applyMiddleware.js

function applyMiddleware(...middlewares) {
  return (createStore) => (reducer, ...args) => {
    const store = createStore(reducer, ...args)
    let dispatch;
    
    const middlewareAPI = {
      getState: store.dispatch,
      dispatch: (action, ...args) => dispatch(action, ...args)
    }
    
    const chain = middlewares.map(middleware => middleware(middlewareAPI))
    dispatch = compose(...chain)(store.dispatch)
    
    return {
      ...store,
      dispatch
    }
  }
}

compose.js

function compose(...funcs) {
  if (funcs.length === 0) return arg => arg
  if (funcs.length === 1) return funcs[0]
  return funcs.reduce( (a, b) => (...args) => a(b(...args)))
}