redux-reuse

redux-reuse is a Redux-related project, which provides extendReducer() helper function, which helps to make your reducers code DRY, based on approach of composable higher-order reducers.

extendReducer() allows you to add logic to reducer after it has been created, in other words you extend reducers. Using extendReducer() it is pretty simple to write composable higher-order reducers for your Redux application, at the end of the day most reducers can be presented as composition of higher-order reducers.

Usage

Install via NPM

npm install redux-reuse --save

Import

import { extendReducer } from 'redux-reuse';
// or
var reduxReuse = require('redux-reuse');

If you need ES6 module

import { extendReducer } from 'redux-reuse/es6';

Use this if you are using rollup.js or webpack 2, or any ES2015 modules-compatible bundler which can eliminate unused library code with tree-shaking.

It is recommended to import the library from redux-reuse/es6 instead of redux-reuse/src because the source code depends on experimental presets from babel (stage 1-3) and may be incompatible with your bundler or settings.

Other environments

Use the Universal Module Definition (UMD)

• reduxReuse.js
• reduxReuse.min.js (minified)

API

extendReducer()

extendReducer(
  handlers: {
    [actionType: string] => (state: any, action: Object) => any
  },
  propagate: ?boolean = true
): (reducer) => reducer

// Uncurried
extendReducer(
  reducer,
  handlers: {
    [actionType: string] => (state: any, action: Object) => any
  },
  propagate: ?boolean = true
): reducer

Extends a reducer with additional action handlers. It means that new reducer will be returned, which wraps the original one and once an action with one of specified action types occurs, the appropriate handler will be executed first and then based on propagate argument the result will or will not be passed on to the original reducer.

In other words extendReducer() extends the existent reducer with some logic, which will be performed on specified actions. With extendReducer() it is pretty easy to write composable higher-order reducers which are specific to your application and reuse them in you reducers. See below examples for how to do it.

initialReducer()

initialReducer(initialState: any): reducer

Creates a reducer with passed value as initial state.

nullReducer

nullReducer: reducer

A reducer which returns null as the initial state.

Overview of existent approaches to reuse code in reducers

There are several approaches for how to organize your reducers code as reusable pieces:

• Organize your common logic in functions and use that functions in you reducers.

Basic approach to reuse pieces of code :)

• Create you own reducer for each usecase and use combineReducers() helper in order to combine separate reducers.

This approach has two disadvantages: first of all it will produce deeper state tree, so once your application will grow it can lead you to huge selectors, and secondly this way will not cover case when you need to manage one piece of state.

• Split logic to separate maps of handlers, where keys are action types and values are appropriate reducers, then use Object.assign(oneReducerMap, anotherReducerMap) or spread operator to combine maps and use their combination in some helpers like handleActions() from redux-actions or createReducer() from redux-create-reducer to initialize your reducer, which will be superset of combined reducer maps.

This approach also has disadvantage: if you have two reducer maps, which should perform some logic on same actions, then as result of merging maps, one of them will overwrite another one, so only logic from one reducer map will be performed when contradicted action occurs.

• Organize common logic in separate reducers, cummulate needed reducers in array and reduce them into single reducer via reduceReducers() helper from reduce-reducers.

This approach let you achieve the same result, as extendReducer(), but last one is more Redux-oriented (like using handleActions() or createReducer() to create reducers) and it helps to operate in terms of composable higher-order reducers, which means control over execution of base reducer. So choose what you need :)

Example of how to use extendReducer() to write your own higher-order reducers.

You can look at ready-to-use higher-order reducers, which has been implemented using extendReducer() helper:

• redux-payload
• redux-reset-reducer

And start organizing your reducers code in reusable pieces like in the following example:

Let's imagine that we have two separate counters and they should provide different behaviours on each tick that occurs. One should only increment its own by 1 and the other one should also change sign of a number it holds. And you have two separate widgets with such a counters, which work independently and start with different numbers.

import { compose, combineReducers } from 'redux';
import { extendReducer, initialReducer } from 'redux-reuse';
import { WIDGET1_TICK, WIDGET2_TICK } from 'event-types';

const increment = (actionType) => (reducer) =>
  extendReducer(reducer, {
    [actionType]: (state) => state + 1
  });

const changeSign = (actionType) => (reducer) =>
  extendReducer(reducer, {
    [actionType]: (state) => state * (-1)
  });

const widgetReducer = (tickActionType, startingNumber) =>
  combineReducers({
    smoothCounter: increment(tickActionType)(initialReducer(startingNumber)),
    jumpingCounter: compose(
      increment(tickActionType),
      changeSign(tickActionType)
    )(initialReducer(startingNumber))
  });

const app = combineReducers({
  widget1: widgetReducer(WIDGET1_TICK, 0),
  widget2: widgetReducer(WIDGET2_TICK, 100)
})

As you can see everything is reused! :)

Another interesting example is when you need to plug-in and plug-out some functionality. Lets imagine we have counter with checkboxes, indicating what exact functionality should be turned on.

import { extendReducer, initialReducer } from 'redux-reuse';
import { PUSH_MODE, POP_MODE, TICK } from 'event-types';
import { INCREMENT, SQUARE, CHANGE_SIGN } from 'modes';

const increment = (actionType) => (reducer) =>
  extendReducer(reducer, {
    [actionType]: (state) => state + 1
  });

const square = (actionType) => (reducer) =>
  extendReducer(reducer, {
    [actionType]: (state) => state * state
  });

const changeSign = (actionType) => (reducer) =>
  extendReducer(reducer, {
    [actionType]: (state) => state * (-1)
  });

const modesMap = {
  [INCREMENT]: increment,
  [SQUARE]: square,
  [CHANGE_SIGN]: changeSign,
};

const counter = (state = { modes: [], value: null }, { payload: mode }) => {
  switch (action.type) {
    case PUSH_MODE:
      state.modes.push(mode);
      break;
    case POP_MODE:
      state.modes.pop();
      break;
    case TICK:
      const reducers = state.modes.map((mode) => modesMap[mode]);
      state.value = compose(...reducers)(initialReducer(0))(state.value, action);
      break;
  }

  return state;
};

And again, we can reuse our higher-order reducers in every place we wish.

Thanks

Big thanks to ilyagelman for reviewing and acting as opponent, which helped a lot in preparing the overview of existent approaches.