Allows to return a script evaluation (based on different fields) for each hit, for example:

GET /_search
{
    "query" : {
        "match_all": {}
    },
    "script_fields" : {
        "test1" : {
            "script" : {
                "lang": "painless",
                "source": "doc['price'].value * 2"
            }
        },
        "test2" : {
            "script" : {
                "lang": "painless",
                "source": "doc['price'].value * params.factor",
                "params" : {
                    "factor"  : 2.0
                }
            }
        }
    }
}

Script fields can work on fields that are not stored (my_field_name in the above case), and allow to return custom values to be returned (the evaluated value of the script).

Script fields can also access the actual _source document and extract specific elements to be returned from it by using params['_source']. Here is an example:

GET /_search
    {
        "query" : {
            "match_all": {}
        },
        "script_fields" : {
            "test1" : {
                "script" : "params['_source']['message']"
            }
        }
    }

Note the _source keyword here to navigate the json-like model.

It’s important to understand the difference between doc['my_field'].value and params['_source']['my_field']. The first, using the doc keyword, will cause the terms for that field to be loaded to memory (cached), which will result in faster execution, but more memory consumption. Also, the doc[...] notation only allows for simple valued fields (you can’t return a json object from it) and makes sense only for non-analyzed or single term based fields. However, using doc is still the recommended way to access values from the document, if at all possible, because _source must be loaded and parsed every time it’s used. Using _source is very slow.