How do you handle complex state updates with nested objects in RTK?

With Redux Toolkit you update nested data by assigning to it directly, like state.profile.address.city = action.payload. createSlice runs reducers through Immer, which turns those writes into an immutable update and copies only the objects on the changed path.

Common patterns:

• Replace a nested object by assigning a new value to its property, as updateAddress does below.
• Update one deep field by assigning to that field, as updateCity does.
• Update an item in an array by finding it first and then changing it, like state.items.find(item => item.id === id).

Arrays also support push and index assignment directly on the draft.

Two rules prevent bugs. Either mutate the draft or return a new state, never both in one reducer. And never reassign state itself, because that only changes the local variable.

For data nested many levels deep, normalizing it by ID keeps reducers flat. You can still return a new state object when that logic reads more clearly.

const userSlice = createSlice({
  name: 'user',
  initialState: {
    profile: {
      name: '',
      address: {
        street: '',
        city: ''
      }
    }
  },
  reducers: {
    updateAddress: (state, action) => {
      // Direct assignment works with Immer
      state.profile.address = action.payload;
    },
    updateCity: (state, action) => {
      state.profile.address.city = action.payload;
    }
  }
});