This article will give you a detailed understanding of deep copying in JavaScript

There are many articles about deep copying on the Internet, but the quality varies, many of them are not well thought out, and the writing methods are relatively crude and unsatisfactory. This article aims to complete a perfect deep copy. If you have any questions after reading it, please feel free to add and improve it.

To evaluate whether a deep copy is complete, please check whether the following questions have been implemented:

• Can basic type data be copied?

• Keys and values ​​are both basic typesOrdinary This article will give you a detailed understanding of deep copying in JavaScriptectsCan they be copied?

• SymbolCan the key of an This article will give you a detailed understanding of deep copying in JavaScriptect be copied? Can

• Date and RegExp This article will give you a detailed understanding of deep copying in JavaScriptect types be copied? Can

• Map and Set This article will give you a detailed understanding of deep copying in JavaScriptect types be copied?

• FunctionCan the This article will give you a detailed understanding of deep copying in JavaScriptect type be copied? (We generally don’t use deep copying for functions)

• Can the prototype of an This article will give you a detailed understanding of deep copying in JavaScriptect be copied?

• Non-enumerable propertiesCan it be copied?

• Circular referenceCan it be copied?

how? Is the deep copy you wrote perfect enough?

The final implementation of deep copy

The final code version is given directly here for the convenience of those who want to know quickly. Of course, if you want to understand step by step, you can continue to view the rest of the article:

function deepClone(target) {
    const map = new WeakMap()
    
    function isObject(target) {
        return (typeof target === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && target ) || typeof target === 'function'
    }

    function clone(data) {
        if (!isObject(data)) {
            return data
        }
        if ([Date, RegExp].includes(data.constructor)) {
            return new data.constructor(data)
        }
        if (typeof data === 'function') {
            return new Function('return ' + data.toString())()
        }
        const exist = map.get(data)
        if (exist) {
            return exist
        }
        if (data instanceof Map) {
            const result = new Map()
            map.set(data, result)
            data.forEach((val, key) => {
                if (isObject(val)) {
                    result.set(key, clone(val))
                } else {
                    result.set(key, val)
                }
            })
            return result
        }
        if (data instanceof Set) {
            const result = new Set()
            map.set(data, result)
            data.forEach(val => {
                if (isObject(val)) {
                    result.add(clone(val))
                } else {
                    result.add(val)
                }
            })
            return result
        }
        const keys = Reflect.ownKeys(data)
        const allDesc = Object.getOwnPropertyDescriptors(data)
        const result = Object.create(Object.getPrototypeOf(data), allDesc)
        map.set(data, result)
        keys.forEach(key => {
            const val = data[key]
            if (isObject(val)) {
                result[key] = clone(val)
            } else {
                result[key] = val
            }
        })
        return result
    }

    return clone(target)
}

1. The copy principle of JavaScript data types

First look at the JS data type diagram ( Except Object, all others are basic types):

In JavaScript, the copying of basic type values ​​is to directly copy a new and identical data. The two data are mutually exclusive. Independent and not affecting each other. The copying of reference type values ​​(Object type) is to pass the reference of the This article will give you a detailed understanding of deep copying in JavaScriptect (that is, the memory address where the This article will give you a detailed understanding of deep copying in JavaScriptect is located, that is, the pointer to the This article will give you a detailed understanding of deep copying in JavaScriptect), which is equivalent to multiple variables pointing to the same This article will give you a detailed understanding of deep copying in JavaScriptect. Then as long as one of the variables has a reference to the This article will give you a detailed understanding of deep copying in JavaScriptect When modified, the This article will give you a detailed understanding of deep copying in JavaScriptects pointed to by other variables will also be modified (because they point to the same This article will give you a detailed understanding of deep copying in JavaScriptect). As shown below:

2. Dark and shallow copy

Deep and shallow copy mainly targets the Object type, and the value of the basic type itself is copied exactly the same One copy, no distinction between dark and light copies. Here we first give the copy This article will give you a detailed understanding of deep copying in JavaScriptect for testing. You can use this This article will give you a detailed understanding of deep copying in JavaScript This article will give you a detailed understanding of deep copying in JavaScriptect to test whether the deep copy function you wrote is perfect:

// 测试的This article will give you a detailed understanding of deep copying in JavaScript对象
const This article will give you a detailed understanding of deep copying in JavaScript = {
    // =========== 1.基础数据类型 ===========
    num: 0, // number
    str: '', // string
    bool: true, // boolean
    unf: undefined, // undefined
    nul: null, // null
    sym: Symbol('sym'), // symbol
    bign: BigInt(1n), // bigint

