redux, react. I can't push to the state array, which already has data

Question

I can't push to the fetchedSpendings state array, which already has data after fetching.

Reducer.js

import {FETCH_SPENDINGS, PUSH_SPENDING } from './types'

const initialState = {
    fetchedSpendings: []
}

export const dataReducer = (state = initialState, action) => {

    switch(action.type){


        case PUSH_SPENDING:

             return {...state, fetchedSpendings: state.fetchedSpendings.push(action.payload)}

            //return {...state, fetchedSpendings: state.fetchedSpendings = [action.payload]}



        case FETCH_SPENDINGS:
            
            return { ...state, fetchedSpendings: action.payload }

        default: return state
    }

}

Commented out line in case PUSH_SPENDING is working but it does not add data to the array that already contains objects, but replaces them.

//return {...state, fetchedSpendings: state.fetchedSpendings = [action.payload]}

the other in this case does not.

 return {...state, fetchedSpendings: state.fetchedSpendings.push(action.payload)}

actions.js

export function fetchSpendings(){
    return async dispatch => {

        const userid = userDataGetter();

        try{
            const response = await axios.post('/api/getSpendings', { userID: userid})
            const json = await response["data"];

            dispatch( {type: FETCH_SPENDINGS, payload: json})
              console.log('Returned data:', json);
         } catch (e) {
         console.log(`Axios request failed in fetchSpendings: ${e}`);
         } 
     }
}


export function pushSpending(cost, category, note){
    return async dispatch => {

        const userData = userDataGetter();
        const userID = userData.userId;


        const dataSpend = {
        _id: userID, 
        date: String(new Date()),
        cost: cost, 
        category: category, 
        note: note
        }


        try{
            const response = await axios.post('/api/addSpending', {cost, category, note, userID})
            const json = await response;


            dispatch( {type: PUSH_SPENDING, payload: dataSpend})

        } catch(e){
        console.log(`Axios request failed in pushSpendings: ${e}`);

        M.toast({html: "incorrectly entered data"});

        }
    }
}

in react i am getting error. props.Spending.map is not a function.

Because this line not push data in array.

return {...state, fetchedSpendings: state.fetchedSpendings.push(action.payload)}

Answer 1

Use the spread operator to create a new array with action.payload as its final element:

return {
  ...state,
  fetchedSpendings: [...state.fetchedSpendings, action.payload],
}

.push modifies the array and returns the added element, so fetchedSpendings is no longer an array when you use .push . Also, React needs to see a new array to update.