SOURCE

//https://blog.csdn.net/weixin_34326558/article/details/93181568
var parents = [];//专门用来记录parent里出现循环引用的位置
var children = [];//记录已被克隆的内容
var clone = (parent)=>{
    if(parent == null)return null;
    if(typeof parent !== 'object'){return parent;}
    let child,proto;
    if(isType(parent,'Array')){
        child = [];
    }else if(isType(parent,'Function')){
        child = parent;
        return child;
    }else if(isType(parent,'Date')){
        child = new Date(parent.getTime());
        return child;
    }else if(isType(parent,'RegExp')){
        child = new Date(parent.source,getRegExp(parent));
        return child;
    }else{
        proto = Object.getPrototypeOf(parent);
        child = Object.create(proto);
    }
    
    //处理循环引用、递归
    let index = parents.indexOf(parent);
    if(index != -1){
        //递归过程中发现循环引用，这个引用一定是之前递归里已经克隆的内容，存在了children中
        return children[index];
    }
    parents.push(parent);
    children.push(child);
    for(let i in parent){
        //let in语法，对数组、对象都适用
        child[i] = clone(parent[i]);
    }
    return child
    
}