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React component does not re-render under Jest on state change

Component:
const MyComponent = props => {
const {price} = props;
const result1 = useResult(price);
return (
<div>...</div>
)
}
Custom Hook:
export const useResult = (price) => {
const [result, setResult] = useState([]);
useEffect(() => {
const data = [{price: price}]
setResult(data);
}, [price]);
return result;
};
Jest test:
it('should ...', async () => {
render(
<MyComponent price={300}/>)
)
await waitFor(() => {
expect(...).toBeInTheDocument();
});
});
What it does happen with the above code is that MyComponent, when running the test, renders only once instead of two (when the application runs). After the initial render where result1 is an empty array, useEffect of useResult is running and since there is a state change due to setResult(data), I should expect MyComponent to be re-rendered. However, that's not the case and result1 still equals to [] whereas it should equal to [{price:300}].
Hence, it seems custom hooks under testing behave differently than the real app. I thought it would be okay to test them indirectly through the component that calls them.
Any explanation/thoughts for the above?
UPDATE
The issue that invoked the above erroneous behaviour was state mutation!! It worked with the app but not with the test! My mistake was to attempt to use push in order to add an element to an array that was a state variable...

Well, it seems that you are asking a very specific thing about testing a custom hook. In that case, I also had some issues in the past testing custom hooks through #testing-library and a different package was created (and recently incorporated into the #testing-library) that provides the renderHook() function for testing custom hooks. I suggest you to test that.
Original Package (do not use it. Use directly the TL one)
Docs about the renderHook() call inside the TL docs
You can read more about it in this blog post from Kent C. Dodds.
I also suggest you create a "state change" to test your component and test the hook with the renderHook().
Here is a simple codesandbox with some tests for a component similar to your case.
Original Answer
Essentially, your test is not waiting for the component to perform the side effects. There are 2 ways of waiting for that:
Using waitFor()
import { waitFor, screen } from '#testing-library/react'
// ...
// add the `async` before the callback function
it('should ...', async () => {
render(<MyComponent price={300}/>);
await waitFor(() =>
expect(screen.getByText('your-text-goes-here')).toBeInTheDocument()
)
});
Using the findBy* query from RTL, that returns a Promise (read the Docs here) and is a combination from the waitFor and getBy* query (read docs here)
import { screen } from '#testing-library/react'
// ...
// add the `async` before the callback function
it('should ...', async () => {
render(<MyComponent price={300}/>);
expect(await screen.findByText('your-text-goes-here')).toBeInTheDocument();
});

Step 1: the code being tested
If, as mentioned in the comments of the question, the operation inside the effect is synchronous, then using useEffect for setting this state based on the props is undesirable in all cases. Not only for testing.
The component will render, update the DOM and immediately need to re render the following frame because it's state was updated. It causes a flash effect for the user and needlessly slows the app down.
If the operation is cheap, it's way more efficient to just execute it on every render.
If the operation can be more expensive, you can wrap it in useMemo to ensure it only happens when there's changes to the inputs.
export const useResult = (price) => {
return useMemo(
// I assume this is a stub for a expensive operation.
() => [{price: price}],
[price]
);
};
If, for some obscure reason, you do need to do this in an effect anyway (you probably don't but there's edge cases), you can use a layoutEffect instead. It will be processed synchronously and avoid the flashing frame. Still wouldn't recommend it but it's a slight improvement over a regular effect.
Step 2: Testing
If you changed the component to not use an effect, it should now be correct from the first render, and you don't have the problem anymore. Avoiding having a problem in the first place is also a valid solution :D
If you do find the need to flush something synchronously in a test, there's now the flushSync function which does just that.
Perhaps it would also flush the state update in the effect, causing your test to work with no other changes. I guess it should, as new updates triggered by effects while flushing should continue to be processed before returning.
flushSync(() => {
render(
<MyComponent price={300}/>)
)
})
In any case there's no point doing this if you can instead improve the component to fix the additional render introduced by setting state in an effect.

you can do:
The test will have to be async: it('should ...', async() => { ....
await screen.findByText('whatever');
This is async so it will wait to find whatever and fail if it can't find it
or you can do
await waitFor (() => {
const whatever = screen.getByText('whatever');
expect(whatever).toBeInTheDocument();
})

You are not waiting for the component to be rerendered
import { waitFor, screen } from 'testing-library/react'
it('should ...', async () => {
render(
<MyComponent price={300}/>)
)
await waitFor (() => {
// check that props.price is shown
screen.debug() // check what's renderered
expect(screen.getByText(300)).toBeInTheDocument();
});
});

