JavaScript實現繼承

如下內容均基於本人對《JavaScript高級程序設計》第三版6.3小節的理解

先看一下父類

function Animal(name) {
    var name = name;             //'私有（受保護）'成員，只容許在父類的構造函數中賦值
    this.food = undefined;       //'公有'成員

    //引用類型的成員
    this.birthday = {            
      year: undefined
    };

    //構造函數中的方法，打印一些基本信息
    this.greeting = function() {
      console.log('Hi, my name is {' + name + '} I like eat {' + this.food + '} and my birth year is {' + this.birthday.year + '}');
    };

    //原型中的方法，將構造函數中的非函數成員以JSON格式打印
    Animal.prototype.briefInfo = function() {
      var brief = {
        name: name,
        food: this.food,
        birthday: this.birthday
      };
      console.log(JSON.stringify(brief));
    };
  }

---

方式一：原型鏈繼承

實現方式：子類的原型指向父類的實例

子類：

function Dog() {}
Dog.prototype = new Animal();        //原型指向父類的實例

測試：

var dog1 = new Dog();
dog1.food = 'shit';
dog1.birthday.year = 2015;

var dog2 = new Dog();
dog2.food = 'bones';
dog2.birthday.year = 2016;

dog1.greeting();  //console: Hi, my name is {undefined} I like eat {shit} and my birth year is {2016}
dog2.greeting();  //console: Hi, my name is {undefined} I like eat {bones} and my birth year is {2016}

dog1.briefInfo();  //console: {"food":"shit","birthday":{"year":2016}}
dog2.briefInfo();  //console: {"food":"bones","birthday":{"year":2016}}

//以上，
//birthday是引用類型的屬性，因此dog1的birthday.year被dog2覆蓋了;
//沒法給dog1和dog2的name賦值

存在的問題：

• 引用類型的對象會被子類的全部實例共享(1.1)

• 沒法在建立子類的實例時，給父類的構造函數傳遞參數(1.2)

---

方式二：借用構造函數（僞造對象、經典繼承）

實現方式：在子類的構造函數中利用call(或者apply)方法執行父類構造函數(問題1.2解決)，將執行對象設爲子類的this,至關於把父類構造函數中的成員拷貝了一份到子類(問題1.1解決)

子類

function Dog(name) {
  Animal.call(this, name);
}

測試

var dog1 = new Dog('tom');
dog1.food = 'shit';
dog1.birthday.year = 2015;

var dog2 = new Dog('mike');
dog2.food = 'bones';
dog2.birthday.year = 2016;

dog1.greeting();  //console: Hi, my name is {tom} I like eat {shit} and my birth year is {2015}
dog2.greeting();  //console: Hi, my name is {mike} I like eat {bones} and my birth year is {2016}

//briefInfo是父類原型中的屬性，並無被繼承，如下語句會報錯
dog1.briefInfo();  //error: dog1.briefInfo is not a function
dog2.briefInfo();  //error: dog2.briefInfo is not a function

存在的問題：

1. 父類原型中定義的屬性沒法被繼承

---

綜合方式一方式二，一種很明顯的方式呼之欲出了：

方式三：組合繼承

實現方式：結合原型鏈繼承和經典繼承

子類

function Dog(name) {
  Animal.call(this, name);    //經典繼承
}
Dog.prototype = new Animal();    //原型鏈繼承

測試

var dog1 = new Dog('tom');
dog1.food = 'shit';
dog1.birthday.year = 2015;

var dog2 = new Dog('mike');
dog2.food = 'bones';
dog2.birthday.year = 2016;

dog1.greeting();  //console: Hi, my name is {tom} I like eat {shit} and my birth year is {2015}
dog2.greeting();  //console: Hi, my name is {mike} I like eat {bones} and my birth year is {2016}
dog1.briefInfo();  //console: {"name":"tom","food":"shit","birthday":{"year":2015}}
dog2.briefInfo();  //console: {"name":"mike","food":"bones","birthday":{"year":2016}}

//終於獲得了預期的結果，算是較好的實現了繼承。
//可是，並不完美

存在的問題

• 父類的構造函數被調用了兩次

---

爲了引出下一個繼承方式，先將函數的繼承放在一邊，看一下js中對象的繼承

(摘抄原文)

1. 原型式繼承

var person = {
  name: 'martin'
  firend: ['bob', 'steven']
};
  
function object(o) {
  function F() {};
  F.prototype = o;
  return new F();
}
var anotherPerson = object(person);    //antherPerson繼承了person

2. 寄生式繼承

function createAnother(original) {
  var clone = object(original);    //原型式繼承定義的方法
  //擴展對象
  clone.sayHi = function() {
    console.log('hi');
  }
  return clone;
}

---

回到正題，接下來介紹一顆 「銀彈」

方式四：寄生組合式繼承

實現方式：

1. 利用經典繼承拷貝父類構造中的屬性到子類

2. 利用原型式繼承建立一個繼承父類原型的對象

3. 將該對象的constructor屬性指向子類的構造函數(寄生式繼承:擴展對象)

4. 將子類的prototype指向該對象

子類

function Dog(name) {
  Animal.call(this, name);
}

function F() {}
var supProto = Animal.prototype;
F.prototype = supProto;
var subProto = new F();
subProto.constructor = Dog;
Dog.prototype = subProto;

測試

var dog1 = new Dog('tom');
dog1.food = 'shit';
dog1.birthday.year = 2015;

var dog2 = new Dog('mike');
dog2.food = 'bones';
dog2.birthday.year = 2016;

dog1.greeting();   //console: Hi, my name is {tom} I like eat {shit} and my birth year is {2015}
dog2.greeting();   //console: Hi, my name is {mike} I like eat {bones} and my birth year is {2016}
dog1.briefInfo();  //console: {"name":"tom","food":"shit","birthday":{"year":2015}}
dog2.briefInfo();  //console: {"name":"mike","food":"bones","birthday":{"year":2016}}