const store = configureStore({ reducer: { posts: postsSlice.reducer, }, })

type RootState = ReturnType

const selectPostsState = (state: RootState) => state.posts const selectVisiblePosts = createSelector([selectPostsState], (postsState) => postsState.filter === 'all' ? postsState.items : postsState.items.filter((post) => post.published), )

store.dispatch(postsSlice.actions.postAdded({ id: 'p1', title: 'Draft' })) store.dispatch(postsSlice.actions.postPublished({ id: 'p1' }))

const visiblePosts = selectVisiblePosts(store.getState()) console.log(visiblePosts)


## Core Patterns

### Dispatch events, not setters

```ts
const postsSlice = createSlice({
  name: 'posts',
  initialState: [] as { id: string; title: string }[],
  reducers: {
    postAdded(state, action: { payload: { id: string; title: string } }) {
      state.push(action.payload)
    },
    postRemoved(state, action: { payload: { id: string } }) {
      return state.filter((post) => post.id !== action.payload.id)
    },
    postUpdated(
      state,
      action: { payload: { id: string; changes: Partial<{ title: string }> } },
    ) {
      const post = state.find((item) => item.id === action.payload.id)
      if (post && action.payload.changes.title) {
        post.title = action.payload.changes.title
      }
    },
  },
})

postsSlice.actions.postAdded({ id: 'p1', title: 'Draft' })

Event-style actions explain what happened in the UI instead of hiding the transition behind a generic setter.

Let reducers combine old store data with new outside data

import { createEntityAdapter, createSlice } from '@reduxjs/toolkit'

const postsAdapter = createEntityAdapter<{ id: string; title: string }>()

const postsSlice = createSlice({
  name: 'posts',
  initialState: postsAdapter.getInitialState(),
  reducers: {
    postsReceived(state, action: { payload: { id: string; title: string }[] }) {
      postsAdapter.upsertMany(state, action.payload)
    },
  },
})

const incomingPosts = [
  { id: 'p1', title: 'Draft' },
  { id: 'p2', title: 'Published' },
]

postsSlice.actions.postsReceived(incomingPosts)

If a transition mixes current store state with new external data, dispatch the new external data and let the reducer own the merge.

Derive values with selectors instead of storing duplicates

import { createSelector } from '@reduxjs/toolkit'

const selectPosts = (state: RootState) => state.posts.items
const selectFilter = (state: RootState) => state.posts.filter

export const selectVisiblePosts = createSelector(
  [selectPosts, selectFilter],
  (posts, filter) =>
    filter === 'all' ? posts : posts.filter((post) => post.published),
)

Selectors keep a single source of truth in state while still exposing the shapes the UI needs.

Common Mistakes

CRITICAL Mutating selected state outside reducers

Wrong:

const post = selectPostById(store.getState(), 'p1')

if (post) {
  post.title = 'Changed in place'
}

Correct:

store.dispatch(
  postUpdated({ id: 'p1', changes: { title: 'Changed in place' } }),
)

Objects read from the store are still store state; mutating them outside reducers breaks immutability and stale-render assumptions.

Source: reduxjs/redux:docs/style-guide/style-guide.md

HIGH Using setter-style actions instead of event-style actions

Wrong:

const nextPosts = [
  ...selectPosts(store.getState()),
  { id: 'p2', title: 'Write docs' },
]
store.dispatch(setPosts(nextPosts))

Correct:

store.dispatch(postAdded({ id: 'p2', title: 'Write docs' }))

Actions should describe events, not ask reducers to blindly replace state with a precomputed value.

Source: reduxjs/redux:docs/style-guide/style-guide.md

HIGH Combining store state before dispatch

Wrong:

const currentPosts = selectPosts(store.getState())
const mergedPosts = [
  ...currentPosts.filter(
    (currentPost) =>
      !incomingPosts.some((incomingPost) => incomingPost.id === currentPost.id),
  ),
  ...incomingPosts,
]

store.dispatch(postsReplaced(mergedPosts))

Correct:

store.dispatch(postsReceived(incomingPosts))

If the next state depends on current store state, the reducer should own that combination logic; only authoritative external snapshots should replace state wholesale.

Source: maintainer interview

HIGH Ignoring current state in async reducers

Wrong:

builder.addCase(fetchPosts.fulfilled, (state, action) => {
  state.status = 'succeeded'
  state.items = action.payload
})

Correct:

builder.addCase(fetchPosts.fulfilled, (state, action) => {
  if (state.status === 'pending') {
    state.status = 'succeeded'
    state.items = action.payload
  }
})

Reducers that treat every lifecycle action as valid can move the slice into impossible states or let stale requests win.

Source: reduxjs/redux:docs/tutorials/essentials/part-5-async-logic.md

MEDIUM Storing derived values in state

Wrong:

const initialState = {
  items: [] as Post[],
  visiblePosts: [] as Post[],
}

Correct:

const selectVisiblePosts = createSelector(
  [selectPosts, selectFilter],
  (posts, filter) =>
    filter === 'all' ? posts : posts.filter((post) => post.published),
)

Derived values drift out of sync quickly; keep the raw state and derive the view shape.

Source: reduxjs/redux:docs/style-guide/style-guide.md

build-modern-redux-apps/redux-dataflow

Redux Dataflow

Setup