redux/src/applyMiddleware.js

import compose from './compose'

/**
 * Creates a store enhancer that applies middleware to the dispatch method
 * of the Redux store. This is handy for a variety of tasks, such as expressing
 * asynchronous actions in a concise manner, or logging every action payload.
 *
 * See `redux-thunk` package as an example of the Redux middleware.
 *
 * Because middleware is potentially asynchronous, this should be the first
 * store enhancer in the composition chain.
 *
 * Note that each middleware will be given the `dispatch` and `getState` functions
 * as named arguments.
 *
 * @param {...Function} middlewares The middleware chain to be applied.
 * @returns {Function} A store enhancer applying the middleware.
 */
export default function applyMiddleware(...middlewares) {
  return (createStore) => (reducer, preloadedState, enhancer) => {
    var store = createStore(reducer, preloadedState, enhancer)
    var dispatch = store.dispatch
    var chain = []

    var middlewareAPI = {
      getState: store.getState,
      dispatch: (action) => dispatch(action)
    }
    chain = middlewares.map(middleware => middleware(middlewareAPI))
    dispatch = compose(...chain)(store.dispatch)

    return {
      ...store,
      dispatch
    }
  }
}