https://www.elastic.co/guide/cn/elasticsearch/guide/current/data-in-data-out.html
An object is a language-specific, in-memory data structure. To send it across the network or store it, we need to be able to represent it in some standard format. JSON is a way of representing objects in human-readable text. It has become the de facto standard for exchanging data in the NoSQL world. When an object has been serialized into JSON, it is known as a JSON document.