What are `extraReducers` and when would you use them?

extraReducers allow a slice to respond to actions that weren't defined in its own reducers field:

• External actions: Handle actions from other slices
• Async thunks: Handle pending/fulfilled/rejected states from createAsyncThunk
• Action matching: Use builder pattern or map object notation

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;
      });
  }
});