Redux => To web worker

0_concept.js

const composedCreateStore = compose(
  applyMiddleware(
    thunkMiddleware, 
    dispatchIdMiddleware, 
    webWorkerMiddleware('/build/worker.js'),
    () => promiseMiddleware, 
    loggerMiddleware
  ),
  createStore
);

const redux = composedCreateStore(reducers, {});

redux.dispatch({
  type: "COMPUTE_DATA",
  payload: 10,
  meta: {
    background: true
  }
});

// Logged actions:
[
  {"type": "COMPUTE_DATA", "payload": 10, "meta": {"background": true, "id": "d1", "backgroundTaskId": "j2", "sequence": "begin"}},
  {"type": "COMPUTE_DATA", "payload": 20, "meta": {"background": false, "id": "d1", "backgroundTaskId": "j2", "sequence": "complete"}}
]

1_worker.js

import { isFSA } from 'flux-standard-action';
import assign from 'lodash/object/assign'

const dispatch = global.postMessage;

function mergeMeta(action, meta) {
  return {...action.meta, ...meta};
}

global.onmessage = function (e) {
  if (isFSA(e.data)) {
    const action = e.data;

    setTimeout(() => dispatch({
      ...action,
      payload: action.payload * 2,
      meta: mergeMeta(action, {
        sequence: 'complete'
      })
    }), 5000);
  }
};

2_webWorkerMiddleware.js

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

function mergeMeta(action, meta) {
  return {...action.meta, ...meta};
}

export default function webWorkerMiddleware(scriptURL) {
  let dispatch = null;
  const worker = new Worker(scriptURL);

  worker.onmessage = (e) => {
    if (dispatch && isFSA(e.data)) {
      dispatch(e.data);
    }
  };

  return methods => {
    dispatch = methods.dispatch;

    return next => action => {
      if (!isFSA(action) || !action.meta || !action.meta.background) {
        return next(action);
      }

      const backgroundTaskId = uniqueId('bt');

      worker.postMessage({ 
        ...action, 
        meta: mergeMeta(action, { 
          backgroundTaskId, 
          background: false 
        })
      });

      next({ 
        ...action, 
        meta: mergeMeta(action, { 
          backgroundTaskId, 
          sequence: 'begin' 
        }) 
      });

      return backgroundTaskId;
    }
  }
}