Idiomatic way to await background tasks with redux + redux thunk

In my application, we have a somewhat long-running "provisioning" task that asynchronously completes that we essentially have to use polling to detect that it has finished. So that a user isn't blocked from using the app while this takes place, we kick that off at the beginning of the user interaction flow. However, at the end of the interaction flow, the user takes an action that needs to await that provisioning task if it hasn't completed. Here's a sort of sequence diagram to illustrate.
[provisioning start][poll for completion....]
[user interactions] [blocked action....][post provisioning task]
The problem I'm having is figuring out an idiomatic way to do this properly in Redux. Here are some things I've considered doing:
Have a "requests" reducer, where I'd store the Promise for that long-running provisioning task, then when [blocked action] executes, it simply awaits the promise. The problem with this is that Redux specifically requests that all store state is serializable, so it seems that things like Promises are not welcome there.
Subscribe to the store in the [blocked action] action creator and look for data that signals the provisioning has completed. Unfortunately, redux-thunk, which I'm pretty fond of, doesn't contain a reference to the store, just its dispatch and getState methods.
Having my React component wait for the provisioning to complete before executing [blocked action]. I don't like this because it adds too much action sequencing logic awareness to my view layer.
Since none of these seem like great options, I'm currently going with:
Store the provisioning Promise in a module property and essentially do option #1 without getting the Promise from the store. I'm not wild about this, since it moves state outside of the redux loop, but it seems like the easiest option.
Is there a more idiomatic way to achieve this with redux-thunk? If there's another asynchronous middleware that makes this cleaner, I'm willing to consider, though I'm already quite invested in redux-thunk.

There are several redux-promise-middleware options that let you dispatch promise actions and have the middleware consume them and emit PENDING,RESOLVED|REJECTED actions as the promise state changes. If you are working with promises and Redux, you might want to look at one of them. However they won't help you with this specific problem.
I think redux-saga is a middleware well suited to help you with this scenario. I've not yet used it personally so I can't provide an example.
Another, perhaps simpler, option is redux-tap. Use tap to expose the stream of actions that your app can subscribe to. Have a blocked action creator thunk subscribe to that stream and await the action that signals that the promise has completed (it should of course first check the contents of the store via getState to see if the promise completed before this blocked action was dispatched). Something like this:
// ========= configureStore.js
import ee from 'event-emitter';
// ...
export const actionStream = ee();
// ...
const emitActions = tap(({type} => type, (type, action) => actionStream.emit(type, action);
// install emitActions middleware to run *after* thunk (so that it does not see thunk actions but only primitive actions)
// =========== your action module.js
import {actionStream} from './configureStore';
export function blockedAction(arg) {
return (dispatch, getState) => {
if (getState().initDone) {
return dispatch({type: "blockedAction", payload: arg});
}
// wait for init action
actionStream.once("initAction", initAction => dispatch({type: "blockedAction", payload: arg}));
};
}
Keep in mind that it is not a binary "either thunk or xxx middleware" choice. You can load thunk as well as many other middlewares. In one app, I use:
thunk
a promise middleware
console logging middleware
custom middleware for batching updates
custom middleware for persisting actions to IndexedDB
custom middleware for pausing the action stream
middleware to prevent recursive store notifications
Without any middleware other than thunk, here is another option:
Option 5 - combine your option 2 with your option 4: Store the promise somewhere globally. Have the blocked action creator thunk await the promise before dispatching the raw action. To minimize the "state outside of the store", you can also have the promise action emit actions into the store to keep the store updated (possibly using a promise middleware)

I think this is simpler—unless I'm missing something—if we're more rigorous in thinking about the UI as a function of state. At every point in your interaction flow there are a few things that are either true or false:
Provisioning has started/not started
Provisioning has finished/not finished
User action is pending/is not pending
These three facts are enough to tell us what the user should be seeing on their screen at a given moment, so our state should include these three facts:
const initialState = {
provisioningStarted: false,
provisioningDone: false,
laterActionIsPending: false,
// ...
};
Using redux-thunk we can handle the provisioning action:
function startProvisioning() {
return dispatch => {
dispatch({ type: START_PROVISIONING });
pollProvisioning().then(postSetupInfo => {
dispatch({ type: DONE_PROVISIONING, postSetupInfo });
});
};
}
...and in our reducer:
function appReducer(state, action) {
// ...
switch (action.type) {
case START_PROVISIONING:
return { ...state, provisioningStarted: true };
case DONE_PROVISIONING:
return { ...state, provisioningDone: true, postSetupInfo: action.postSetupInfo };
case LATER_ACTION_PENDING:
return { ...state, laterActionIsPending: true };
// ...
}
}
As I said, this should be enough to tell us what the user should be seeing:
const Provisioner = ({provisioningStarted, provisioningDone, /* ... */}) => {
if (provisioningStarted) {
if (laterActionIsPending) {
if (provisioningDone) {
return <p>Finished! Here's your info: {postSetupInfo}</p>;
}
return <p>Provisioning...</p>;
}
return <button type="button" onClick={doAction}>Do action</button>;
}
return <button type="button" onClick={startProvisioning}>Start provisioning</button>;
};
export default connect(mapStateToProps, mapDispatchToProps)(Provisioner);

Not exactly redux-thunk but functionally equivalent is using
redux-background. A package I created to make the process easier.
It goes something like this:
import { startJob } from 'redux-background';
import store from './store';
store.dispatch(startJob('provision', function(job, dispatch, getState) {
const { progress, data } = job;
return new Promise((resolve, reject) => {
// Do some async stuff
// Report progress
progress(10);
// Return value
return 10;
})
}, { data: '...' } );
On your state when it ends you should have something like this
{
background: {
provision: {
active: true,
running: false,
value: 10,
error: null,
/* and lot of more metadata */
....
},
}
}

Why should I use redux async actions? [duplicate]

