Parsing of custom objects in js

This article mainly introduces the analysis of custom objects in js, which has certain reference value. Now I share it with everyone. Friends in need can refer to it

1. Objects

　1. Literal created object

var person = {
    name: "sun",
    age: 18,
    work: function () {
        console.log(this.name + "is working...");
    },
    address: {
        home: "大屯里xxx路xxx小区xx单元xxx室",
        phone: "123456789",
    }
};
person.work();
console.log(person.address.home);

　2.Data description and access description setting

var person = {
    age: 18,
    address: {
        home: "大屯里xxx路xxx小区xx单元xxx室",
        phone: "123456789",
    }
};
Object.defineProperties(person, {
    name: {
        value: "sun",        // 该属性的值，可被读取
        writable: true,      // 表示能否修改属性的值，默认值为true
        configurable: true,  // 表示能否delete该属性并重新定义，直接在对象上定义的属性默认值为true
        enumerable: true     // 表示能否通过for-in枚举，直接在对象上定义的属性默认值为true
    },
    work: {
        value: function(){
            console.log(this.name + "is working...");
            },
        // 通过Object.defineProperty和Object.defineProperties定义属性，
        // 如果没有声明writable、configurable、enumerable，它们的默认值都是false
    }
});
person.work();
console.log(person.address.home);

　3.get and set

var circle = {
    value: 10,
    get girth(){
        return 2 * 3.14 * this.R
    },
    get area(){
        return 3.4 * this.R * this.R
    },
};
Object.defineProperty(circle, "R", {
    get : function () {
        return this.value;
    },
    set : function (val) {
        console.log("半径被修改了！");
        this.value = val;
    }
});
circle.R = 100;
console.log("girth: " + circle.girth + "area: " + circle.area);

　4.Data description and access description Check

var circle = {
    R: 10,
    // __proto__: null,
    get area(){
        return 3.4 * this.R * this.R
    },
};
Object.defineProperty(circle, "site", {
    value: [0, 2.2, 4.1],
    // enumerable: true,  // 是否可配置（读取），不设置为true时，Object.keys(circle))和Object.values(circle))将获取不到该键值对
});
console.log("R" in circle);  // 检查属性
console.log(circle.hasOwnProperty("R"));  // 检查自有的属性
console.log(circle.propertyIsEnumerable("R"));   // 检查属性是否是可枚举的
// Object对象的方法
console.log(Object.keys(circle));
console.log(Object.values(circle));
console.log(Object.getOwnPropertyNames(circle));  // 检查对象自身所有属性
console.log(Object.getOwnPropertyDescriptor(circle, "R")); // 得到circle对象关于R属性的描述

2. prototype

1.prototype definition

- 每一次创建函数，解析器都会向函数中添加一个属性:prototype
- 如果函数作为普通函数调用prototype，没有任何作用
- 当该函数以构造函数的形式调用时，它会有一个隐含的属性__proto__指向其原型对象
- 每个实例有各自的__proto__指向原型对象的prototype, 也就是原型对象中的属性和方法被调用函数"共享"
- 当类的原型对象prototype指向的内存地址发生改变时，已创建实例的__proto__ !== prototype，也就是不会被覆盖。而新创建的实例仍然是__proto__ === prototyp

function Person(name, age) {
    this.name = name;
    this.age = age;
}
// Person.prototype.gender = "male";
// Person.prototype.sayHello = function () {
//     return this.name + ", " + this.age + "years old."
// };
Person.prototype = {
    gender: "male",
    sayHello: function () {
        return this.name + ", " + this.age + "years old."
    }
};
var p1 = new Person("孙悟空", 2000);
p1.sayHello();
console.log(Person.prototype);
console.log(Person.prototype.constructor === Person);

2.prototype and __proto__

function Person() {}
var obj1 = { gender: "male"};  // 创建两个内存地址
var obj2 = { age: 200 };
Person.prototype = obj1;
var p1 = new Person();
console.log(p1.__proto__ === Person.prototype);
console.log(p1.__proto__.gender);
console.log(Person.prototype);
Person.prototype = obj2;
var p2 = new Person();
console.log(p2.__proto__.age);
console.log(Person.prototype);
console.log(p1.__proto__.age);    // undefined
console.log(p2.__proto__.gender); // undefined
console.log(p1.__proto__ === Person.prototype);  // false，表示当prototype指向的内存地址改变时，已经创建的实例对象的__proto__仍指向原来的内存地址
console.log(p2.__proto__ === Person.prototype);

function Person() {}
Person.prototype = {name: "xxx", age: 100,};
var p1 = new Person();
console.log(p1.__proto__.name);
Person.prototype = { price: 998,};
var p2 = new Person();
console.log(p2.__proto__.price);
console.log(p1.__proto__.price);  // undefined
console.log(p2.__proto__.name);   // undefiend
console.log(p1.__proto__ === Person.prototype);  // false, 原型对象的内存地址引用已发生改变
console.log(p1.__proto__.age);    // __proto__指向的内存地址被保留
console.log(p2.__proto__ === Person.prototype);  // true

function Person() {}
Person.prototype = { price: 60 };
var p1 = new Person();
Person.prototype = { price: 998};
var p2 = new Person();
console.log(p1.__proto__ === Person.prototype);  // 依然是false
console.log(p2.__proto__ === Person.prototype);  // true

