beating impurity in redux applications using dependency injection

gistfile1.js

/* First, since our reducers need to be free of side-effects, we move them to an
action creator. Note the double arrow function: The outer function is only there
dependency injection. It's a function that creates an action creator, but I
don't really want to call it an action creator creator ;) */
const impureActionCreator = (idGenerator) => () => ({type: 'FOO_ACTION', id: randomSource.randomId()});

// Our reducer will now stay free of side effects, and we already have `action.id` when it's called
const reducer = (state, action) => // ...

// we can unit test our action creator by injecting a pure stub
test('impure action creator', () => {
  const idGeneratorStub = () => 42;
  const previousState = // ...
  const newState = reducer(previousState,  impureActionCreator(idGeneratorStub));
  expect(newState) // ...
});

// in the app entry point we inject the actually impure implementation
const app = createApp({
  // ...
  idGenerator: createCountingIdGeneratorStartingAt(1)
});