What are extraReducers and when would you use them?

extraReducers lets a slice respond to actions it did not define in its own reducers field. Unlike reducers, it generates no action creators or action types. It only adds handlers for actions that already exist.

Use it for:

• the pending, fulfilled and rejected actions of a createAsyncThunk
• actions defined by another slice, like resetting state on logout
• RTK Query endpoint matchers

You write it with the builder callback. addCase handles one action type, addMatcher handles a range of actions, and addDefaultCase handles the rest. The old object notation was removed in Redux Toolkit 2.0, so the builder is the only form.

const userSlice = createSlice({
  name: 'user',
  initialState: { data: null, loading: false },
  reducers: {},
  extraReducers: (builder) => {
    builder
      .addCase(fetchUser.pending, (state) => {
        state.loading = true;
      })
      .addCase(fetchUser.fulfilled, (state, action) => {
        state.data = action.payload;
        state.loading = false;
      });
  }
});