ES5继承的六种方式

（2018-09-06）

1.原型链

将一个类型的实例赋值给另一个构造函数的原型。

    function SuperType(){
            this.property = true;
        }
        
        SuperType.prototype.getSuperValue = function(){
            return this.property;
        };
        
        function SubType(){
            this.subproperty = false;
        }
        
        //inherit from SuperType
        SubType.prototype = new SuperType();
        
        SubType.prototype.getSubValue = function (){
            return this.subproperty;
        };
        
        var instance = new SubType();
        alert(instance.getSuperValue());   //true
       
        alert(instance instanceof Object);      //true
        alert(instance instanceof SuperType);   //true
        alert(instance instanceof SubType);     //true

        alert(Object.prototype.isPrototypeOf(instance));    //true
        alert(SuperType.prototype.isPrototypeOf(instance)); //true
        alert(SubType.prototype.isPrototypeOf(instance));   //true

2.构造函数继承

在子类型构造函数的内部调用超类型构造函数。

function SuperType(){
            this.colors = ["red", "blue", "green"];
        }

        function SubType(){  
            //inherit from SuperType
            SuperType.call(this);
        }

        var instance1 = new SubType();
        instance1.colors.push("black");
        alert(instance1.colors);    //"red,blue,green,black"
        
        var instance2 = new SubType();
        alert(instance2.colors);    //"red,blue,green"

3.组合式继承

使用原型链实现对原型方法的继承，借用构造函数来实现对实例属性的继承。

function hasPrototypeProperty(object, name) {
            return !object.hasOwnProperty(name) && (name in object);
        }

        function SuperType(name){
            this.name = name;
            this.colors = ["red", "blue", "green"];
        }
        
        SuperType.prototype.sayName = function(){
            console.log(this.name);
        };

        function SubType(name, age){  
            SuperType.call(this, name);
            
            this.age = age;
        }

        SubType.prototype = new SuperType("hehe");
        
        SubType.prototype.sayAge = function(){
            console.log(this.age);
        };
        
        var instance1 = new SubType("Nicholas", 29);
        instance1.colors.push("black");
        console.log(instance1.colors);  //"red,blue,green,black"
        instance1.sayName();      //"Nicholas";
        instance1.sayAge();       //29
        
       
        var instance2 = new SubType("Greg", 27);
        console.log(instance2.colors);  //"red,blue,green"
        instance2.sayName();      //"Greg";
        instance2.sayAge();       //27

        //delete instance1.name;
        console.log(hasPrototypeProperty(instance1, "colors"));
        console.log(hasPrototypeProperty(instance1, "name"));

4.原型式继承

function object(o){
            function F(){}
            F.prototype = o;
            return new F();
        }
        
        var person = {
            name: "Nicholas",
            friends: ["Shelby", "Court", "Van"]
        };
        
        var anotherPerson = object(person);
        anotherPerson.name = "Greg";
        anotherPerson.friends.push("Rob");
        
        var yetAnotherPerson = object(person);
        yetAnotherPerson.name = "Linda";
        yetAnotherPerson.friends.push("Barbie");
        
        alert(person.friends);   //"Shelby,Court,Van,Rob,Barbie"

在object()函数内部，先创建了一个临时性构造函数，然后将传入的对象作为这个构造函数的原型，最后返回了这个临时类型的一个新实例。

5.寄生式继承

创建一个仅用于封装继承过程的函数，该函数在内部以某种方式来增强对象，最后再像真地做了所有工作一样返回对象。

function createAnother(original){
  var clone = object(original);
  clone.sayHi = function(){
      alert("hi);
    };
return clone;
}

6.寄生组合式继承

通过借用构造函数来继承属性，通过原型链的混成形式来继承方法。基本思路：不必为了指定子类型的原型而调用超类型的构造函数。
本质：使用寄生式继承来继承超类型的原型，然后再将结果指定给子类型的原型。

function inheritPrototype(subType, superType){
            var prototype = object(superType.prototype);   //create object
            prototype.constructor = subType;               //augment object
            subType.prototype = prototype;                 //assign object
        }
                                
        function SuperType(name){
            this.name = name;
            this.colors = ["red", "blue", "green"];
        }
        
        SuperType.prototype.sayName = function(){
            alert(this.name);
        };

        function SubType(name, age){  
            SuperType.call(this, name);
            
            this.age = age;
        }

        inheritPrototype(SubType, SuperType);
        
        SubType.prototype.sayAge = function(){
            alert(this.age);
        };
        
        var instance1 = new SubType("Nicholas", 29);
        instance1.colors.push("black");
        alert(instance1.colors);  //"red,blue,green,black"
        instance1.sayName();      //"Nicholas";
        instance1.sayAge();       //29
        
       
        var instance2 = new SubType("Greg", 27);
        alert(instance2.colors);  //"red,blue,green"
        instance2.sayName();      //"Greg";
        instance2.sayAge();       //27