[[painless-metric-agg-map-context]]
=== Metric aggregation map context

Use a Painless script to
{ref}/search-aggregations-metrics-scripted-metric-aggregation.html[map]
values for use in a scripted metric aggregation. A map script is run once per
collected document following an optional
<<painless-metric-agg-init-context, initialization script>> and is required as
part of a full metric aggregation.

*Variables*

`params` (`Map`, read-only)::
        User-defined parameters passed in as part of the query.

`params['_agg']` (`Map`)::
        `Map` used to add values for processing in a
        <<painless-metric-agg-map-context, combine script>> or returned
        directly.

`doc` (`Map`, read-only)::
        Contains the fields of the current document where each field is a
        `List` of values.

`_score` (`double` read-only)::
        The similarity score of the current document.

*Side Effects*

`params['_agg']` (`Map`)::
        Use this `Map` to add values for processing in a combine script.
        Additional values must be of the type `Map`, `List`, `String` or
        primitive. If an initialization script is provided as part the
        aggregation then values added from the initialization script are
        available as well.  If no combine script is specified, values must be
        directly stored in `_agg`. If no combine script and no
        <<painless-metric-agg-reduce-context, reduce script>> are specified, the
        values are used as the result.

*Return*

`void`::
        No expected return value.

*API*

The standard <<painless-api-reference, Painless API>> is available.