example of reducer

moviesReducer.js

import {
    RECEIVED_NEW_RELEASES,
    RECEIVED_FREE_MOVIES
} from '../constants/ActionTypes';

import selectn from 'selectn';
import { merge, union } from 'lodash';

const initialState = {
    allMovies: {},
    releases: [],
    free: []
};

/**
 *
 * @param {Object} state Current state
 * @param {Object} action Action object with type and payload
 * @returns {Object} New state
 */
export default function moviesReducer(state = initialState, action) {
    const movies = selectn('payload.entities.movies', action);
    const ids = selectn('payload.result.movies', action);

    if (!movies) return state;

    // we want grab all movies (it's not important with which action it comes)
    const newState = merge({}, state, {
        allMovies: movies
    });

    switch (action.type) {
    case RECEIVED_NEW_RELEASES:
        return merge({}, newState, {
            releases: union(newState.releases, ids).sort()
        });

    case RECEIVED_FREE_MOVIES:
        return merge({}, newState, {
            free: union(newState.free, ids).sort()
        });

    default:
        return newState;
    }
}