According to the docs, "Without middleware, Redux store only supports synchronous data flow". I don't understand why this is the case. Why can't the container component call the async API, and then dispatch the actions?
For example, imagine a simple UI: a field and a button. When user pushes the button, the field gets populated with data from a remote server.
import * as React from 'react';
import * as Redux from 'redux';
import { Provider, connect } from 'react-redux';
const ActionTypes = {
STARTED_UPDATING: 'STARTED_UPDATING',
UPDATED: 'UPDATED'
};
class AsyncApi {
static getFieldValue() {
const promise = new Promise((resolve) => {
setTimeout(() => {
resolve(Math.floor(Math.random() * 100));
}, 1000);
});
return promise;
}
}
class App extends React.Component {
render() {
return (
<div>
<input value={this.props.field}/>
<button disabled={this.props.isWaiting} onClick={this.props.update}>Fetch</button>
{this.props.isWaiting && <div>Waiting...</div>}
</div>
);
}
}
App.propTypes = {
dispatch: React.PropTypes.func,
field: React.PropTypes.any,
isWaiting: React.PropTypes.bool
};
const reducer = (state = { field: 'No data', isWaiting: false }, action) => {
switch (action.type) {
case ActionTypes.STARTED_UPDATING:
return { ...state, isWaiting: true };
case ActionTypes.UPDATED:
return { ...state, isWaiting: false, field: action.payload };
default:
return state;
}
};
const store = Redux.createStore(reducer);
const ConnectedApp = connect(
(state) => {
return { ...state };
},
(dispatch) => {
return {
update: () => {
dispatch({
type: ActionTypes.STARTED_UPDATING
});
AsyncApi.getFieldValue()
.then(result => dispatch({
type: ActionTypes.UPDATED,
payload: result
}));
}
};
})(App);
export default class extends React.Component {
render() {
return <Provider store={store}><ConnectedApp/></Provider>;
}
}
When the exported component is rendered, I can click the button and the input is updated correctly.
Note the update function in the connect call. It dispatches an action that tells the App that it is updating, and then performs an async call. After the call finishes, the provided value is dispatched as a payload of another action.
What is wrong with this approach? Why would I want to use Redux Thunk or Redux Promise, as the documentation suggests?
EDIT: I searched the Redux repo for clues, and found that Action Creators were required to be pure functions in the past. For example, here's a user trying to provide a better explanation for async data flow:
The action creator itself is still a pure function, but the thunk function it returns doesn't need to be, and it can do our async calls
Action creators are no longer required to be pure. So, thunk/promise middleware was definitely required in the past, but it seems that this is no longer the case?

What is wrong with this approach? Why would I want to use Redux Thunk or Redux Promise, as the documentation suggests?
There is nothing wrong with this approach. It’s just inconvenient in a large application because you’ll have different components performing the same actions, you might want to debounce some actions, or keep some local state like auto-incrementing IDs close to action creators, etc. So it is just easier from the maintenance point of view to extract action creators into separate functions.
You can read my answer to “How to dispatch a Redux action with a timeout” for a more detailed walkthrough.
Middleware like Redux Thunk or Redux Promise just gives you “syntax sugar” for dispatching thunks or promises, but you don’t have to use it.
So, without any middleware, your action creator might look like
// action creator
function loadData(dispatch, userId) { // needs to dispatch, so it is first argument
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
);
}
// component
componentWillMount() {
loadData(this.props.dispatch, this.props.userId); // don't forget to pass dispatch
}
But with Thunk Middleware you can write it like this:
// action creator
function loadData(userId) {
return dispatch => fetch(`http://data.com/${userId}`) // Redux Thunk handles these
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_DATA_FAILURE', err })
);
}
// component
componentWillMount() {
this.props.dispatch(loadData(this.props.userId)); // dispatch like you usually do
}
So there is no huge difference. One thing I like about the latter approach is that the component doesn’t care that the action creator is async. It just calls dispatch normally, it can also use mapDispatchToProps to bind such action creator with a short syntax, etc. The components don’t know how action creators are implemented, and you can switch between different async approaches (Redux Thunk, Redux Promise, Redux Saga) without changing the components. On the other hand, with the former, explicit approach, your components know exactly that a specific call is async, and needs dispatch to be passed by some convention (for example, as a sync parameter).
Also think about how this code will change. Say we want to have a second data loading function, and to combine them in a single action creator.
With the first approach we need to be mindful of what kind of action creator we are calling:
// action creators
function loadSomeData(dispatch, userId) {
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
function loadOtherData(dispatch, userId) {
return fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
);
}
function loadAllData(dispatch, userId) {
return Promise.all(
loadSomeData(dispatch, userId), // pass dispatch first: it's async
loadOtherData(dispatch, userId) // pass dispatch first: it's async
);
}
// component
componentWillMount() {
loadAllData(this.props.dispatch, this.props.userId); // pass dispatch first
}
With Redux Thunk action creators can dispatch the result of other action creators and not even think whether those are synchronous or asynchronous:
// action creators
function loadSomeData(userId) {
return dispatch => fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
function loadOtherData(userId) {
return dispatch => fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_OTHER_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_OTHER_DATA_FAILURE', err })
);
}
function loadAllData(userId) {
return dispatch => Promise.all(
dispatch(loadSomeData(userId)), // just dispatch normally!
dispatch(loadOtherData(userId)) // just dispatch normally!
);
}
// component
componentWillMount() {
this.props.dispatch(loadAllData(this.props.userId)); // just dispatch normally!
}
With this approach, if you later want your action creators to look into current Redux state, you can just use the second getState argument passed to the thunks without modifying the calling code at all:
function loadSomeData(userId) {
// Thanks to Redux Thunk I can use getState() here without changing callers
return (dispatch, getState) => {
if (getState().data[userId].isLoaded) {
return Promise.resolve();
}
fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'LOAD_SOME_DATA_SUCCESS', data }),
err => dispatch({ type: 'LOAD_SOME_DATA_FAILURE', err })
);
}
}
If you need to change it to be synchronous, you can also do this without changing any calling code:
// I can change it to be a regular action creator without touching callers
function loadSomeData(userId) {
return {
type: 'LOAD_SOME_DATA_SUCCESS',
data: localStorage.getItem('my-data')
}
}
So the benefit of using middleware like Redux Thunk or Redux Promise is that components aren’t aware of how action creators are implemented, and whether they care about Redux state, whether they are synchronous or asynchronous, and whether or not they call other action creators. The downside is a little bit of indirection, but we believe it’s worth it in real applications.
Finally, Redux Thunk and friends is just one possible approach to asynchronous requests in Redux apps. Another interesting approach is Redux Saga which lets you define long-running daemons (“sagas”) that take actions as they come, and transform or perform requests before outputting actions. This moves the logic from action creators into sagas. You might want to check it out, and later pick what suits you the most.
I searched the Redux repo for clues, and found that Action Creators were required to be pure functions in the past.
This is incorrect. The docs said this, but the docs were wrong.
Action creators were never required to be pure functions.
We fixed the docs to reflect that.

