batching-drawing-events-with-redux-observable.js

const handleDrawingEvent = line => ({
  line,
  type: 'LINE_DRAWN'
})

const drawingSyncSuccess = () =>  ({
  type: 'DRAWING_SYNC_SUCCESS',
})

const drawingSyncFailure = () => ({
  type: 'DRAWING_SYNC_FAILED',
})

const drawingSyncFailure = () => ({
  type: 'DRAWING_MOUSE_UP',
})

function loadDataEpic(action$) {
  return action$
    .ofType('LINE_DRAWN')
    .bufferBy(action$.ofType('DRAWING_MOUSE_UP'))
    .switchMap((lines) =>
      fetch('/a/fake/url', {
        method: 'post',
        body: JSON.stringify(lines)
      })
        .then(response => response.ok ? drawingSyncSuccess() : drawingSyncFailure())
        .catch(() => drawingSyncFailure())
    )
}

const reducer = (state = {}, action) => {
  switch (action.type) {
    case 'DRAWING_SYNC_SUCCESS':
      return { ...state, drawingSyncSuccess: true, drawingSyncFailed: false }
    case 'DRAWING_SYNC_FAILED':
      return { ...state, drawingSyncFailed: true, drawingSyncSuccess: false }
    default:
      return state;
  }
}

drawingSyncFailure variable name has been used twice. I reckon second action creator name it should be drawingMouseUp.