General-purpose reducers for Redux.

general-purpose-reducers.js

/**
 * This gist was inspired by the video course titled "Building React Applications with Idiomatic Redux" 
 * available on Egghead.io by the creator of Redux, Dan Abramov. 
 * 
 * The purpose of this gist is to demonstrate general purpose reducers that can be used via Redux's combineReducers
 * to compose more complex reducers and therefore maximize code reuse.
 * 
 * Feedback is more than welcome!
 * 
 * @author Christoffer Niska <christofferniska@gmail.com>
 * @link https://egghead.io/courses/building-react-applications-with-idiomatic-redux
 */
 
import uniq from 'lodash/uniq';
import { combineReducers } from 'redux';

/**
 * Creates a reducer that manages a single value.
 * 
 * @param {function(state, action)} shouldUpdate
 * @param {function(state, action)} shouldReset
 * @param {function(state, action)} getValue
 * @param {*} defaultValue
 * @returns {function(state, action)}
 */
const createValue = (shouldUpdate = () => true, shouldReset = () => false, getValue = (state, action) => action.payload, defaultValue = null) =>
  (state = defaultValue, action) => {
    if (shouldReset(state, action)) {
      return defaultValue;
    }
    if (shouldUpdate(state, action)) {
      return getValue(state, action);
    }
    return state;
  };

/**
 * Creates a reducer that manages a map.
 * 
 * @param {function(state, action)} shouldUpdate
 * @param {function(state, action)} shouldReset
 * @param {function(state, action)} getValue
 * @returns {function(state, action)}
 */
const createMap = (shouldUpdate = () => true, shouldReset = () => false, getValues = (state, action) => action.payload) =>
  createValue(shouldUpdate, shouldReset, (state, action) => ({ ...state, ...getValues(state, action) }), {});

/**
 * Creates a reducer that manages a set.
 * 
 * @param {function(state, action)} shouldUpdate
 * @param {function(state, action)} shouldReset
 * @param {function(state, action)} getValue
 * @returns {function(state, action)}
 */
const createSet = (shouldUpdate = () => true, shouldReset = () => false, getValues = (state, action) => action.payload) =>
  createValue(shouldUpdate, shouldReset, (state, action) => uniq([...state, ...getValues(state, action)]), []);

/**
 * Creates a reducer that manages a flag.
 * 
 * @param {function(state, action)} shouldTurnOn
 * @param {function(state, action)} shouldTurnOff
 * @param {bool} defaultValue
 * @returns {function(state, action)}
 */
const createFlag = (shouldTurnOn, shouldTurnOff, defaultValue = false) => 
  (state = defaultValue, action) => {
    if (shouldTurnOn(state, action)) {
      return true;
    }
    if (shouldTurnOff(state, action)) {
      return false;
    }
    return state;
  };

/**
 * Example usage.
 * 
 * This example assumes that the data is set as a payload property on the action object.
 * In the real world this might not be the case, in that case use the third parameter to customize this.
 */

const UserActions = {
  FETCH_REQUEST: 'user/FETCH_REQUEST',
  FETCH_SUCCESS: 'user/FETCH_SUCCESS',
  FETCH_FAILURE: 'user/FETCH_FAILURE'
};

const reducer = combineReducers({
  isFetching: createFlag(
    (state, action) => action.type === UserActions.FETCH_REQUEST,
    (state, action) => [UserActions.FETCH_SUCCESS, UserActions.FETCH_FAILURE].indexOf(action.type) !== -1
  ),
  byId: createMap(
    (state, action) => action.type === UserActions.FETCH_SUCCESS,
    (state, action) => action.type === UserActions.FETCH_REQUEST
  ),
  ids: createSet(
    (state, action) => action.type === UserActions.FETCH_SUCCESS,
    (state, action) => action.type === UserActions.FETCH_REQUEST
  ),
  errorMessage: createValue(
    (state, action) => action.type === UserActions.FETCH_FAILURE,
    (state, action) => action.type === UserActions.FETCH_DEPARTMENT
  )
});