You don't.
But... you should use redux-saga :)
Dan Abramov's answer is right about redux-thunk but I will talk a bit more about redux-saga that is quite similar but more powerful.
Imperative VS declarative
DOM: jQuery is imperative / React is declarative
Monads: IO is imperative / Free is declarative
Redux effects: redux-thunk is imperative / redux-saga is declarative
When you have a thunk in your hands, like an IO monad or a promise, you can't easily know what it will do once you execute. The only way to test a thunk is to execute it, and mock the dispatcher (or the whole outside world if it interacts with more stuff...).
If you are using mocks, then you are not doing functional programming.
Seen through the lens of side-effects, mocks are a flag that your code is impure, and in the functional programmer's eye, proof that something is wrong. Instead of downloading a library to help us check the iceberg is intact, we should be sailing around it.
A hardcore TDD/Java guy once asked me how you do mocking in Clojure. The answer is, we usually don't. We usually see it as a sign we need to refactor our code.
Source
The sagas (as they got implemented in redux-saga) are declarative and like the Free monad or React components, they are much easier to test without any mock.
See also this article:
in modern FP, we shouldn’t write programs — we should write descriptions of programs, which we can then introspect, transform, and interpret at will.
(Actually, Redux-saga is like a hybrid: the flow is imperative but the effects are declarative)
Confusion: actions/events/commands...
There is a lot of confusion in the frontend world on how some backend concepts like CQRS / EventSourcing and Flux / Redux may be related, mostly because in Flux we use the term "action" which can sometimes represent both imperative code (LOAD_USER) and events (USER_LOADED). I believe that like event-sourcing, you should only dispatch events.
Using sagas in practice
Imagine an app with a link to a user profile. The idiomatic way to handle this with each middleware would be:
redux-thunk
<div onClick={e => dispatch(actions.loadUserProfile(123)}>Robert</div>
function loadUserProfile(userId) {
return dispatch => fetch(`http://data.com/${userId}`)
.then(res => res.json())
.then(
data => dispatch({ type: 'USER_PROFILE_LOADED', data }),
err => dispatch({ type: 'USER_PROFILE_LOAD_FAILED', err })
);
}
redux-saga
<div onClick={e => dispatch({ type: 'USER_NAME_CLICKED', payload: 123 })}>Robert</div>
function* loadUserProfileOnNameClick() {
yield* takeLatest("USER_NAME_CLICKED", fetchUser);
}
function* fetchUser(action) {
try {
const userProfile = yield fetch(`http://data.com/${action.payload.userId }`)
yield put({ type: 'USER_PROFILE_LOADED', userProfile })
}
catch(err) {
yield put({ type: 'USER_PROFILE_LOAD_FAILED', err })
}
}
This saga translates to:
every time a username gets clicked, fetch the user profile and then dispatch an event with the loaded profile.
As you can see, there are some advantages of redux-saga.
The usage of takeLatest permits to express that you are only interested to get the data of the last username clicked (handle concurrency problems in case the user click very fast on a lot of usernames). This kind of stuff is hard with thunks. You could have used takeEvery if you don't want this behavior.
You keep action creators pure. Note it's still useful to keep actionCreators (in sagas put and components dispatch), as it might help you to add action validation (assertions/flow/typescript) in the future.
Your code becomes much more testable as the effects are declarative
You don't need anymore to trigger rpc-like calls like actions.loadUser(). Your UI just needs to dispatch what HAS HAPPENED. We only fire events (always in the past tense!) and not actions anymore. This means that you can create decoupled "ducks" or Bounded Contexts and that the saga can act as the coupling point between these modular components.
This means that your views are more easy to manage because they don't need anymore to contain that translation layer between what has happened and what should happen as an effect
For example imagine an infinite scroll view. CONTAINER_SCROLLED can lead to NEXT_PAGE_LOADED, but is it really the responsibility of the scrollable container to decide whether or not we should load another page? Then he has to be aware of more complicated stuff like whether or not the last page was loaded successfully or if there is already a page that tries to load, or if there is no more items left to load? I don't think so: for maximum reusability the scrollable container should just describe that it has been scrolled. The loading of a page is a "business effect" of that scroll
Some might argue that generators can inherently hide state outside of redux store with local variables, but if you start to orchestrate complex things inside thunks by starting timers etc you would have the same problem anyway. And there's a select effect that now permits to get some state from your Redux store.
Sagas can be time-traveled and also enables complex flow logging and dev-tools that are currently being worked on. Here is some simple async flow logging that is already implemented:
Decoupling
Sagas are not only replacing redux thunks. They come from backend / distributed systems / event-sourcing.
It is a very common misconception that sagas are just here to replace your redux thunks with better testability. Actually this is just an implementation detail of redux-saga. Using declarative effects is better than thunks for testability, but the saga pattern can be implemented on top of imperative or declarative code.
In the first place, the saga is a piece of software that permits to coordinate long running transactions (eventual consistency), and transactions across different bounded contexts (domain driven design jargon).
To simplify this for frontend world, imagine there is widget1 and widget2. When some button on widget1 is clicked, then it should have an effect on widget2. Instead of coupling the 2 widgets together (ie widget1 dispatch an action that targets widget2), widget1 only dispatch that its button was clicked. Then the saga listen for this button click and then update widget2 by dispaching a new event that widget2 is aware of.
This adds a level of indirection that is unnecessary for simple apps, but make it more easy to scale complex applications. You can now publish widget1 and widget2 to different npm repositories so that they never have to know about each others, without having them to share a global registry of actions. The 2 widgets are now bounded contexts that can live separately. They do not need each others to be consistent and can be reused in other apps as well. The saga is the coupling point between the two widgets that coordinate them in a meaningful way for your business.
Some nice articles on how to structure your Redux app, on which you can use Redux-saga for decoupling reasons:
http://jaysoo.ca/2016/02/28/organizing-redux-application/
http://marmelab.com/blog/2015/12/17/react-directory-structure.html
https://github.com/slorber/scalable-frontend-with-elm-or-redux
A concrete usecase: notification system
I want my components to be able to trigger the display of in-app notifications. But I don't want my components to be highly coupled to the notification system that has its own business rules (max 3 notifications displayed at the same time, notification queueing, 4 seconds display-time etc...).
I don't want my JSX components to decide when a notification will show/hide. I just give it the ability to request a notification, and leave the complex rules inside the saga. This kind of stuff is quite hard to implement with thunks or promises.
I've described here how this can be done with saga
Why is it called a Saga?
The term saga comes from the backend world. I initially introduced Yassine (the author of Redux-saga) to that term in a long discussion.
Initially, that term was introduced with a paper, the saga pattern was supposed to be used to handle eventual consistency in distributed transactions, but its usage has been extended to a broader definition by backend developers so that it now also covers the "process manager" pattern (somehow the original saga pattern is a specialized form of process manager).
Today, the term "saga" is confusing as it can describe 2 different things. As it is used in redux-saga, it does not describe a way to handle distributed transactions but rather a way to coordinate actions in your app. redux-saga could also have been called redux-process-manager.
See also:
Interview of Yassine about Redux-saga history
Kella Byte: Claryfing the Saga pattern
Microsoft CQRS Journey: A Saga on Sagas
Medium response of Yassine
Alternatives
If you don't like the idea of using generators but you are interested by the saga pattern and its decoupling properties, you can also achieve the same with redux-observable which uses the name epic to describe the exact same pattern, but with RxJS. If you're already familiar with Rx, you'll feel right at home.
const loadUserProfileOnNameClickEpic = action$ =>
action$.ofType('USER_NAME_CLICKED')
.switchMap(action =>
Observable.ajax(`http://data.com/${action.payload.userId}`)
.map(userProfile => ({
type: 'USER_PROFILE_LOADED',
userProfile
}))
.catch(err => Observable.of({
type: 'USER_PROFILE_LOAD_FAILED',
err
}))
);
Some redux-saga useful resources
Redux-saga vs Redux-thunk with async/await
Managing processes in Redux Saga
From actionsCreators to Sagas
Snake game implemented with Redux-saga
2017 advises
Don't overuse Redux-saga just for the sake of using it. Testable API calls only are not worth it.
Don't remove thunks from your project for most simple cases.
Don't hesitate to dispatch thunks in yield put(someActionThunk) if it makes sense.
If you are frightened of using Redux-saga (or Redux-observable) but just need the decoupling pattern, check redux-dispatch-subscribe: it permits to listen to dispatches and trigger new dispatches in listener.
const unsubscribe = store.addDispatchListener(action => {
if (action.type === 'ping') {
store.dispatch({ type: 'pong' });
}
});

