system 2025-07-12T09:13:00Z: You are a expert on javascript and react

user 2025-07-15T14:20:50Z: teach me createAsyncThunk. why is it useful?

assistant 2025-07-15T14:24:13Z: createAsyncThunk is a utility that ships with Redux Toolkit (RTK).
It lets you express a “side-effect” (usually an HTTP call) as one action creator that dispatches three separate redux actions for you:

  1. pending
  2. fulfilled (with the server’s response)
  3. rejected (with the thrown error)

You stop writing that boilerplate yourself (dispatching a _REQUEST → do fetch → dispatch _SUCCESS or _FAILURE) and get automatic loading / error flags, strong typing with TS, cancellation, optimistic updates, etc.

1. The problem createAsyncThunk solves

Classic hand-written:

// 1. action types
const FETCH_USER_REQUEST = 'FETCH_USER_REQUEST'
const FETCH_USER_SUCCESS = 'FETCH_USER_SUCCESS'
const FETCH_USER_FAILURE = 'FETCH_USER_FAILURE'

// 2. action creators
const fetchUserRequest = () => ({ type: FETCH_USER_REQUEST })
const fetchUserSuccess = user => ({ type: FETCH_USER_SUCCESS, payload: user })
const fetchUserFailure = error => ({ type: FETCH_USER_FAILURE, payload: error })

// 3. thunk action creator
export const fetchUser = id => async dispatch => {
  dispatch(fetchUserRequest())
  try {
    const res = await axios.get(`/users/${id}`)
    dispatch(fetchUserSuccess(res.data))
  } catch (e) {
    dispatch(fetchUserFailure(e.message))
  }
}

That is 12 lines only for the plumbing.
With createAsyncThunk:

import { createAsyncThunk } from '@reduxjs/toolkit'

export const fetchUser = createAsyncThunk(
  'users/fetchUser',              // action prefix → creates 3 actions with /pending /fulfilled /rejected suffixes
  async (id, { rejectWithValue }) => {
    const res = await axios.get(`/users/${id}`)
    return res.data                // becomes action.payload for fulfilled
  }
)

5 lines. The rest is generated for you.

2. How to use it (step-by-step)

  1. Generate the thunk:
import { createAsyncThunk, createSlice } from '@reduxjs/toolkit'

export const fetchUser = createAsyncThunk(
  'users/fetchUser',
  async (userId, thunkAPI) => {
    try {
      const { data } = await axios.get(`/users/${userId}`)
      return data
    } catch (err) {
      // Optional: pick off the server’s error message
      return thunkAPI.rejectWithValue(err.response.data)
    }
  }
)
  1. Wire it up in a slice:
const usersSlice = createSlice({
  name: 'users',
  initialState: { entity: null, loading: false, error: null },
  reducers: {},                              // optional normal sync reducers
  extraReducers: builder => {
    builder
      .addCase(fetchUser.pending, state => {
        state.loading = true
        state.error = null
      })
      .addCase(fetchUser.fulfilled, (state, action) => {
        state.loading = false
        state.entity = action.payload
      })
      .addCase(fetchUser.rejected, (state, action) => {
        state.loading = false
        state.error = action.payload || action.error.message
      })
  }
})

export default usersSlice.reducer
  1. Dispatch from a React component:
import { useEffect } from 'react'
import { useDispatch, useSelector } from 'react-redux'
import { fetchUser } from './usersSlice'

function User({ id }) {
  const dispatch = useDispatch()
  const { entity, loading, error } = useSelector(s => s.users)

  useEffect(() => {
    dispatch(fetchUser(id))
  }, [id])

  if (loading) return <Spinner />
  if (error) return <p>{error}</p>
  return <div>{entity.name}</div>
}

3. Why it’s more than just “saves lines”

  • Automatic loading states: use pending, fulfilled, rejected to set loading / error – no race conditions.
  • Cancellable: you get an AbortController.
    thunkAPI.signal lets you wire fetch(url, { signal }).
const fetchWithAbort = createAsyncThunk('todos/get', async (_, thunkAPI) => {
  const res = await fetch('/todos', { signal: thunkAPI.signal })
  return res.json()
})
  • Serializing/thunk environment:
    • getState() → read the whole store.
    • dispatch() → dispatch any other action or thunk.

  • Serializable check passes – all error objects are made serializable for Redux DevTools.

  • TypeScript: createAsyncThunk<User, string, { state: RootState }>() gives you correct action & payload types everywhere.

4. Common mini-patterns

  1. Transform the server payload before it hits reducers:
createAsyncThunk('todos/fetch', async () => {
  const data = await api.fetchTodos()
  return data.map(t => ({ ...t, isSelected: false }))
})
  1. Reject with a shaped error so your reducer can draw UI:
return rejectWithValue({ status: 422, errors: err.response.data.errors })
  1. Chain dependent calls:
const initApp = createAsyncThunk('app/init', async (_, thunkAPI) => {
  await thunkAPI.dispatch(authenticate())
  return thunkAPI.dispatch(fetchProfile()).unwrap()   // returns the payload
})

TL;DR
createAsyncThunk is the RTK tool that kills 90 % of Redux async boilerplate while giving you strong typing, cancellation, error serialization and a clean separation between async side-effects and your pure reducers.

user 2025-07-15T14:25:27Z: when it will go to rejected? please tell me the exact situation.

assistant 2025-07-15T14:25:51Z: There are only two strictly-defined code paths that make a createAsyncThunk jump into the rejected action:

Location Exact code the thunk executes Outcome
1️⃣ Your async payloadCreator throws synchronously OR an awaited Promise rejects and you do NOT catch it. Anything that ends up like throw … or a rejected Promise bubbling out. Redux Toolkit automatically catches it → rejected action with action.error filled.
2️⃣ You explicitly return rejectWithValue(payload) (after catching an error, validation failure, business rule, etc.). catch (e) { return rejectWithValue(e) } inside the payloadCreator. Toolkit immediately shortcuts to rejected action with action.payload === whatever you passed to rejectWithValue.

That’s the whole list.
Any other branch (return someData, return undefined, even return null) lands in fulfilled.