    // =========== 2.Object类型 ===========
    // 普通对象
    This article will give you a detailed understanding of deep copying in JavaScript: {
        name: '我是一个对象',
        id: 1
    },
    // 数组
    arr: [0, 1, 2],
    // 函数
    func: function () {
        console.log('我是一个函数')
    },
    // 日期
    date: new Date(0),
    // 正则
    reg: new RegExp('/我是一个正则/ig'),
    // Map
    map: new Map().set('mapKey', 1),
    // Set
    set: new Set().add('set'),
    // =========== 3.其他 ===========
    [Symbol('1')]: 1  // Symbol作为key
};

// 4.添加不可枚举属性
Object.defineProperty(This article will give you a detailed understanding of deep copying in JavaScript, 'innumerable', {
    enumerable: false,
    value: '不可枚举属性'
});

// 5.设置原型对象
Object.setPrototypeOf(This article will give you a detailed understanding of deep copying in JavaScript, {
    proto: 'proto'
})

// 6.设置loop成循环引用的属性
This article will give you a detailed understanding of deep copying in JavaScript.loop = This article will give you a detailed understanding of deep copying in JavaScript

This article will give you a detailed understanding of deep copying in JavaScript The This article will give you a detailed understanding of deep copying in JavaScriptect is in Result in Chrome:

##2.1 Shallow copy

Shallow copy: Create a new Object to accept the This article will give you a detailed understanding of deep copying in JavaScriptect value that you want to re-copy or reference. If the This article will give you a detailed understanding of deep copying in JavaScriptect attribute is a basic data type, the value of the basic type is copied to the new This article will give you a detailed understanding of deep copying in JavaScriptect; but if the attribute is a reference data type, the address in the memory is copied. If one of the This article will give you a detailed understanding of deep copying in JavaScriptects changes the address pointed to by the memory, Object will definitely affect another This article will give you a detailed understanding of deep copying in JavaScriptect.

First let’s take a look at some shallow copy methods (for details, click on the hyperlinks to the corresponding methods):

MethodUse MethodNotes##Object.assign()Object.assign(target, ... sources) 2. The non-enumerable properties of the This article will give you a detailed understanding of deep copying in JavaScriptect will not be copied; Expand syntaxlet This article will give you a detailed understanding of deep copying in JavaScriptClone = { ...This article will give you a detailed understanding of deep copying in JavaScript };Object.assign ()Array.prototype.concat() copy arrayconst new_array = old_array.concat(value1[, value2[, ...[, valueN] ]])Array.prototype.slice() copy arrayarr.slice([begin[, end]])

这里只列举了常用的几种方式，除此之外当然还有其他更多的方式。注意，我们直接使用=赋值不是浅拷贝，因为它是直接指向同一个对象了，并没有返回一个新对象。

手动实现一个浅拷贝：

function shallowClone(target) {
    if (typeof target === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && target !== null) {
        const cloneTarget = Array.isArray(target) ? [] : {};
        for (let prop in target) {
            if (target.hasOwnProperty(prop)) {
                cloneTarget[prop] = target[prop];
            }
        }
        return cloneTarget;
    } else {
        return target;
    }
}


// 测试
const shallowCloneObj = shallowClone(This article will give you a detailed understanding of deep copying in JavaScript)

shallowCloneObj === This article will give you a detailed understanding of deep copying in JavaScript  // false，返回的是一个新对象
shallowCloneObj.arr === This article will give you a detailed understanding of deep copying in JavaScript.arr  // true，对于对象类型只拷贝了引用

从上面这段代码可以看出，利用类型判断（查看typeof），针对引用类型的对象进行 for 循环遍历对象属性赋值给目标对象的属性（for...in语句以任意顺序遍历一个对象的除Symbol以外的可枚举属性，包含原型上的属性。查看for…in），基本就可以手工实现一个浅拷贝的代码了。

2.2 深拷贝

深拷贝：创建一个新的对象，将一个对象从内存中完整地拷贝出来一份给该新对象，并从堆内存中开辟一个全新的空间存放新对象，且新对象的修改并不会改变原对象，二者实现真正的分离。

看看现存的一些深拷贝的方法：

方法1：JSON.stringify()

JSON.stringfy() 其实就是将一个 JavaScript 对象或值转换为 JSON 字符串，最后再用 JSON.parse() 的方法将JSON 字符串生成一个新的对象。（点这了解：JSON.stringfy()、JSON.parse()）

使用如下：

function deepClone(target) {
    if (typeof target === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && target !== null) {
        return JSON.parse(JSON.stringify(target));
    } else {
        return target;
    }
}

// 开头的测试This article will give you a detailed understanding of deep copying in JavaScript存在BigInt类型、循环引用，JSON.stringfy()执行会报错，所以除去这两个条件进行测试
const clonedObj = deepClone(This article will give you a detailed understanding of deep copying in JavaScript)