The short answer: seems like a totally reasonable approach to the asynchrony problem to me. With a couple caveats.
I had a very similar line of thought when working on a new project we just started at my job. I was a big fan of vanilla Redux's elegant system for updating the store and rerendering components in a way that stays out of the guts of a React component tree. It seemed weird to me to hook into that elegant dispatch mechanism to handle asynchrony.
I ended up going with a really similar approach to what you have there in a library I factored out of our project, which we called react-redux-controller.
I ended up not going with the exact approach you have above for a couple reasons:
The way you have it written, those dispatching functions don't have access to the store. You can somewhat get around that by having your UI components pass in all of the info the dispatching function needs. But I'd argue that this couples those UI components to the dispatching logic unnecessarily. And more problematically, there's no obvious way for the dispatching function to access updated state in async continuations.
The dispatching functions have access to dispatch itself via lexical scope. This limits the options for refactoring once that connect statement gets out of hand -- and it's looking pretty unwieldy with just that one update method. So you need some system for letting you compose those dispatcher functions if you break them up into separate modules.
Take together, you have to rig up some system to allow dispatch and the store to be injected into your dispatching functions, along with the parameters of the event. I know of three reasonable approaches to this dependency injection:
redux-thunk does this in a functional way, by passing them into your thunks (making them not exactly thunks at all, by dome definitions). I haven't worked with the other dispatch middleware approaches, but I assume they're basically the same.
react-redux-controller does this with a coroutine. As a bonus, it also gives you access to the "selectors", which are the functions you may have passed in as the first argument to connect, rather than having to work directly with the raw, normalized store.
You could also do it the object-oriented way by injecting them into the this context, through a variety of possible mechanisms.
Update
It occurs to me that part of this conundrum is a limitation of react-redux. The first argument to connect gets a state snapshot, but not dispatch. The second argument gets dispatch but not the state. Neither argument gets a thunk that closes over the current state, for being able to see updated state at the time of a continuation/callback.

