normalizing.js

const x = {
  entities: {
    countries: {
      1: { ... }
      2: { ... }
    },
    regions: {
      1: { ... },
      2: { ... },
    }
  }
}

// REST response data for GET /country/:id
{
  id: 1,
  name: 'Japan',
  regions: [{
    id: 3,
    name: 'Kanto',
  }, {
    id: 4,
    name: 'Kansai',
  }]
}

// normalizr will output the found entities 
// (that you can use to update your store):
data.entities = {
  countries: {
    1: {
      id: 1,
      name: 'Japan',
    }
  },
  regions: {
    3: {
      id: 3,
      name: 'Kanto',
    },
    4: {
      id: 4,
      name: 'Kansai',
    }
  }
}

// and the normalized original data (that you can use for UI)
data.result = {
  id: 1,
  name: 'Japan',
  regions: [3,4]
}

// store update could be done like this:
// (this can be done more elegantly)
(store) => {
  return {
    ...store,
    entities: {
      countries: {
        ...store.countries,
        ...data.entities,
      },
      regions: {
        ...store.regions,
        ...data.regions,
      }
    }
  }
}

// So any component that is consuming, let's say, the region with the "Kansai" name, will be updated even though you've fetched from the country endpoint