// 测试
clonedObj === This article will give you a detailed understanding of deep copying in JavaScript  // false，返回的是一个新对象
clonedObj.arr === This article will give you a detailed understanding of deep copying in JavaScript.arr  // false，说明拷贝的不是引用

浏览器执行结果：

从以上结果我们可知JSON.stringfy() 存在以下一些问题：

• 执行会报错：存在BigInt类型、循环引用。

• 拷贝Date引用类型会变成字符串。

• 键值会消失：对象的值中为Function、Undefined、Symbol 这几种类型，。

• 键值变成空对象：对象的值中为Map、Set、RegExp这几种类型。

• 无法拷贝：不可枚举属性、对象的原型链。

• 补充：其他更详细的内容请查看官方文档：JSON.stringify()

由于以上种种限制条件，JSON.stringfy() 方式仅限于深拷贝一些普通的对象，对于更复杂的数据类型，我们需要另寻他路。

方法2：递归基础版深拷贝

手动递归实现深拷贝，我们只需要完成以下2点即可：

• 对于基础类型，我们只需要简单地赋值即可（使用=）。

• 对于引用类型，我们需要创建新的对象，并通过遍历键来赋值对应的值，这个过程中如果遇到 Object 类型还需要再次进行遍历。

function deepClone(target) {
    if (typeof target === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && target) {
        let cloneObj = {}
        for (const key in target) { // 遍历
            const val = target[key]
            if (typeof val === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && val) {
                cloneObj[key] = deepClone(val) // 是对象就再次调用该函数递归
            } else {
                cloneObj[key] = val // 基本类型的话直接复制值
            }
        }
        return cloneObj
    } else {
        return target;
    }
}

// 开头的测试This article will give you a detailed understanding of deep copying in JavaScript存在循环引用，除去这个条件进行测试
const clonedObj = deepClone(This article will give you a detailed understanding of deep copying in JavaScript)

// 测试
clonedObj === This article will give you a detailed understanding of deep copying in JavaScript  // false，返回的是一个新对象
clonedObj.arr === This article will give you a detailed understanding of deep copying in JavaScript.arr  // false，说明拷贝的不是引用

浏览器执行结果：

该基础版本存在许多问题：

• 不能处理循环引用。

• 只考虑了Object对象，而Array对象、Date对象、RegExp对象、Map对象、Set对象都变成了Object对象，且值也不正确。

• 丢失了属性名为Symbol类型的属性。

• 丢失了不可枚举的属性。

• 原型上的属性也被添加到拷贝的对象中了。

如果存在循环引用的话，以上代码会导致无限递归，从而使得堆栈溢出。如下例子：

const a = {}
const b = {}
a.b = b
b.a = a
deepClone(a)

对象 a 的键 b 指向对象 b，对象 b 的键 a 指向对象 a，查看a对象，可以看到是无限循环的：

对对象a执行深拷贝，会出现死循环，从而耗尽内存，进而报错：堆栈溢出

如何避免这种情况呢？一种简单的方式就是把已添加的对象记录下来，这样下次碰到相同的对象引用时，直接指向记录中的对象即可。要实现这个记录功能，我们可以借助 ES6 推出的 WeakMap 对象，该对象是一组键/值对的集合，其中的键是弱引用的。其键必须是对象，而值可以是任意的。（WeakMap相关见这：WeakMap）

针对以上基础版深拷贝存在的缺陷，我们进一步去完善，实现一个完美的深拷贝。

方法3：递归This article will give you a detailed understanding of deep copying in JavaScript

对于基础版深拷贝存在的问题，我们一一改进：

	Description: Used to assign the values ​​of all enumerable properties from one or more source This article will give you a detailed understanding of deep copying in JavaScriptects to the target This article will give you a detailed understanding of deep copying in JavaScriptect. It will return the target This article will give you a detailed understanding of deep copying in JavaScriptect. 1. The inherited properties of the This article will give you a detailed understanding of deep copying in JavaScriptect will not be copied;	3. Symbol type properties can be copied.
	Defects and	Almost the same, but if the attributes are all basic type values, it will be more convenient to use the spread operator to perform shallow copy.
	Shallow copy, suitable for arrays of basic type values
	Shallow copy, suitable for arrays of basic type values