3. Sharability of prototype

// prototype非常类似python中的静态属性和静态方法。每个实例都可以访问同一块内存空间。
function Person() {}
Person.prototype = {price: 60};
var p1 = new Person();
var p2 = new Person();
console.log(p1.__proto__.price);
console.log(p2.__proto__.price);
console.log(Person.prototype.price);

4. Inheritance of prototype

// 当访问实例对象的一个属性或方法时，它会先在对象自身中查找，如果有则直接使用；如果没有则在原型对象中继续查找，如果有则直接使用
function Person() {}
Person.prototype = {price: 60};
var p1 = new Person();
var p2 = new Person();
console.log(p1.price);
console.log(p2.price);
console.log(Person.prototype.price);

3. Class

1. Class encapsulation

// 字面量方法(工厂方法) -- 直接在var obj = {}内部写代码，缺点是只实例化一次
// 构造函数方法        -- 只用构造函数声明this，缺点是可扩展性差，数据重复
// 原型方法           -- 只用prototype声明共有的属性和方法，缺点是实例的数据相同，不满足多态

1. Mixed constructor/prototype method

// 最广泛的使用方法
function Person(name, age) {
    this.name = name;
    this.age = age;
}
// prototype写在外面是为了保证其动态增加公共属性和方法
Person.prototype.sayHello = function () {
    console.log(this.name + ", " + this.age + " years old.");  // 把共有的属性和方法封装到prototype中
};
var p = new Person("孙悟空", 2000);
p.sayHello();

// 我把它写给Person的属性，让父类也能够访问
function Person(name, age) {
    Person.group = Person.prototype.group = "西天取经组";
    Person.toString = Person.prototype.toString = function (){
        console.log("Person: " + Person.group)
    };
    this.name = name;
    this.age = age;
    this.sayHello = function () {
      console.log(this.name + ", " + this.age + "years old.")
    };
}
var person = new Person("孙悟空", 2000);
console.log(person.constructor); // 检查构造器函数
console.log(person instanceof Person);  // 检查是否为其原型类
person.sayHello();
Person.toString();

2. Dynamic prototype method

// 也是常用的方法
function Person(name, age) {
    this.name = name;
    this.age = age;
    if (typeof Person._initialized === "undefined"){
        Person.prototype.sayHello = function () {
            console.log(this.name + ", " + this.age + " years old.");
        };
        Person._initialized = true;
    }
}
var p = new Person("孙悟空", 2000);
p.sayHello();

3. Mixed factory Method

// 混合工厂方法  -- 存在与工厂方法类似的问题，不建议使用
function Person(name, age) {
    var obj = {};
    obj.name = name;
    obj.age = age;
    obj.sayHello = function () {
        console.log(this.name + ", " + this.age + " years old.");
    };
    return obj
}
var p = new Person("孙悟空", 2000);
p.sayHello();

　4. Discuss the class structure again

function Person(name, age) {
    // 静态属性
    Person.group = "西天取经四人组，暗合金木水火土";
    // 静态方法
    Person.introduce = function () {
        console.log("贫僧自东土大唐而来")
    };
    // 实例属性
    this.name = name;
    this.age = age;
    // 实例方法，应该写在prototype中
    this.say = function () {
        console.log("hello, i'm " + this.name);
    };
    Person.prototype.introduce = Person.introduce; // 此时Person类和其实例都可以使用introduce方法
    // 父类使用实例方法
    Person.example = Person.prototype.example = function (self) {
        self = self || this;
        console.log(self.name + " " + self.age);
    }
}
// 在python中，实例可以访问父类的属性和方法，父类也可以使用实例方法
// 在java和js中，实例不能调用父类的静态属性和静态方法，父类不能使用实例方法
// 如果想让实例和父类共享一个属性或者方法，就只能放到方法区并创建引用
var sun = new Person("孙悟空", 2000);
Person.introduce();  // 父类调用静态方法
sun.say();
sun.introduce();     // 实例调用静态方法
Person.example(sun); // 父类调用实例方法
sun.example();       // 子类调用实例方法
// 可见，prototype是父类和实例的沟通桥梁

