关于javascript:js深浅拷贝梳理

const originArray = [1, 2, [6, 7], 3, 4, 5];
let shallowCopyArr = [];
let deepCopyArr = [];

const originObject = {a: 1, b: {c: 3} };
let shallowCopyObj = {};
let deepCopyObj = {}

const arrayLike = {
  0: "AAA",
  1: [1, 2],
  2: "BBB",
  length: 3
};
let shallowCopyArrLike = [];
let deepCopyArrLike = [];

1. 浅拷贝

援用的复制

• 浅拷贝实现：赋值操作符
• “ 首层浅拷贝 ” 实现：
  对象：遍历、Object.assign、扩大运算符
  数组（类数组对象）：遍历、扩大运算符、Array.from、slice、concat、filter、map、reduce 等数组办法
  ( 留神：所有的数组规范办法都实用于数组和类数组对象，只有 concat 例外, 具体应用参考 js 伪数组对象)

对象以遍历为例，数组以 concat 为例

for (const prop in originObject) {if (originObject.hasOwnProperty(prop)) {shallowCopyObj[prop] = originObject[prop];
    }
}
shallowCopyObj.a = 2;
shallowCopyObj.b.c = 4;
shallowCopyObj  // {a: 2, b: {c: 4} }
originObject    // {a: 1, b: {c: 4} }

----------

arrayLike[Symbol.isConcatSpreadable] = true;
shallowCopyArrLike = [].concat.call(arrayLike, [7, 8]);
shallowCopyArrLike.push(8)
shallowCopyArrLike[2].push(3)
shallowCopyArrLike   // ['AAA', [1,2,3], 'BBB', 7, 8, 3]
arrayLike
// const arrayLike = {
//  0: "AAA",
//  1: [1, 2, 3],
//  2: "BBB",
//  length: 3
// };

2. 深拷贝

堆内存的重新分配

实现：对象、数组、类数组对象通用

• 递归
• JSON.parse, JSON.stringify
• jquery.extend()

JSON.parse, JSON.stringify 实现：

const originObject = {a: 1, b: { c: 3}, d: undefined, e: Array, f: Symbol('foo') };
const originArray = [1, 2, [6, 7], 3, 4, 5, undefined, Array];
deepCopyObj = JSON.parse(JSON.stringify(originObject));
deepCopyArr = JSON.parse(JSON.stringify(originArray));

deepCopyObj.a = 2;
deepCopyObj.b.c = 4;
deepCopyObj  // {a: 2, b: {c: 4} }
deepCopyArr  // [1, 2, [6, 7], 3, 4, 5, null, null]

实现限度：1. 非数组对象的属性值为 undefined、函数、Symbol 值时在序列化时被疏忽, 数组中会转化为 null
2. 无奈解决循环援用的对象

递归实现：
关键点在于

• 拷贝环问题
• Date、Reg 等援用类型的拷贝
• 函数的拷贝

(1) 解决拷贝环问题: 应用 Map/WeakMap，举荐应用 WeakMap

const obj = {
    a:1,
    c: [1,2],
    d: {e: 1,},
    f: null,
};
obj.g = obj;
function isObject(target) {return (typeof target === 'object' || typeof target === 'function') && target !== null
}
function clone(target) {
    // WeakMap 对键名的援用为弱援用，有利于垃圾回收机制
    const vm = new WeakMap();
    function deepClone(target) {if (isObject(target)) {if (vm.has(target)) return vm.get(target);
            let cloneTarget = Array.isArray(target) ? [] : {};
            vm.set(target, cloneTarget);
            for (const key in target) {cloneTarget[key] = deepClone(target[key]);
            }
            return cloneTarget;
        } else {return target;}
    };
    return deepClone(target);
}
const obj1 = clone(obj)
obj1.c[1] = 5;
obj1.f = [1,2];
console.log(obj1)  // {a: 1, c: [1,5], d: {e: 1}, f: [1,2], g: {...} }

(2) Date、Reg 等援用类型的拷贝
应用 constructor/Object.prototype.toString.call 判断类型，依据不同援用类型做相应解决
未完待续……..

(3) 函数、不可枚举属性等的拷贝
有这工夫钻研不如应用现成的工具库 lodash.cloneDeep