JavaScript浅拷贝与深拷贝

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1.  什么是浅拷贝，深拷贝？

浅拷贝：只复制一层对象，即只复制引用，没有复制真正的值。
( 浅拷贝仅仅是指向被复制的内存地址，如果原对象被改变了，那么新对象也会相应改变 )

深拷贝：彻底复制一个对象，拷贝多层，而不是只复制对象的引用。
（ 深拷贝建一个新的对象，将原对象的各属性的值拷贝过来，改变原对象的值时，不会改变新对象的值 ）

举个例子：

let drinking = {
     
    name : "milk",
    volume : "250mL",
    pride : {
     
        value : 8
    }
};

function deepCopy(object) {
     
    let obj = object instanceof Array ? [] : {
     };
    for (const [key, val] of Object.entries(object)) {
     
      obj[key] = typeof val === "object" ? deepCopy(val) : val;
    }
    return obj;
};

let drinking1 = drinking;
let drinking2 = deepCopy(drinking);
console.log("drinking: ", drinking);
console.log("drinking1: ", drinking1);
console.log("drinking2: ", drinking2);

drinking.name = "coffee";
console.log("drinking: ", drinking);
console.log("drinking1: ", drinking1);
console.log("drinking2: ", drinking2);

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2. 常用的浅拷贝方法

2.1） ‘=’ 赋值

let drinking = {
     
	name : "milk",
    pride : 8
};
let drinking1 = drinking;

drinking1.name = "coffee";
console.log("drinking: ", drinking);

2.2) 扩展运算符

我们可以使用扩展运算符 … 来进行浅拷贝
需要注意的是：
扩展运算符 … 可以进行一层的深拷贝

let drinking = {
     
    name : "milk",
    pride : 8,
    volume : {
     
        value : "250mL"
    }
};

let drinking1 = {
     ...drinking};
drinking1.name = "water";
drinking1.volume.value = "550mL";
console.log("drinking_name: ", drinking.name); // milk，因为进行了一层的深拷贝
console.log("drinking_volume_value: ", drinking.volume.value); // 到了一下层，所以是浅拷贝

2.3) Object.assign()

Object.assign() 将一个或多个对象的属性复制给目标对象。

• 接受一个参数时：浅拷贝

let drinking = {
     
    name : "milk",
    pride : 8,
    volume : {
     
        value : "250mL"
    }
};

let drinking2 = {
     
    name2 : "tea"
}

let drinking1 = Object.assign(drinking2,drinking);
drinking1.name = "water";
drinking1.volume.value = "550mL";
console.log("drinking_name: ", drinking.name);
console.log("drinking_volume_value: ", drinking.volume.value);

• 接受多个参数时：

需要注意的是：
Object.assign() 会进行一层的深拷贝，剩下层都是浅拷贝。

let drinking = {
     
    name : "milk",
    pride : 8,
    volume : {
     
        value : "250mL"
    }
};

let drinking1 = Object.assign({
     }, drinking);
drinking1.name = "water";
drinking1.volume.value = "550mL";
console.log("drinking_name: ", drinking.name); // milk， 因为是第一层，进行了一次深拷贝
console.log("drinking_volume_value: ", drinking.volume.value); // 550mL，因为到了下一层，进行浅拷贝

2.4) for … in

会进行一层深拷贝，剩下层都是浅拷贝。

let drinking = {
     
    name : "milk",
    pride : 8,
    volume : {
     
        value : "250mL"
    }
};

let drinking1 = {
     };
for (const key in drinking) {
     
    drinking1[key] = drinking[key];
};

drinking1.name = "coffee";
drinking1.volume.value = "550mL";
console.log("drinking_name: ", drinking.name);
console.log("drinking_volume_value: ", drinking.volume.value);

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3. 常用的深拷贝方法

3.1) 递归

let drinking = {
     
    name : "milk",
    volume : "250mL",
    pride : {
     
        value : 8
    }
};

function deepCopy(object) {
     
    let obj = object instanceof Array ? [] : {
     };
    for (const [key, val] of Object.entries(object)) {
     
      obj[key] = typeof val === "object" ? deepCopy(val) : val;
    }
    return obj;
};

let drinking1 = deepCopy(drinking);
drinking1.name = "coffee";
drinking1.pride.value = 12;

console.log("drinking1: ", drinking1);
console.log("drinking: ", drinking);

3.2) JSON做字符串转换

利用 JSON.stringify() 和 JSON.parse()
需要注意的是：
只有可以转成JSON格式的对象才可以这样用，像function没办法转成JSON，就不可以。

let drinking = {
     
    name : "milk",
    volume : "250mL",
    pride : {
     
        value : 8
    }
};

let strDrinking = JSON.stringify(drinking);
let drinking1 = JSON.parse(strDrinking);
console.log("drinking1", drinking1);

drinking1.name = "coffee";
drinking1.pride.value = 12;
console.log("drinking1", drinking1);
console.log("drinking", drinking);