存在的问题	改进方案
1. 不能处理循环引用	使用 WeakMap 作为一个Hash表来进行查询
2. 只考虑了Object对象	当参数为 Date、RegExp 、Function、Map、Set，则直接生成一个新的实例返回
3. 属性名为Symbol的属性 4. 丢失了不可枚举的属性	针对能够遍历对象的不可枚举属性以及 Symbol 类型，我们可以使用 Reflect.ownKeys() 注：Reflect.ownKeys(This article will give you a detailed understanding of deep copying in JavaScript)相当于[...Object.getOwnPropertyNames(This article will give you a detailed understanding of deep copying in JavaScript), ...Object.getOwnPropertySymbols(This article will give you a detailed understanding of deep copying in JavaScript)]
4. 原型上的属性	Object.getOwnPropertyDescriptors()设置属性描述对象，以及Object.create()方式继承原型链

代码实现：

function deepClone(target) {
    // WeakMap作为记录对象Hash表（用于防止循环引用）
    const map = new WeakMap()

    // 判断是否为This article will give you a detailed understanding of deep copying in JavaScriptect类型的辅助函数，减少重复代码
    function isObject(target) {
        return (typeof target === 'This article will give you a detailed understanding of deep copying in JavaScriptect' && target ) || typeof target === 'function'
    }

    function clone(data) {

        // 基础类型直接返回值
        if (!isObject(data)) {
            return data
        }

        // 日期或者正则对象则直接构造一个新的对象返回
        if ([Date, RegExp].includes(data.constructor)) {
            return new data.constructor(data)
        }

        // 处理函数对象
        if (typeof data === 'function') {
            return new Function('return ' + data.toString())()
        }

        // 如果该对象已存在，则直接返回该对象
        const exist = map.get(data)
        if (exist) {
            return exist
        }

        // 处理Map对象
        if (data instanceof Map) {
            const result = new Map()
            map.set(data, result)
            data.forEach((val, key) => {
                // 注意：map中的值为This article will give you a detailed understanding of deep copying in JavaScriptect的话也得深拷贝
                if (isObject(val)) {
                    result.set(key, clone(val))
                } else {
                    result.set(key, val)
                }
            })
            return result
        }

        // 处理Set对象
        if (data instanceof Set) {
            const result = new Set()
            map.set(data, result)
            data.forEach(val => {
                // 注意：set中的值为This article will give you a detailed understanding of deep copying in JavaScriptect的话也得深拷贝
                if (isObject(val)) {
                    result.add(clone(val))
                } else {
                    result.add(val)
                }
            })
            return result
        }

        // 收集键名（考虑了以Symbol作为key以及不可枚举的属性）
        const keys = Reflect.ownKeys(data)
        // 利用 Object 的 getOwnPropertyDescriptors 方法可以获得对象的所有属性以及对应的属性描述
        const allDesc = Object.getOwnPropertyDescriptors(data)
        // 结合 Object 的 create 方法创建一个新对象，并继承传入原对象的原型链， 这里得到的result是对data的浅拷贝
        const result = Object.create(Object.getPrototypeOf(data), allDesc)

        // 新对象加入到map中，进行记录
        map.set(data, result)

        // Object.create()是浅拷贝，所以要判断并递归执行深拷贝
        keys.forEach(key => {
            const val = data[key]
            if (isObject(val)) {
                // 属性值为 对象类型 或 函数对象 的话也需要进行深拷贝
                result[key] = clone(val)
            } else {
                result[key] = val
            }
        })
        return result
    }

    return clone(target)
}



// 测试
const clonedObj = deepClone(This article will give you a detailed understanding of deep copying in JavaScript)
clonedObj === This article will give you a detailed understanding of deep copying in JavaScript  // false，返回的是一个新对象
clonedObj.arr === This article will give you a detailed understanding of deep copying in JavaScript.arr  // false，说明拷贝的不是引用
clonedObj.func === This article will give you a detailed understanding of deep copying in JavaScript.func  // false，说明function也复制了一份
clonedObj.proto  // proto，可以取到原型的属性

详细的说明见代码中的注释，更多测试希望大家自己动手尝试验证一下以加深印象。

在遍历 Object 类型数据时，我们需要把 Symbol 类型的键名也考虑进来，所以不能通过 Object.keys 获取键名或 for...in 方式遍历，而是通过Reflect.ownKeys()获取所有自身的键名（getOwnPropertyNames 和 getOwnPropertySymbols 函数将键名组合成数组也行：[...Object.getOwnPropertyNames(This article will give you a detailed understanding of deep copying in JavaScript), ...Object.getOwnPropertySymbols(This article will give you a detailed understanding of deep copying in JavaScript)]），然后再遍历递归，最终实现拷贝。

浏览器执行结果：

可以发现我们的cloneObj对象和原来的This article will give you a detailed understanding of deep copying in JavaScript对象一模一样，并且修改cloneObj对象的各个属性都不会对This article will give you a detailed understanding of deep copying in JavaScript对象造成影响。其他的大家再多尝试体会哦！

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