React-Redux: Actions must be plain objects. Use custom middleware for async actions

> Unhandled Rejection (Error): Actions must be plain objects. Use custom middleware for async actions.

I wanted to add comments with every posts. So when fetch posts are run I want to call fetch comment API for all post.

export function bindComments(postId) {
  return API.fetchComments(postId).then(comments => {
    return {
      type: BIND_COMMENTS,
      comments,
      postId
    }
  })
}

You have to dispatch after the async request ends.

This would work:

export function bindComments(postId) {
	return function(dispatch) {
		return API.fetchComments(postId).then(comments => {
			// dispatch
			dispatch({
				type: BIND_COMMENTS,
				comments,
				postId
			});
		});
	};
}

For future seekers who might have dropped simple details like me, in my case I just have forgotten to call my action function with parentheses.

actions.js:

export function addNewComponent() {
  return {
    type: ADD_NEW_COMPONENT,
  };
}

myComponent.js:

import React, { useEffect } from 'react';
import { addNewComponent } from '../../redux/actions';

  useEffect(() => {
    dispatch(refreshAllComponents); // <= Here was what I've missed.
  }, []);

I've forgotten to dispatch the action function with (). So doing this solved my issue.

  useEffect(() => {
    dispatch(refreshAllComponents());
  }, []);

Again this might have nothing to do with OP's problem, but I hope I helps people with the same problem as mine.

The error is simply asking you to insert a Middleware in between which would help to handle async operations.

You could do that by :

>npm i redux-thunk

        Inside index.js

import thunk from "redux-thunk" 
import { createStore, applyMiddleware } from 'redux';
        
...createStore(rootReducers, applyMiddleware(thunk));

Now, async operations will work inside your functions.

You can't use fetch in actions without middleware. Actions must be plain objects. You can use a middleware like redux-thunk or redux-saga to do fetch and then dispatch another action.

Here is an example of async action using redux-thunk middleware.

export function checkUserLoggedIn (authCode) {
 let url = `${loginUrl}validate?auth_code=${authCode}`;
  return dispatch => {
    return fetch(url,{
      method: 'GET',
      headers: {
        "Content-Type": "application/json"
      }
      }
    )
      .then((resp) => {
        let json = resp.json();
       if (resp.status >= 200 && resp.status < 300) {
          return json;
        } else {
          return json.then(Promise.reject.bind(Promise));
        }
      })
      .then(
        json => {
          if (json.result && (json.result.status === 'error')) {
            dispatch(errorOccurred(json.result));
            dispatch(logOut());
          }
          else{
            dispatch(verified(json.result));
          }
        }
      )
      .catch((error) => {
        dispatch(warningOccurred(error, url));
      })
  }
}

Make use of Arrow functions it improves the readability of code. No need to return anything in API.fetchComments, Api call is asynchronous when the request is completed then will get the response, there you have to just dispatch type and data.

Below code does the same job by making use of Arrow functions.

export const bindComments = postId => {
  return dispatch => {
    API.fetchComments(postId).then(comments => {
      dispatch({
        type: BIND_COMMENTS,
        comments,
        postId
      });
    });
  };
};

Change:

export const <youractionName> = async (dispatch) => {}

to,

export const <youractionName> = () => async (dispatch) => {}

This fixed my issue. Missed a '() =>'

I had same issue as I had missed adding composeEnhancers. Once this is setup then you can take a look into action creators. You get this error when this is not setup as well.

const composeEnhancers = window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;

const store = createStore(
  rootReducer,
  composeEnhancers(applyMiddleware(thunk))
);

Use redux-thunk, setup with redux & create action like this

export const actionName = (data) => dispatch => {
  dispatch({
    type:"ACTION_TYPE"
    payload:"my payload"
  })
}

You might also have forgotten to getDefaultMiddleware() in the middlewares' array, as I did. No further installations required:

export const store = configureStore({
  reducer: GlobalReducer,
  middleware: (getDefaultMiddleware) => [
    ...getDefaultMiddleware(),
    mainMiddleware,
  ],
});

Action Definition

const selectSlice = () => {
  return {
    type: 'SELECT_SLICE'
  }
};

Action Dispatch

store.dispatch({
  type:'SELECT_SLICE'
});

Make sure the object structure of action defined is same as action dispatched. In my case, while dispatching action, type was not assigned to property type.

