# Reducer Plugin

For advanced users, who want to add additional action handler to kepler.gl reducer, kepler.gl provides a reducer plugin function. `Reducer.plugin` will take additional action handlers and return a new reducer function. Plugin is only meant to be called where the store is initialized. The state passed into the additional action handler is the instance state.

`Reducer.plugin` will allow advanced users to extend the kepler.gl reducer behavior. Here is an example of adding an additional action `HIDE_AND_SHOW_SIDE_PANEL` handler that modifies the `uiState`.

```js
import {combineReducers} from 'redux';
import keplerGlReducer from '@kepler.gl/reducers';

const customizedKeplerGlReducer = keplerGlReducer
 .plugin({
   HIDE_AND_SHOW_SIDE_PANEL: (state, action) => ({
     ...state,
     uiState: {
       ...state.uiState,
       readOnly: !state.uiState.readOnly
     }
   })
 });

const reducers = combineReducers({
 keplerGl: customizedKeplerGlReducer,
 app: appReducer
});
```

Note that, reducer plugin **should not be used to override default kepler.gl actions** The following code will not change `SET_FILTER`, because plugins are handled after default actions.

```js
const customizedKeplerGlReducer = keplerGlReducer
 .plugin({
   [ActionTypes.SET_FILTER]: (state, action) => state
 });
```

For full implementation, take a look at the [custom reducer example](https://github.com/keplergl/kepler.gl/tree/master/examples/custom-reducer)


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.kepler.gl/docs/api-reference/advanced-usages/reducer-plugin.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.