extends 继承

借助构造函数实现继承

原理： 将父级构造函数的 this 指向子类

function Parent () {
    this.name = 'parent';
}
Parent.prototype.say = function () {}
function Child () {
   // call改变执行上下文，this指向
   Parent.call(this);//在子类中调用父类
   this.type = 'child';
}

总结：实现部分继承，但是无法继承父类的原型方法

借助原型链实现继承

原理：每个函数都有prototype属性(是个Object对象)，prototype是子类构造函数的属性，Parent的实例赋值给这个属性，即new Child().__proto__ = Child.prototype = new Parent

function Parent () {
    this.name = 'parent';
    this.play = [1, 2, 3];
}
Parent.prototype.say = function () {}
function Child () {
   this.type = 'child';
}
Child.prototype = new Parent();
console.log(Child.prototype === new Child().__proto__);
var c1 = new Child();
var c2 = new Child();
console.log(c1.play, c2.play);
c1.play.push(4);
console.log(c1.play, c2.play);

总结：继承的原型链引用同一个原型对象，多个实例更改其中一个会引起其他实例也发生更改。

组合方式继承

组合构造函数继承和原型链继承方式继承

function Parent () {
    this.name = 'parent';
    this.play = [1, 2, 3];
}
Parent.prototype.say = function () {}
function Child () {
   Parent.call(this);
   this.type = 'child';
}
Child.prototype = new Parent();
console.log(new Child());
var c1 = new Child();
var c2 = new Child();
console.log(c1.play, c2.play);
c1.play.push(4);
console.log(c1.play, c2.play);

总结：父类被执行了两次，出现不必要的冗余

组合继承优化方式

function Parent () {
    this.name = 'parent';
    this.play = [1, 2, 3];
}
Parent.prototype.say = function () {}
function Child () {
   Parent.call(this);
   this.type = 'child';
}
Child.prototype = Parent.prototype;
console.log(new Child());
var c1 = new Child();
var c2 = new Child();
console.log(c1.play, c2.play);
c1.play.push(4);
console.log(c1.play, c2.play);

总结：子类没有constructor，子类、父类共用一个原型类

组合继承优化方式2

Object.create() 方法会使用指定的原型对象及其属性去创建一个新的对象。

function Parent () {
    this.name = 'parent';
    this.play = [1, 2, 3];
}
Parent.prototype.say = function () {}
function Child () {
   Parent.call(this);
   this.type = 'child';
}
Child.prototype = Object.create(Parent.prototype);
Child.prototype.constructor = Child;
console.log(new Child());
var c1 = new Child();
var c2 = new Child();
console.log(c1.play, c2.play);
c1.play.push(4);
console.log(c1.play, c2.play);

总结: 完美解决上述所有的问题