redux-pre-thunk.js

// thunk based action creator example, cannot be tested while calling mapDispatchToProps beside checking if dispatch was called
function saveScore (score) {
  return (dispatch) => {
    fetch('/scores', { method: 'POST', { score } })
      .then(() => {
        dispatch({ type: SAVE_SCORE_SUCCEEDED })
      })
  }
}

// used to created dispatchable object that is easy to test
function createThunkObject (type, payload, actionCreator) {
  return {
    type: `@@THUNK_OBJECT@@_${type}`,
    payload,
    actionCreator
  };
}

// middleware that inject payload to action creator
const preThunkMiddleware = store => next => action => {
  if (action.type && action.type.indexOf('@@THUNK_OBJECT@@')  === 0) {
    next(action);
  } else {
    next(action.actionCreator(action.payload));
  }
}

// usage example in mapDispatchToProps
function mapDispatchToProps(dispatch) {
  return {
    saveScore: (score) => { dispatch(createThunkObject('SAVE_SCORE', score, saveScore)) }
  }
}