示例数据：

let jsonObj = 
{ "1234":
   [ { "_id": "5cad99fe0506582a4430491c",
       "created": "2019-04-10T07:20:20.600Z" },
     { "_id": "5cad9cd46a97d8470c09b817",
       "created": "2019-04-10T07:35:48.273Z" } ],
  "2345":
   [ { "_id": "5cad9d066a97d8470c09b819",
       "created": "2019-04-10T07:36:38.448Z" },
     { "_id": "5cad9d0e6a97d8470c09b81b",
       "created": "2019-04-10T07:36:46.250Z" } ] }

使用for in循环JSON对象：

for(let key in jsonObj){
	console.info(key);
	console.info(jsonObj[key]);
    console.info('================');
}

结果：

1234
[
	{ _id: "5cad99fe0506582a4430491c", created: "2019-04-10T07:20:20.600Z" }
	​{ _id: "5cad9cd46a97d8470c09b817", created: "2019-04-10T07:35:48.273Z" }
]
================
2345
[
	{ _id: "5cad9d066a97d8470c09b819", created: "2019-04-10T07:36:38.448Z" }
	{ _id: "5cad9d0e6a97d8470c09b81b", created: "2019-04-10T07:36:46.250Z" }
]
================