ex:递归实现对象深拷贝
function deepCopy(target,source){
for(let index in source){
if(typeof source[index] === "object"){
target[index] = {};
deepCopy(target[index],source[index])
}else{
target[index] = source[index];
}
}
}
let b = {a:1,d:{b:1}};
let a = {};
deepCopy(a,b);
a.d.b = 4;
console.log(b)`