index.js

import { isFSA } from 'flux-standard-action';
import uniqueId from 'lodash/utility/uniqueId';

function isPromise(val) {
  return val && typeof val.then === 'function';
}

export default function promiseMiddleware({ dispatch }) {
  return next => action => {
    if (!isFSA(action)) {
      return isPromise(action)
        ? action.then(dispatch)
        : next(action);
    }

    if (isPromise(action.payload)) {
      const sequenceId = uniqueId();

      dispatch({
        ...action,
        payload: undefined,
        sequence: {
          type: 'start',
          id: sequenceId
        }
      });

      return action.payload.then(
        result => dispatch({
          ...action,
          payload: result,
          sequence: {
            type: 'next',
            id: sequenceId
          }
        }),
        error => dispatch({
          ...action,
          payload: error,
          error: true,
          sequence: {
            type: 'next',
            id: sequenceId
          }
        })
      );
    }

    return next(action);
  };
}