深拷贝对象

function deepClone(obj) {
  if (obj === null) return null;
  if (typeof obj !== 'object') return obj;
  if (obj instanceof Date) {
    let date = new Date();
    date.setTime(obj.getTime());
    return date;
  }
  if (obj instanceof RegExp) {
    let re = new RegExp(obj.source);
    re.lastIndex = obj.lastIndex;
    return re;
  }
  let newObj = new obj.constructor();
  for (let key in obj) {
    if (obj.hasOwnProperty(key)) {
      newObj[key] = deepClone(obj[key]);
    }
  }
  return newObj;
}