createList.ts

function createMapById<PropName extends string>(
  $list: Store<{ [key in PropName]: any }[]>,
  propName: PropName,
  id: string
) {
  return $list.map(list =>
    list.reduce(
      (propByIdMap, element) => (
        (propByIdMap[element[id]] = element[propName]), propByIdMap
      ),
      {}
    )
  );
}

export function createList(propsList: string[], uidPropName: string) {
  const $list = createStore([]);

  const $listNorm = propsList.reduce(
    (propsMap, propName) => (
      (propsMap[propName] = createMapById($list, propName, uidPropName)),
      propsMap
    ),
    {}
  );

  function createListElement(id: string) {
    return combine(propsList.reduce(
      (elementShape, propName) => (
        (elementShape[propName] = $listNorm[propName].map(propByIdMap => propByIdMap[id]),
          elementShape
        )),
      {}
    ));
  }

  $list.createListElement = createListElement

  return $list
}

// USAGE

const $users = createList(['id', 'email', 'name'], 'id')
  .on(fetchUsers.done, (_, payload) => payload)

fetchUsers.done([{ id: 1, email: 'a@b.c', name: 'name' }])

const $firstUser = $users.createListElement(1)

$firstUser.getState()
// { id: 1, email: 'a@b.c', name: 'name' }