Export the store from the module you called createStore with. Then you are assured it will both be created and will not pollute the global window space.

MyStore.js

const store = createStore(myReducer);
export store;

or

const store = createStore(myReducer);
export default store;

MyClient.js

import {store} from './MyStore'
store.dispatch(...)

or if you used default

import store from './MyStore'
store.dispatch(...)

For Multiple Store Use Cases

If you need multiple instances of a store, export a factory function. I would recommend making it async (returning a promise).

async function getUserStore (userId) {
   // check if user store exists and return or create it.
}
export getUserStore

On the client (in an async block)

import {getUserStore} from './store'

const joeStore = await getUserStore('joe')