source code for applyMiddleware of redux

applyMiddleware.js

/* v3.5.2*/
export default function (...middlewares) {
  return (createStore) => (reducers, initialState, enhancer) => {
    const store = createStore(reducers, initialState, enhancer)
    const dispatch = store.dispatch
    const chain = []
    const middleWareAPI = {
      getState: store.getState,
      dispatch: action => dispatch(action)
    }
    
    chain = middlewares.map(middleware => middleware(middlewareAPI))
    dispatch = compose(...chain)(store.dispatch) // store.dispatch or dispatch both work
    // compose will do following thing:
    /*
     * a, b, c ==> a(b(c())), indeed, it is just a reduce and store.dispatch will be an initial value
     */
    return {
      ...store,
      dispatch
    }
  }
}