elasticsearch/docs/reference/scripting-languages/painless/painless-debugging.md

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https://www.elastic.co/guide/en/elasticsearch/painless/current/painless-debugging.html

Painless debugging [painless-debugging]

Debug.Explain [_debug_explain]

Painless doesn’t have a REPL and while it’d be nice for it to have one day, it wouldn’t tell you the whole story around debugging painless scripts embedded in Elasticsearch because the data that the scripts have access to or "context" is so important. For now the best way to debug embedded scripts is by throwing exceptions at choice places. While you can throw your own exceptions (throw new Exception('whatever')), Painless’s sandbox prevents you from accessing useful information like the type of an object. So Painless has a utility method, Debug.explain which throws the exception for you. For example, you can use _explain to explore the context available to a script query.

PUT /hockey/_doc/1?refresh
{"first":"johnny","last":"gaudreau","goals":[9,27,1],"assists":[17,46,0],"gp":[26,82,1]}

POST /hockey/_explain/1
{
  "query": {
    "script": {
      "script": "Debug.explain(doc.goals)"
    }
  }
}

Which shows that the class of doc.first is org.elasticsearch.index.fielddata.ScriptDocValues.Longs by responding with:

{
   "error": {
      "type": "script_exception",
      "to_string": "[1, 9, 27]",
      "painless_class": "org.elasticsearch.index.fielddata.ScriptDocValues.Longs",
      "java_class": "org.elasticsearch.index.fielddata.ScriptDocValues$Longs",
      ...
   },
   "status": 400
}

You can use the same trick to see that _source is a LinkedHashMap in the _update API:

POST /hockey/_update/1
{
  "script": "Debug.explain(ctx._source)"
}

The response looks like:

{
  "error" : {
    "root_cause": ...,
    "type": "illegal_argument_exception",
    "reason": "failed to execute script",
    "caused_by": {
      "type": "script_exception",
      "to_string": "{gp=[26, 82, 1], last=gaudreau, assists=[17, 46, 0], first=johnny, goals=[9, 27, 1]}",
      "painless_class": "java.util.LinkedHashMap",
      "java_class": "java.util.LinkedHashMap",
      ...
    }
  },
  "status": 400
}

Once you have a class you can go to Painless API Reference to see a list of available methods.