source: node_modules/@reduxjs/toolkit/src/entities/tests/state_adapter.test.ts

import type { EntityAdapter } from '../index'
import { createEntityAdapter } from '../index'
import type { PayloadAction } from '../../createAction'
import { configureStore } from '../../configureStore'
import { createSlice } from '../../createSlice'
import type { BookModel } from './fixtures/book'

describe('createStateOperator', () => {
  let adapter: EntityAdapter<BookModel, string>

  beforeEach(() => {
    adapter = createEntityAdapter({
      selectId: (book: BookModel) => book.id,
    })
  })
  it('Correctly mutates a draft state when inside `createNextState', () => {
    const booksSlice = createSlice({
      name: 'books',
      initialState: adapter.getInitialState(),
      reducers: {
        // We should be able to call an adapter method as a mutating helper in a larger reducer
        addOne(state, action: PayloadAction<BookModel>) {
          // Originally, having nested `produce` calls don't mutate `state` here as I would have expected.
          // (note that `state` here is actually an Immer Draft<S>, from `createReducer`)
          // One woarkound was to return the new plain result value instead
          // See https://github.com/immerjs/immer/issues/533
          // However, after tweaking `createStateOperator` to check if the argument is a draft,
          // we can just treat the operator as strictly mutating, without returning a result,
          // and the result should be correct.
          const result = adapter.addOne(state, action)
          expect(result.ids.length).toBe(1)
          //Deliberately _don't_ return result
        },
        // We should also be able to pass them individually as case reducers
        addAnother: adapter.addOne,
      },
    })

    const { addOne, addAnother } = booksSlice.actions

    const store = configureStore({
      reducer: {
        books: booksSlice.reducer,
      },
    })

    const book1: BookModel = { id: 'a', title: 'First' }
    store.dispatch(addOne(book1))

    const state1 = store.getState()
    expect(state1.books.ids.length).toBe(1)
    expect(state1.books.entities['a']).toBe(book1)

    const book2: BookModel = { id: 'b', title: 'Second' }
    store.dispatch(addAnother(book2))

    const state2 = store.getState()
    expect(state2.books.ids.length).toBe(2)
    expect(state2.books.entities['b']).toBe(book2)
  })
})