SOURCE

const myObject = {
    name: 'Harry',
    talk: function() {
        console.log('hello world')
    }
}
myObject.myObject = myObject; // 循环引用

/**
 * 乞丐版
 * 无法拷贝函数、处理循环引用
 */
const deepClone0 = (obj) => JSON.parse(JSON.stringify(obj));

/**
 * for-in
 * 没有处理循环引用、函数没有深拷贝
 */
const deepClone1 = (obj) => {
    if (typeof obj === 'object') {
        let newObj = Array.isArray(obj) ? [] : {};
        for (const key in obj) {
            newObj[key] = deepClone1(obj[key]);
        }
        return newObj;
    } else {
        return obj;
    }
};

const deepClone2 = (obj, map = new Map()) => {
    if (typeof obj === 'object') {
        let newObj = Array.isArray(obj) ? [] : {};
        if (map.get(obj)) {
            return map.get(obj);
        }
        map.set(obj, newObj);
        for (const key in obj) {
            newObj[key] = deepClone2(obj[key], map);
        }    
        return newObj;
    } else {
        return obj;
    }
};
const old = {a:1, b:{a:123}, c:[1,2,3], d:function(){console.log(1)}};
old.old = old;
const aa = deepClone2(old);
console.log(aa.d());