Abramov's goal - and everyone's ideally - is simply to encapsulate complexity (and async calls) in the place where it's most appropriate and reusable.
Where's the best place to do that in the standard Redux dataflow? How about:
Reducers? No way. They should be pure functions with no side-effects. Updating the store is serious, complicated business. Don't contaminate it.
Dumb View Components? Definitely No. They have one concern: presentation and user-interaction, and should be as simple as possible.
Container Components? Possible, but sub-optimal. It makes sense in that the container is a place where we encapsulate some view related complexity and interact with the store, but:
Containers do need to be more complex than dumb components, but it's still a single responsibility: providing bindings between view and state/store. Your async logic is a whole separate concern from that.
By placing it in a container, you'd be locking your async logic into a single context, coupled to one or more views/routes. Bad idea. Ideally it's all reusable, and totally decoupled from the views.
(Like all rules, there could be an exception if you have stateful binding logic that happens to be reusable across multiple contexts, or if you can somehow generalize all of your state into something like an integrated GraphQL schema. OK, fine, that could be cool. But... most of the time the bindings seem to end up pretty context/view specific.)
Some other Service Module? Bad idea: you'd need to inject access to the store, which is a maintainability/testability nightmare. Better to go with the grain of Redux and access the store only using the APIs/models provided.
Actions and the Middlewares that interpret them? Why not?! For starters, it's the only major option we have left. :-) More logically, the action system is decoupled execution logic that you can use from anywhere. It's got access to the store and can dispatch more actions. It has a single responsibility which is to organize the flow of control and data around the application, and most async fits right into that.
What about the Action Creators? Why not just do async in there, instead of in the actions themselves, and in Middleware?
First and most important, the creators don't have access to the store, as middleware does. That means you can't dispatch new contingent actions, can't read from the store to compose your async, etc.
So, keep complexity in a place that's complex of necessity, and keep everything else simple. The creators can then be simple, relatively pure functions that are easy to test.

To answer the question that is asked in the beginning:
Why can't the container component call the async API, and then dispatch the actions?
Keep in mind that those docs are for Redux, not Redux plus React. Redux stores hooked up to React components can do exactly what you say, but a Plain Jane Redux store with no middleware doesn't accept arguments to dispatch except plain ol' objects.
Without middleware you could of course still do
const store = createStore(reducer);
MyAPI.doThing().then(resp => store.dispatch(...));
But it's a similar case where the asynchrony is wrapped around Redux rather than handled by Redux. So, middleware allows for asynchrony by modifying what can be passed directly to dispatch.
That said, the spirit of your suggestion is, I think, valid. There are certainly other ways you could handle asynchrony in a Redux + React application.
One benefit of using middleware is that you can continue to use action creators as normal without worrying about exactly how they're hooked up. For example, using redux-thunk, the code you wrote would look a lot like
function updateThing() {
return dispatch => {
dispatch({
type: ActionTypes.STARTED_UPDATING
});
AsyncApi.getFieldValue()
.then(result => dispatch({
type: ActionTypes.UPDATED,
payload: result
}));
}
}
const ConnectedApp = connect(
(state) => { ...state },
{ update: updateThing }
)(App);
which doesn't look all that different from the original — it's just shuffled a bit — and connect doesn't know that updateThing is (or needs to be) asynchronous.
If you also wanted to support promises, observables, sagas, or crazy custom and highly declarative action creators, then Redux can do it just by changing what you pass to dispatch (aka, what you return from action creators). No mucking with the React components (or connect calls) necessary.

