原型對(duì)象繼承

• ==原理：原型對(duì)象的屬性可以經(jīng)由對(duì)象實(shí)例訪問(wèn)==
• 下面的例子，所含【對(duì)象繼承】和【構(gòu)造函數(shù)繼承】，用于理解【原型屬性經(jīng)由對(duì)象實(shí)例訪問(wèn)】

//對(duì)象繼承
var person = {
  name:"gs",
  age:19
}
console.log(person.prototype === undefined );
function Student(){}
Student.prototype = person;//原型屬性經(jīng)由對(duì)象實(shí)例訪問(wèn)
console.log(new Student());
//構(gòu)造函數(shù)繼承
function Person(){
  this.name = "gs";
  this.age = 19;
}
function Student(){
  this.name = "zk"
}
Student.prototype = new Person();//原型屬性經(jīng)由對(duì)象實(shí)例訪問(wèn)
var zk = new Student();
console.log(zk);

Object是頂層對(duì)象

• 所有對(duì)象都繼承自 Object.prototype，除非顯示指定
• 以對(duì)象字面量定義的對(duì)象，其 prototype 的值都被設(shè)為 Object.prototype

定義在 Object.prototype 上的方法

• hasOwnProperty
• propertyIsEnumerable
• isPrototypeOf
• valueOf | 返回對(duì)象的值表達(dá)
• toString | 返回對(duì)象的字符串表達(dá)

valueOf

• 每當(dāng)一個(gè)操作符被用于一個(gè)對(duì)象時(shí)（自動(dòng)）調(diào)用

toString

• 在進(jìn)行運(yùn)算時(shí)，有可能發(fā)生類型轉(zhuǎn)換

let obj = { name:"gs" };
console.log(obj+111)//;[object Object]111
console.log(obj.toString());//[object Object]
console.log(obj.valueOf());//{ "name": "gs" }

最好不要去修改 Object.prototype

最簡(jiǎn)單的一個(gè)對(duì)象繼承

• 聲明一個(gè)字面量對(duì)象，相當(dāng)于自動(dòng)繼承自 Object
• 聲明字面量對(duì)象，可以用 Object.create 方法替代，它的繼承作用更加明顯
• ==注意：Object.prototype 實(shí)際上值是 null==

var student1 = {
  name:"gs"
}
var student2 = Object.create(Object.prototype,{
  name:{
    configurable:true,
    enumerable:true,
    value:"gs",
    writable:true,
  }
})

實(shí)現(xiàn)一個(gè)基本的對(duì)象繼承

//對(duì)象繼承
var person = {
  name:"gs",
  age:19
}
console.log(person.prototype === undefined );
function Student(){}
Student.prototype = person;//原型屬性經(jīng)由對(duì)象實(shí)例訪問(wèn)
console.log(new Student());

實(shí)現(xiàn)一個(gè)稍復(fù)雜的構(gòu)造函數(shù)繼承

• 因?yàn)閖s中的繼承是通過(guò)原型對(duì)象鏈來(lái)實(shí)現(xiàn)的，因此沒(méi)有調(diào)用對(duì)象父類的構(gòu)造函數(shù)
• 所以，要注意！最好重寫(xiě)構(gòu)造器屬性，否則將被設(shè)為父類的值

function Person(school,name,age){
  this.school = "beijing univercity";
  this.name = name;
  this.age = age;
}
Person.prototype.sayName = function(){
  console.log(this.name);
}
Person.prototype.sayAge = function(){
  console.log(this.age);
}

function Student(name,age){
  this.name = name;
  this.age = age;
}
Student.prototype = new Person();
var gs = new Student("gs",19);
console.log(gs.constructor);//Person
Student.prototype.constructor = Student;//注意！要重寫(xiě)構(gòu)造器屬性，否則將被設(shè)為父類的值
console.log(gs.constructor);//Student
Student.prototype.sayName = function(){
  console.log("student is "+this.name);
}
console.log(gs.sayName());
console.log(gs.sayAge());

實(shí)現(xiàn)一個(gè)偽類繼承（構(gòu)造函數(shù)竊?。?/h4>

• 實(shí)際上就是使用 call 的方式，將父類的屬性“拷貝”到子類中，同時(shí)還包括了constructor屬性
• 仍然需要用prototype方式繼承方法，同時(shí)還要設(shè)置constructor

function Person(school,name,age){
  this.school = "beijing univercity";
  this.name = name;
  this.age = age;
}
Person.prototype.sayName = function(){
  console.log(this.name);
}
Person.prototype.sayAge = function(){
  console.log(this.age);
}

function Teacher(school){
  Person.call(this,...arguments);//因?yàn)閠his指向Teacher，所以constructor屬性仍將是Teacher
  this.school = school;//注意，新的屬性，應(yīng)置于繼承發(fā)生之后，以做覆蓋
}
var zk = new Teacher("haver","zk",23);
console.log(zk.constructor);//constructor屬性正確，為T(mén)eacher
console.log(zk.sayName === undefined);//true，但是，沒(méi)有繼承原型方法和屬性
Teacher.prototype = new Person();
var zk_2 = new Teacher("haver","zk_2",23);//沒(méi)有繼承原型方法
console.log(zk_2.sayName());//拿到方法了
console.log(zk_2.constructor);//Person。不是吧，constructor屬性又不對(duì)了
Teacher.prototype.constructor = Teacher;
console.log(zk_2.constructor);//Teacher。使用call方式繼承屬性，仍然需要用prototype方式繼承方法，同時(shí)還要設(shè)置constructor

父類方法被子類覆蓋，如何仍然可以調(diào)用？

function Person(name,age){
  this.name = name;
  this.age = age;
}
Person.prototype.sayName = function(){
  console.log("father method " + this.name);
}
Person.prototype.sayAge = function(){
  console.log(this.age);
}

function Student(){
  Person.call(this,...arguments);
}
Student.prototype = Object.create(Person.prototype,{
  constructor:{
    configurable:true,
    enumerable:true,
    value:Student,
    writable:true,
  }
})
Student.prototype.sayName = function(){
  console.log("child method");
}
Student.prototype.sayName_1 = function(){
  Person.prototype.sayName.call(this);//這是唯一的訪問(wèn)父類的方法
}
var gs = new Student("gs",20);
console.log(gs);

console.log(gs.sayName_1());

創(chuàng)建一個(gè)沒(méi)有原型對(duì)象的對(duì)象

• 它不會(huì)發(fā)生和繼承來(lái)的屬性名相沖突的問(wèn)題，所以它是一個(gè)完美的【哈希容器】
• 哈希容器也可以理解為是一種映射容器，采用哈希算法（映射算法，散列算法），將不定長(zhǎng)的數(shù)據(jù)壓縮成定長(zhǎng)的數(shù)據(jù)，這串定長(zhǎng)值我們稱為 哈希值

var obj = Object.create(null);
console.log(obj.prototype === undefined);
console.log( valueOf in obj);
console.log( toString in obj);