　2. Custom class

function Person(name, age) {
    this.name = name;
    this.age = age;
    this.sayHello = function () {
      console.log(this.name + ", " + this.age + "years old.")
    };
}
function New(Person) {
    return function () {
        var obj = {"__proto__": Person.prototype};  // 必须写在这里
        Person.apply(obj, arguments);  // arguments同this一样，是默认自带的关键字，用于存储传入的参数
        return obj
    }
}
var temp = New(Person);
var p1 = temp("孙悟空", 2000);
var p2 = temp("猪八戒", 1);
p1.sayHello();
p2.sayHello();

　3. Class inheritance

　1.Copy inheritance literal Object (instance)

var person = {
    name: "Li",
    age: 16,
    address: {
        home: "none",
        city: "none",
    },
    say: function(){
        console.log("hello, guy.")
    }
};
var child = {gender:"female",};
function extendDeeply (p, c){
    var c = c || {};
    for (var prop in p) {
        if (typeof p[prop] === "object") {
            c[prop] = (p[prop].constructor === Array) ? [] : {};
            extendDeeply(p[prop], c[prop]);
        } else {
            c[prop] = p[prop];
        }
    }
}
extendDeeply(person, child);
console.log(child);
child.say();

　2.call and apply implement object inheritance

function Person(name, age) {
    this.name = name;
    this.age = age;
    this.address = {
        home: "none",
        city: "none",
    }
}
Person.prototype.say = function () {
    console.log("hello, guy.")
};
// 它继承的只是实例对象this，无法继承父类原型prototyp
function Child(name, age) {
    Person.call(this, name, age);
    this.gender = "female";
}
var child = new Child("Li", 16);
console.log(child);
// child.say(); 报错: child.say is not a function.

对象继承的缺点：只继承了实例对象的可访问的属性和方法，没有继承原型

　3.Prototype chain inheritance

// 原型链继承
function Person() {}
Person.prototype.name = "Person";
Person.prototype.toString = function () {
    console.log(this.name);
};
function Child(name, age) {
    this.age = age;
    this.name = name;
}
Child.prototype = Person.prototype;
Child.prototype.constructor = Child;
var child = new Child("Li", 16);
console.log(child.name + " " + child.age);
child.toString();
// 其缺点是之继承了原型，没有继承实例

　4.create implements class inheritance

function Person(name, age) {
    this.name = name;
    this.age = age;
    this.address = {
        home: "none",
        city: "none",
    }
}
Person.prototype.say = function () {
    console.log("hello, guy.")
};
function Child(P, name, age) {
    function F() {}
    F.prototype = new P(name, age);
    var c = new F();
    return c;
}
Child.prototype.constructor = Child;  // 无法修正
var child = new Child(Person, "Li", 16);
console.log(child);
console.log(child.name);
child.say();
console.log(child.constructor);  // 结果为[Function: Person]，构造器指向无法修正
console.log(child instanceof Child);  // false
console.log(child instanceof Person); // true

5. Object.create implements class inheritance – the recommended way

// Object.create继承，实现原理和上面的create类似
// 1.创建父类
function Person() {}
Person.prototype.sayPerson = function () {
    console.log("hello, Person.")
};
// 2.创建子类
function Child(gender) {this.gender = gender;}
// 3.create继承
// Object.create的第二个参数是属性描述
Child.prototype = Object.create(Person.prototype, {
    name: {
        value: "Li",
        writable: true,
        enumerable: true,
        configurable: true,
    },
    age: {
        value: 16,
        writable:true,
        configurable:true,
        enumerable:true,
    },
});  // 重写子类prototype
Child.prototype.constructor = Child;  // constructor 修正
// 4.在create之后写子类的prototype
Child.prototype.sayChild = function () {
    console.log("hello, Child.")
};
var child = new Child("female");
console.log(child);
console.log(child.name + " " + child.age);
child.sayChild();
child.sayPerson();

5. Combination inheritance – the recommended way

function Person(name, age) {
    this.name =name;
    this.age = age;
}
Person.prototype.toString = function () {
    console.log(this.name + " " + this.age);
};
function Child(name, age, gender) {
    Person.call(this, name, age);
    this.gender = gender;
}
Child.prototype = new Person();  // new时不传参数，是为了只继承原型，即Child.prototype = Person.prototype
// Child.prototype = Person.prototype;  // 两者等价
Child.prototype.constructor = Child;
var child = new Child("Li", 16, "female");
console.log(child);
child.toString();
console.log(child instanceof Child);   // true
console.log(child instanceof Person);  // true

6. Inheritance summary

js继承需要继承两部分内容：　　
- 一部分是父类构造函数中的this定义属性和方法，相当于继承初始化的数据　　
- 另一部分是父类的prototype，相当于继承实例方法- 要实现this的继承，可以用call(apply)；要实现prtotype的继承，可以用原型链
- 要实现两者的继承，可以用this+prototype的组合方式，Object.create本质上也是这种思路

7. The relationship between prototype, constructor and __proto__ in inheritance

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