OK, let's start to see how middleware working first, that quite answer the question, this is the source code applyMiddleWare function in Redux:
function applyMiddleware() {
for (var _len = arguments.length, middlewares = Array(_len), _key = 0; _key < _len; _key++) {
middlewares[_key] = arguments[_key];
}
return function (createStore) {
return function (reducer, preloadedState, enhancer) {
var store = createStore(reducer, preloadedState, enhancer);
var _dispatch = store.dispatch;
var chain = [];
var middlewareAPI = {
getState: store.getState,
dispatch: function dispatch(action) {
return _dispatch(action);
}
};
chain = middlewares.map(function (middleware) {
return middleware(middlewareAPI);
});
_dispatch = compose.apply(undefined, chain)(store.dispatch);
return _extends({}, store, {
dispatch: _dispatch
});
};
};
}
Look at this part, see how our dispatch become a function.
...
getState: store.getState,
dispatch: function dispatch(action) {
return _dispatch(action);
}
Note that each middleware will be given the dispatch and getState functions as named arguments.
OK, this is how Redux-thunk as one of the most used middlewares for Redux introduce itself:
Redux Thunk middleware allows you to write action creators that return
a function instead of an action. The thunk can be used to delay the
dispatch of an action, or to dispatch only if a certain condition is
met. The inner function receives the store methods dispatch and
getState as parameters.
So as you see, it will return a function instead an action, means you can wait and call it anytime you want as it's a function...
So what the heck is thunk? That's how it's introduced in Wikipedia:
In computer programming, a thunk is a subroutine used to inject an
additional calculation into another subroutine. Thunks are primarily
used to delay a calculation until it is needed, or to insert
operations at the beginning or end of the other subroutine. They have
a variety of other applications to compiler code generation and in
modular programming.
The term originated as a jocular derivative of "think".
A thunk is a function that wraps an expression to delay its
evaluation.
//calculation of 1 + 2 is immediate
//x === 3
let x = 1 + 2;
//calculation of 1 + 2 is delayed
//foo can be called later to perform the calculation
//foo is a thunk!
let foo = () => 1 + 2;
So see how easy the concept is and how it can help you manage your async actions...
That's something you can live without it, but remember in programming there are always better, neater and proper ways to do things...

There are synchronous action creators and then there are asynchronous action creators.
A synchronous action creator is one that when we call it, it immediately returns an Action object with all the relevant data attached to that object and its ready to be processed by our reducers.
Asynchronous action creators is one in which it will require a little bit of time before it is ready to eventually dispatch an action.
By definition, anytime you have an action creator that makes a network request, it is always going to qualify as an async action creator.
If you want to have asynchronous action creators inside of a Redux application you have to install something called a middleware that is going to allow you to deal with those asynchronous action creators.
You can verify this in the error message that tells us use custom middleware for async actions.
So what is a middleware and why do we need it for async flow in Redux?
In the context of redux middleware such as redux-thunk, a middleware helps us deal with asynchronous action creators as that is something that Redux cannot handle out of the box.
With a middleware integrated into the Redux cycle, we are still calling action creators, that is going to return an action that will be dispatched but now when we dispatch an action, rather than sending it directly off to all of our reducers, we are going to say that an action will be sent through all the different middleware inside the application.
Inside of a single Redux app, we can have as many or as few middleware as we want. For the most part, in the projects we work on we will have one or two middleware hooked up to our Redux store.
A middleware is a plain JavaScript function that will be called with every single action that we dispatch. Inside of that function a middleware has the opportunity to stop an action from being dispatched to any of the reducers, it can modify an action or just mess around with an action in any way you which for example, we could create a middleware that console logs every action you dispatch just for your viewing pleasure.
There are a tremendous number of open source middleware you can install as dependencies into your project.
You are not limited to only making use of open source middleware or installing them as dependencies. You can write your own custom middleware and use it inside of your Redux store.
One of the more popular uses of middleware (and getting to your answer) is for dealing with asynchronous action creators, probably the most popular middleware out there is redux-thunk and it is about helping you deal with asynchronous action creators.
There are many other types of middleware that also help you in dealing with asynchronous action creators.

To use Redux-saga is the best middleware in React-redux implementation.
Ex:
store.js
import createSagaMiddleware from 'redux-saga';
import { createStore, applyMiddleware } from 'redux';
import allReducer from '../reducer/allReducer';
import rootSaga from '../saga';
const sagaMiddleware = createSagaMiddleware();
const store = createStore(
allReducer,
applyMiddleware(sagaMiddleware)
)
sagaMiddleware.run(rootSaga);
export default store;
And then saga.js
import {takeLatest,delay} from 'redux-saga';
import {call, put, take, select} from 'redux-saga/effects';
import { push } from 'react-router-redux';
import data from './data.json';
export function* updateLesson(){
try{
yield put({type:'INITIAL_DATA',payload:data}) // initial data from json
yield* takeLatest('UPDATE_DETAIL',updateDetail) // listen to your action.js
}
catch(e){
console.log("error",e)
}
}
export function* updateDetail(action) {
try{
//To write store update details
}
catch(e){
console.log("error",e)
}
}
export default function* rootSaga(){
yield [
updateLesson()
]
}
And then action.js
export default function updateFruit(props,fruit) {
return (
{
type:"UPDATE_DETAIL",
payload:fruit,
props:props
}
)
}
And then reducer.js
import {combineReducers} from 'redux';
const fetchInitialData = (state=[],action) => {
switch(action.type){
case "INITIAL_DATA":
return ({type:action.type, payload:action.payload});
break;
}
return state;
}
const updateDetailsData = (state=[],action) => {
switch(action.type){
case "INITIAL_DATA":
return ({type:action.type, payload:action.payload});
break;
}
return state;
}
const allReducers =combineReducers({
data:fetchInitialData,
updateDetailsData
})
export default allReducers;
And then main.js
import React from 'react';
import ReactDOM from 'react-dom';
import App from './app/components/App.jsx';
import {Provider} from 'react-redux';
import store from './app/store';
import createRoutes from './app/routes';
const initialState = {};
const store = configureStore(initialState, browserHistory);
ReactDOM.render(
<Provider store={store}>
<App /> /*is your Component*/
</Provider>,
document.getElementById('app'));
try this.. is working

