面向对象

类的声明

使用构造函数声明

function Animal(name){
    this.name = name;
}

使用 class 声明类

class Animal {
    constructor(name) {
        this.name = name;
    }
}

类的实例化

使用 new 操作符来实例化一个类

const animal = new Animal('dog');

类的继承

借助构造函数实现继承

function Animal(name){
    this.name = name;
}

Animal.prototype.say = function() {
    console.log("I'm Animal");
};

function Dog() {
    Animal.call(this);
    this.eyes = 2;
}
console.log(new Dog().say()); // 报错

缺点：通过这种方式实现的继承是部分继承，只能继承构造函数中的内容，不能继承父类原型链上的属性。

借助原型链实现继承

function Animal(name){
    this.name = name;
    this.numbers = [1, 2, 3];
}

Animal.prototype.say = function() {
    console.log("I'm Animal");
};

function Dog() {
    this.eyes = 2;
}

Dog.prototype = new Animal();

缺点：如果通过 Dog 实例化两个对象，当其中一个对象对原型中的数据发生修改时，因为两个实例拥有同一个原型，所以第二个实例也会发生对应的变化。

const dog1 = new Dog();
const dog2 = new Dog();
dog1.numbers.push(4); // [1, 2, 3, 4]
console.log(dog2.numbers); // [1, 2, 3, 4]

组合方式

function Animal(name){
    this.name = name;
    this.numbers = [1, 2, 3];
}

Animal.prototype.say = function() {
    console.log("I'm Animal");
};

function Dog() {
    Animal.call(this);
    this.eyes = 2;
}

Dog.prototype = new Animal();

const dog1 = new Dog();
const dog2 = new Dog();
dog1.numbers.push(4); // [1, 2, 3, 4]
console.log(dog2.numbers); // [1, 2, 3]

缺点：父类声明了两次

组合方式优化

function Animal(name){
    this.name = name;
    this.numbers = [1, 2, 3];
}

Animal.prototype.say = function() {
    console.log("I'm Animal");
};

function Dog() {
    Animal.call(this);
    this.eyes = 2;
}

Dog.prototype = Animal.prototype;

const dog1 = new Dog();
const dog2 = new Dog();
dog1.numbers.push(4); // [1, 2, 3, 4]
console.log(dog2.numbers); // [1, 2, 3]

缺点：实例对象的 constructor 指向父类而不是指向构造函数

组合方式2

function Animal(name){
    this.name = name;
    this.numbers = [1, 2, 3];
}

Animal.prototype.say = function() {
    console.log("I'm Animal");
};

function Dog() {
    Animal.call(this);
    this.eyes = 2;
}

Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;

const dog1 = new Dog();
const dog2 = new Dog();
dog1.numbers.push(4); // [1, 2, 3, 4]
console.log(dog2.numbers); // [1, 2, 3]