injectableMiddleware.js

import { createStore, compose, applyMiddleware } from 'redux'

export default function createInjectableStore(reducers, preloadedState, enhancer) {
  const injectedMiddleware = []

  const injectableMiddleware = store => next => action => {
    const chain = injectedMiddleware.map(m => m(store))
    const dispatchThroughMiddleware = chain.reduceRight((next, f) => {
      return f(next)
    }, action => next(action))

    return dispatchThroughMiddleware(action)
  }

  function injectMiddleware(middleware) {
    injectedMiddleware.push(store => next => action => {
      return middleware(store)(next)(action)
    })
  }

  const store = createStore(
    reducers,
    preloadedState,
    compose(enhancer, applyMiddleware(injectableMiddleware))
  )

  return Object.assign(store, {
    injectMiddleware,
  })
}

Usage:

const store = createInjectableStore(reducers, preloadedState, applyMiddleware(thunk))

store.injectMiddleware(todoMiddleware)

Updated gist which scopes the injected middleware into "local" middleware. https://gist.github.com/goatslacker/511a92a4fa154ad4dc9e6b844da6b310