@reduxjs/toolkit/src/entities/entity_state.ts

import type { EntityState } from './models'

export function getInitialEntityState<V>(): EntityState<V> {
  return {
    ids: [],
    entities: {},
  }
}

export function createInitialStateFactory<V>() {
  function getInitialState(): EntityState<V>
  function getInitialState<S extends object>(
    additionalState: S
  ): EntityState<V> & S
  function getInitialState(additionalState: any = {}): any {
    return Object.assign(getInitialEntityState(), additionalState)
  }

  return { getInitialState }
}