A port of elm-effects and the Elm Architecture to Redux that allows you to sequence your effects naturally and purely by returning them from your reducers.

Isn't it incorrect to cause side-effects in a reducer?

Yes! Absolutely.

Doesn't redux-loop put side-effects in the reducer?

It doesn't. The values returned from the reducer when scheduling an effect with redux-loop only describe the effect. Calling the reducer will not cause the effect to run. The value returned by the reducer is just an object that the store knows how to interpret when it is enhanced by redux-loop. You can safely call a reducer in your tests without worrying about waiting for effects to finish and what they will do to your environment.

What are the environment requirements for redux-loop?

redux-loop requires polyfills for ES6 Promise and Symbol to be included if the browsers you target don't natively support them.

Quick Example

import { createStore } from 'redux';
import { install, loop, Effects } from 'redux-loop';
import { fromJS } from 'immutable';

const firstAction = {
  type: 'FIRST_ACTION',
};

const doSecondAction = (value) => {
  return new Promise((resolve) => {
    setTimeout(() => {
      resolve({
        type: 'SECOND_ACTION',
        payload: value,
      });
    });
  });
}

const thirdAction = {
  type: 'THIRD_ACTION',
};

// immutable store state allowed by default, but not required
const initialState = fromJS({
  firstRun: false,
  secondRun: false,
  thirdRun: false,
});

function reducer(state, action) {
  switch(action.type) {

  case 'FIRST_ACTION':
    // Enter a sequence at FIRST_ACTION, SECOND_ACTION and THIRD_ACTION will be
    // dispatched in the order they are passed to batch
    return loop(
      state.set('firstRun', true),
      Effects.batch([
        Effects.promise(doSecondAction, 'hello'),
        Effects.constant(thirdAction)
      ])
    );

  case 'SECOND_ACTION':
    return state.set('secondRun', action.payload);

  case 'THIRD_ACTION':
    return state.set('thirdRun', true);

  default:
    return state;
  }
}

// Note: passing enhancer as the last argument to createStore requires redux@>=3.1.0
const store = createStore(reducer, initialState, install());

store
  .dispatch(firstAction)
  .then(() => {
    // dispatch returns a promise for when the current sequence is complete
    // { firstRun: true, secondRun: 'hello', thirdRun: true }
    console.log(store.getState().toJS());
  });

Why use this?

Having used and followed the progression of Redux and the Elm Architecture, and after trying other effect patterns for Redux, we came to the following conclusion:

Synchronous state transitions caused by returning a new state from the reducer in response to an action are just one of all possible effects an action can have on application state.

Many other methods for handling effects in Redux, especially those implemented with action-creators, incorrectly teach the user that asynchronous effects are fundamentally different from synchronous state transitions. This separation encourages divergent and increasingly specific means of processing particular types effects. Instead, we should focus on making our reducers powerful enough to handle asynchronous effects as well as synchronous state transitions. With redux-loop, the reducer doesn't just decide what happens now due to a particular action, it decides what happens next. All of the behavior of your application can be traced through one place, and that behavior can be easily broken apart and composed back together. This is one of the most powerful features of the Elm architecture, and with redux-loop it is a feature of Redux as well.

Tutorial

Install the store enhancer

import { createStore, compose, applyMiddleware } from 'redux';
import reducer from './reducers';
import { install } from 'redux-loop';
import someMiddleware from 'some-middleware';

const enhancer = compose(
  applyMiddleware(someMiddleware),
  install()
);

// Note: passing enhancer as the last argument to createStore requires redux@>=3.1.0
const store = createStore(reducer, initialState, enhancer);

Installing redux-loop is as easy as installing any other store enhancer. You can apply it directly over createStore or compose it with other enhancers and middlewares. Composition of enhancers can be confusing, so the order in which install() is applied may matter. If something like applyMiddleware() doesn't work when called before install(), applying after may fix the issue.

Write a reducer with some effects

import { Effects, loop } from 'redux-loop';
import { loadingStart, loadingSuccess, loadingFailure } from './actions';

export function fetchDetails(id) {
  return fetch(`/api/details/${id}`)
    .then((r) => r.json())
    .then(loadingSuccess)
    .catch(loadingFailure);
}

export default function reducer(state, action) {
  switch (action.type) {
    case 'LOADING_START':
      return loop(
        { ...state, loading: true },
        Effects.promise(fetchDetails, action.payload.id)
      );

    case 'LOADING_SUCCESS':
      return {
        ...state,
        loading: false,
        details: action.payload
      };

    case 'LOADING_FAILURE':
      return {
        ...state,
        loading: false,
        error: action.payload.message
      };

    default:
      return state;
  }
}

Any reducer case can return a loop instead of a state object. A loop joins an updated model state with an effect for the store to process. There are several options for effects, all available under the Effects object:

