MusareReact/frontend/app/js/reducers/app.js

import { Map } from "immutable";

import {
	INCREMENT,
} from "actions/app";

const initialState = Map({
	counter: 0,
});

const actionsMap = {
	[INCREMENT]: (state) => {
		const counter = state.get("counter") + 1;

		return state.merge({
			counter,
		});
	},
};

export default function reducer(state = initialState, action = {}) {
	const fn = actionsMap[action.type];
	return fn ? fn(state, action) : state;
}