ES5和ES6中的继承以及单例模式的实现

关于ES5实现继承的方式，之前的一篇文章《JavaScript 面向对象的那些事儿》已经总结过，这里为了对比和ES6的写法区别，重新再写一个例子。

一、继承

1、ES5 实现继承

var Car = function (color, name, model) {
    this.color = color
    this.name = name
    this. model = model
    this.print = function () {
        console.log('My car is ' + this.color + ' ' +this.name + ' ' +this. model)
    }
}

var myCar = function (color, name, model) {
     Car.call(this, color, name, model)
}

myCar.prototype = Object.create(Car.prototype)

var myAudi = new myCar('red', 'Audi', 'A4L')
myAudi.print()  // My car is red Audi A4L

2、ES6 实现继承

class Car {
    constructor (color, name, model) {
        this.color = color
        this.name = name
        this. model = model
    }

    print () {
        console.log(`My car is ${this.color}  ${this.name}  ${this.model}`)
    }
}

class MyCar extends Car {   // 通过extends实现继承
    constructor (color, name, model, price) {
        super(color, name, model)  // 实例化子类的时候把子类的数据传给父类
        this.price = price
    }

    getPrice () {
        console.log(`My car's price is  ${this.price}`)
    }
}

let myAudi = new MyCar('white', 'Audi', 'A4L', '$35000')
myAudi.getPrice()  // My car's price is $35000
myAudi.print()  // My car is white  Audi  A4L

二、单例模式的实现

单例就是保证一个类只有一个实例，实现的方法一般是先判断实例存在与否，如果存在直接返回，如果不存在就创建了再返回，这就确保了一个类只有一个实例对象。在JavaScript里，单例作为一个命名空间提供者，从全局命名空间里提供一个唯一的访问点来访问该对象。

单例模式的核心：保证一个类仅有一个一个实例，并提供一个访问它的全局访问点。

1、ES5的单例模式写法

var Car = function (color, name, model) {
    this.color = color
    this.name = name
    this. model = model
    this.instance = null
    this.print = function () {
        console.log('My car is ' + this.color + ' ' +this.name + ' ' +this. model)
    }
}

Car.getInstance = function(color, name, model) {
    if(!this.instance) {
        this.instance = new Car(color, name, model);
    }
    return this.instance;
}

var Audi = Car.getInstance('red','Audi', 'A4');
var Benz = Car.getInstance('white', 'Benz', 'C200');
Audi.print()  // My car is red Audi A4
Benz.print() // My car is red Audi A4

说明Audi和Benz指向的是唯一实例化的对象

2、ES6的单例模式写法

class Car {
    constructor (color, name, model) {
        this.color = color
        this.name = name
        this. model = model
        console.log('实例化会触发构造函数')
        this.getCarInfo()
         this.instance = null
    }

    static getInstance (color, name, model) {
        if (!this.instance) {
            this.instance = new Car(color, name, model)
        }
        return this.instance
    }

    print () {
        console.log(`My car is ${this.color}  ${this.name}`)
    }

    getCarInfo () {
        console.log(`My car is ${this.color}  ${this.name}  ${this.model}`)
    }
}

let myCar1 = Car.getInstance('red', 'Benz', 'C200L')
let myCar2 = Car.getInstance('white', 'Benz', 'E200L')
let myCar3 = Car.getInstance('black', 'Benz', 'C200L')
let myCar4 = Car.getInstance('graw', 'Benz', 'E200L')
// 只打印第一次
// 实例化会触发构造函数
// My car is red  Benz  C200L

myCar1.print()
myCar2.print()
myCar3.print()
myCar4.print()
// 会打印4次
// My car is red  Benz
//  My car is red  Benz
// My car is red  Benz
// My car is red  Benz

单例只执行一次构造函数，如上面例子中的打印结果可以看到，在一些实际开发需求中，如连接数据库操作，点击登录按钮弹出登录框操作等等。