• promise(factory, ...args)
  • Accepts a factory function that returns a Promise for an action instance when called with args.
• constant(action)
  • Accepts an action instance to dispatch immediately once the current dispatch cycle is completed.
• batch(effects)
  • Accepts an array of other effects and runs them in parallel, dispatching the resulting actions once all effects are resolved.
• none()
  • A no-op action, for convenience when writing custom effect creators

Easily test reducer results

import test from 'tape';
import reducer, { fetchDetails } from './reducer';
import { loadingStart } from './actions';
import { Effects, loop } from 'redux-loop';

test('reducer works as expected', (t) => {
  const state = { loading: false };

  const result = reducer(state, loadingStart(1));

  t.deepEqual(result, loop(
    { loading: true },
    Effects.promise(fetchDetails, 1)
  ));
});

Effects are declarative specifications of the next behavior of the store. They are only processed by an active store, pushing effecting behavior to the edge of the application. You can call a reducer as many times with a given action and state and always get a result which is deepEqual.

CAVEAT For testing sanity, always pass a referenceable function to Effects.promise. Functions curried or bound from the same function with the same arguments are not equal within JavaScript, and so are best to avoid if you want to compare effects in your tests.

Use the custom combineReducers if you need it

import { createStore } from 'redux';
import { combineReducers, install } from 'redux-loop';

import { firstReducer, secondReducer } from './reducers';

const reducer = combineReducers({
  first: firstReducer,
  second: secondReducer,
});

// Note: passing enhancer as the last argument to createStore requires redux@>=3.1.0
const store = createStore(reducer, initialState, install());

The combineReducers implementation in redux-loop is aware that some of your reducers might return effects, and it knows how to properly compose them and forward them to the store. The built-in createStore in redux will not properly identify effects from your nested reducers' results and execute them, and the redux-loop implementation is completely compatible with the behavior of the built-in version so there should be no problem with exchanging it.

Using redux-loop's combineReducers with Immutable.js (or any other data structure)

import { combineReducers } from 'redux-loop';
import { Map } from 'immutable';

import { firstReducer, secondReducer } from './reducers';

const reducers = {
  first: firstReducer,
  second: secondReducer,
}

//Map() is now used as the new root state, and custom accessor and mutator properties are provided
const reducer = combineReducers(
    reducers,
    Map(),
    (child, key) => child.get(key),
    (child, key, value) => child.set(key, value)
);

Our combineReducers can also handle states made of data structures other than the default {}, you simply pass it in the root state, an accessor function (which returns a value for that key), and a mutator function (which returns a new version of the object with a value set at a given key). The example above demonstrates using Immutable.js' Map() data structure, but you can use any key => value data structure as long as you provide your own accessor and mutator functions.

Avoid circular loops!

function reducer(state, action) {
  switch (action.type) {
    case 'FIRST':
      return loop(
        state,
        Effects.constant(second())
      );

    case 'SECOND':
      return loop(
        state,
        Effects.constant(first())
      );
  }
}

This minimal example will cause perpetual dispatching! While it is also possible to make this mistake with large, complicated networks of redux-thunk action creators, it is much easier to spot the mistake before it is made. It helps to keep your reducers small and focused, and use combineReducers or manually compose reducers so that the number of actions you deal with at one time is small. A small set of actions which initiate a loop will help reduce the likelihood of causing circular dispatches.

Support

Potential bugs, generally discussion, and proposals or RFCs should be submitted as issues to this repo, we'll do our best to address them quickly. We use this library as well and want it to be the best it can! For questions about using the library, submit questions on StackOverflow with the redux-loop tag.

Don't see a feature you want?

If you're interested in adding something to redux-loop but don't want to wait for us to incorporate the idea you can follow these steps to get your own installable version of redux-loop with your feature included:

1. Fork the main repo here
2. Add your feature or change
3. Change the package "name" in package.json to be "@<your-npm-username>/redux-loop
4. Commit to master and npm publish
5. npm install @<your-npm-username>/redux-loop

We are always interested in new ideas, but sometimes we get a little busy and fall behind on responding and reviewing PRs. Hopefully this process will allow you to continue making progress on your projects and also provide us with more context if and when you do decide to make a PR for your new feature or change. The best way to verify new features for a library is to use them in real-world scenarios!

Contributing

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms. Multiple language translations are available at contributor-covenant.org