深拷贝、浅拷贝

浅拷贝：将值的引用指向新的变量，当新的变量的值有改动，那么被拷贝的变量的值也会跟着 变动

深拷贝：申请一个新的内存空间，将被拷贝的值复制到新的内存中。拷贝变量的值改变，被拷贝的变量的值 不会改变

实现深度拷贝

    1、JSON.stringify()

function deepClone(obj) {

    let temp = JSON.stringify(obj);
    let result = JSON.parse(temp);
    
    return result;
}

    2、for...in 遍历和复制

function deepClone(obj) {
    let result = typeof  obj.splice === "function" ? [] : {};
    if( obj && typeof obj === 'object' ) {
        for( let key in obj) {
            if (obj[key] && typeof obj[key] === 'object') {
                result[key] = deepClone(obj[key]);
            } else {
                result[key] = obj[key];
            }
        }
        return result;
    }
    return obj;
}

    3、Array.prototype.forEach

function deepClone(obj) {
    let copy = Object.create(Object.getPrototypeOf(obj));
    let propNames = Object.getOwnPropertyNames(obj);
    propNames.forEach(function (items) {
        let item = Object.getOwnPropertyDescriptor(obj, items);
        Object.defineProperty(copy, items, item);

    });
    return copy;
}

实现浅拷贝

    Object.assign()

        如果对象或者数组只有一级属性，没有二级属性，那么assign()方法是深拷贝。像下面的第一个例子

        如果有二级属性，那么二级以后的属性就是浅拷贝。下面的第二个例子

let a = ["ASD", "666", "ECC", "BCC"];
let b = [];
Object.assign(b, a);
b[0] = "QQQ";
console.log(`Value of a: ${a}, Value of b: ${b}`); 
// Value of a: ["ASD", "666", "ECC", "BCC"], Value of b: ["QQQ", "666", "ECC", "BCC"]

let a = a = ["ASD", "666", "ECC", "BCC", {"name": 1, "age": 2}]
let b = [];
Object.assign(b, a);
b[4].name = "QQQ";
console.log(`Value of a: ${a}, Value of b: ${b}`); 
// Value of a: ["ASD", "666", "ECC", "BCC", {"name": "QQQ", "age": 2}], Value of b: ["ASD", "666", "ECC", "BCC", {"name": "QQQ", "age": 2}]