I am using draftJS as wysiwyg in a dashboard and i use to save the content as HTML, cause i use it for example in a news section of the website.
In first time import the related functions from the draft-js package.
import { EditorState, convertFromRaw, convertToRaw } from 'draft-js';
then when you would like to store it on your database just send it to your back end as html string.
const rawContentState = convertToRaw(this.state.editorState.getCurrentContent());
const HtmlContent = draftToHtml(rawContentState);
Now in HtmlContent you got your html text!