4. 原型链 %26 继承

关于原型链，要注意以下几个公式：

• Function.__proto__ == Function.prototype == Object.__proto__
• Function.prototype.__proto__ == Object.prototype
• Object.prototype.__proto__ == null
• function.__proto__ == Function.prototype
• function.prototype.__proto__ == Object.prototype
• prototype 是函数才有的属性，表示该函数的原型
• __proto__ 是每个对象都有的属性，表示构造器的原型，即：contrusctor.prototype

  1、 阅读代码

  function Person(name) {
    this.name = name
  }
  var p2 = new Person('king');
  console.log(p2.__proto__) 
  console.log(p2.__proto__.__proto__) 
  console.log(p2.__proto__.__proto__.__proto__) 
  console.log(p2.__proto__.__proto__.__proto__.__proto__)
  console.log(p2.__proto__.__proto__.__proto__.__proto__.__proto__)
  console.log(p2.constructor)
  console.log(p2.prototype)
  console.log(Person.constructor)
  console.log(Person.prototype)
  console.log(Person.prototype.constructor)
  console.log(Person.prototype.__proto__)
  console.log(Person.__proto__) 
  console.log(Function.prototype.__proto__)
  console.log(Function.__proto__)
  console.log(Object.__proto__)
  console.log(Object.prototype.__proto__)

  2、 阅读代码

  ```javascript // a function Foo () { getName = function () { console.log(1); } return this; } // b Foo.getName = function () { console.log(2); } // c Foo.prototype.getName = function () { console.log(3); } // d var getName = function () { console.log(4); } // e function getName () { console.log(5); }

Foo.getName();
getName();
Foo().getName();
getName();
new Foo.getName();
new Foo().getName();
new new Foo().getName();

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### 3、 阅读代码
```javascript
var F = function() {};
Object.prototype.a = function() {
  console.log('a');
};
Function.prototype.b = function() {
  console.log('b');
}
var f = new F();
f.a();
f.b();
F.a();
F.b()

4、 阅读代码

function Foo(){
    Foo.a = function(){
        console.log(1);
    }
    this.a = function(){
        console.log(2)
    }
}
Foo.prototype.a = function(){
    console.log(3);
}
Foo.a = function(){
    console.log(4);
}
Foo.a();
let obj = new Foo();
obj.a();
Foo.a();

5、 阅读代码

function Dog() {
  this.name = 'puppy'
}
Dog.prototype.bark = () => {
  console.log('woof!woof!')
}
const dog = new Dog()
console.log(Dog.prototype.constructor === Dog && dog.constructor === Dog && dog instanceof Dog)

6、 阅读代码

var A = {n: 4399};
var B =  function(){this.n = 9999};
var C =  function(){var n = 8888};
B.prototype = A;
C.prototype = A;
var b = new B();
var c = new C();
A.n++
console.log(b.n);
console.log(c.n);

7、 阅读代码

function A(){
}
function B(a){
　　this.a = a;
}
function C(a){
　　if(a){
this.a = a;
　　}
}
A.prototype.a = 1;
B.prototype.a = 1;
C.prototype.a = 1;
console.log(new A().a);
console.log(new B().a);
console.log(new C(2).a);

8、 阅读代码

function Parent() {
    this.a = 1;
    this.b = [1, 2, this.a];
    this.c = { demo: 5 };
    this.show = function () {
        console.log(this.a , this.b , this.c.demo );
    }
}
function Child() {
    this.a = 2;
    this.change = function () {
        this.b.push(this.a);
        this.a = this.b.length;
        this.c.demo = this.a++;
    }
}
Child.prototype = new Parent();
var parent = new Parent();
var child1 = new Child();
var child2 = new Child();
child1.a = 11;
child2.a = 12;
parent.show();
child1.show();
child2.show();
child1.change();
child2.change();
parent.show();
child1.show();
child2.show();

9、 阅读代码

function SuperType(){
    this.property = true;
}
SuperType.prototype.getSuperValue = function(){
    return this.property;
};
function SubType(){
    this.subproperty = false;
}
SubType.prototype = new SuperType();
SubType.prototype.getSubValue = function (){
    return this.subproperty;
};
var instance = new SubType();
console.log(instance.getSuperValue());