If you are working with redux-observable check that your action returns an observable. I had the issue because I used map and not a mergemap

// error
    export const myEpic = (action$: any) =>
      action$.pipe(
        ofType('...'),
        map((x => x.payload),
        map((x) => callAPi(x)),
      )
    
// success
        export const myEpic = (action$: any) =>
          action$.pipe(
            ofType('...'),
            map((x => x.payload),
            mergeMap((x) => callAPi(x)),
          )

Just here to share my case. I had a setLoading action, while also having

const [loading, setLoading] = useState(false)

above which I didn't delete. So it was basically not dispatching the setLoading from redux but the one from useState. Deleting/renaming this solves the problem.

if things were working with this code and this is a new iteration, check to make sure you have your variables in the correct order for the function (this was my mistake)

i.e. code that got this error

export const fetchProjects = newPage => (getState, dispatch) => NOPE

export const fetchProjects = newPage => (dispatch, getState) => OK YEAH

In my case, I just wanted to sent some values to the server without saving them to redux store, so I was not using a type, nor dispatching anything at the end. But I was calling the action with dispatch. So all I had to do, was to remove the dispatch, because it wasn't really an action. It was just a function.

For me, the solution was to add redux-thunk as a middleware, so inside my store configuration file, I passed redux-thunk as middleware.

inside the console:

import reducerData from './store-reducer';
import {applyMiddleware, compose, createStore} from 'redux';
import ReduxThunk from 'redux-thunk';

const middlewares = [ReduxThunk];

const store = createStore(
  reducerData,
  compose(applyMiddleware(...middlewares)),
);
export default store;

Arrow function syntax

export const bindComments = (postId) => dispatch => {
 return API.fetchComments(postId).then(comments => {
   // dispatch
    dispatch({
      type: BIND_COMMENTS,
       comments,
       postId
   })
})}

This error occurs mainly if you are dispatching an action and your action is not returning an object. For example here is an increment function which I use it to increment number value when increment button is clicked. const increment = () => type: INCREMENT and here is my dispatch function onClick={() => dispatch(increment)} because of ommiting parenthesis () inside dispatch function now in your terminal there would be the same error appears. The reason dispatch function expects an object not a function name...

This error occurs when you make an asynchronous api call in your action creator you need to convert your action creator from synchornous action creator to asynchronous action and this conversion can be possible if we use the middleware so let me explain you in detailRedux without middleware

Two types of action creators Sync Action Creator VS Async Action Creator. you need to change sync action to async action in order to get rid of this error and this can be done with middleware

Redux with middleware enter image description here

So now solution is: Dispatch after the async request would befinished.

export function bindComments(postId) {
        return function(dispatch) {
            return API.fetchComments(postId).then(comments => {
                // dispatch
                dispatch({
                    type: BIND_COMMENTS,
                    comments,
                    postId
                });
            });
        };
    }  

Without middleware, redux supports only synchronous data flow. If you need to make ajax request and dispatch the result of this request, then you need to use middlewares that handles the async operations like, redux-promise, redux-thunk or redux-saga. Or you could write your own middleware:

export default ({ dispatch }) =>
  (next) =>
  (action) => {
    // check if there is payload in  action. if not send it to the next middleware
    if (!action.payload || !action.payload.then) {
      return next.action;
    }
    // if we are here means we have action.payload. now check if it is promise
    // wait for the promise to be resolved
    action.payload.then(function (response) {
      // overwrite the action
      const newAction = { ...action, payload: response };
      dispatch(newAction);
    });
  };

I have solved my issue changing :

export const = async (dispatch) => {}

to,

export const = () => async (dispatch) => {}