
console.log(obj2);

var obj = Object.create(null);
obj.num = 1
var obj1 = {
	count:2
}
obj.__proto__ = obj1;
console.log(obj.count); //undefined
//不可以自己指定__proto__