To Answer the question:
Why can't the container component call the async API, and then
dispatch the actions?
I would say for at least two reasons:
The first reason is the separation of concerns, it's not the job of the action creator to call the api and get data back, you have to have to pass two argument to your action creator function, the action type and a payload.
The second reason is because the redux store is waiting for a plain object with mandatory action type and optionally a payload (but here you have to pass the payload too).
The action creator should be a plain object like below:
function addTodo(text) {
return {
type: ADD_TODO,
text
}
}
And the job of Redux-Thunk midleware to dispache the result of your api call to the appropriate action.

When working in an enterprise project, there are many requirements available in middle-ware such as (saga) not available in simple asynchronous flow, below are some:
Running request in parallel
Pulling future actions without the need to wait
Non-blocking calls Race effect, example pickup first
response to initiate the process Sequencing your tasks (first in first call)
Composing
Task cancellation Dynamically forking the task.
Support Concurrency Running Saga outside the redux middleware.
Using channels
The list is long just review the advanced section in saga documentation

Redux can't return a function instead of an action. It's just a fact. That's why people use Thunk. Read these 14 lines of code to see how it allows the async cycle to work with some added function layering:
function createThunkMiddleware(extraArgument) {
return ({ dispatch, getState }) => (next) => (action) => {
if (typeof action === 'function') {
return action(dispatch, getState, extraArgument);
}
return next(action);
};
}
const thunk = createThunkMiddleware();
thunk.withExtraArgument = createThunkMiddleware;
export default thunk;
https://github.com/reduxjs/redux-thunk

I would say for at least two reasons:
The first reason is the separation of concerns, it's not the job of the action creator to call the api and get data back, you have to have to pass two argument to your action creator function, the action type and a payload.
The second reason is because the redux store is waiting for a plain object with mandatory action type and optionally a payload (but here you have to pass the payload too).
The action creator should be a plain object like below:
function addTodo(text) {
return {
type: ADD_TODO,
text
}
}
And the job of Redux-Thunk midleware to dispache the result of your api call to the appropriate action.

react, redux chain a next dispatch

I'm still beginning in react and redux and now I followed this tutorial and it uses this middleware for dispatch. I was wondering how I would do another dispatch after the first one (to chain it)? I have something like this now.
fetchData() {
const { dispatch } = this.props;
const action = PageActions.fetchPage({slug: this.props.params.slug});
dispatch(action);
}
and wondering if I can dispatch(action).then(...) but the return of dispatch is always undefined. Is that possible?

If you would like to use async actions inside of your action creators you need to use middleware. The recommended middleware is thunk.
There is a good post on Stack about it's usage and appropriate situations. Stack Overflow Async Actions
This will allow you to dispatch many actions from a single action. However if you are wanting to "chain" actions together you should simply dispatch the second action at the end of the first actions definition.
ie
function getBookings() {
return (
RequestHelper.fetchEntities(Routes.bookings.all, {}, queryParams)
.then(res => dispatch(getBookingsSuccess(res));
)
}
...
function getBookingsSuccess(data) {
dispatch(showSuccess());
}

Enforce an action to be called before another using Redux

Recently I did that for one app and I'd like to know if this would be considered a bad practice or if it is ok. Let's say I have a reducer listening for two actions:
switch (action.type) {
case 'PRE_FETCH_ACTION':
return Object.assign({}, state, {value: action.value, isAllowed: true})
case 'FETCHED_SUCCESS':
if (!state.isAllowed) {
throw Error('You did not dispatch PRE_FETCH_ACTION before fetching!');
}
return Object.assign({}, state, {data: action.data, isAllowed: false})
}
So the flow is:
I dispatch PRE_FETCH_ACTION
I make a fetch call to an external API
When the service response returns, it dispatches FETCHED_SUCCESS
If somebody tries to fetch data without dispatching the PRE_FETCH_ACTIONfirst, the code will throw an error.
Ok, so this works just fine. My concern is, as I said, if this would be considered a bad pattern. Why do I think so? Because the isAllowed piece of state is kinda internal to the reducer and it does not affect any component render method.

I think it's ok to have such "technical" flags in your app state.
In your case you want to ensure that actions are dispatched in a specific order, which defines a particular behavior of your app (for example, displaying a loader on the screen when data-fetching starts). That's why I think your "technical" flag eventually manages your UI.
To let your reducer "reduce only", you could make your check before dispatching, by testing your state in action creator. This pattern is detailed in Redux's doc about async actions. You would do it in a "thunk" action creator that you would call instead of calling sync action "fetchedSuccess", like this:
function toFetchedSuccess(data) { // "thunk" action creator
return (dispatch, getState) => {
const state = getState(); // current state
if(!state.isAllowed) {
console.error(
'You did not dispatch PRE_FETCH_ACTION before fetching!');
return Promise.resolve(); // nothing to do
} else {
return dispatch(fetchedSuccess(data));
}
};
}