As documented here (http://offlinefirst.org/sync/ http://offlinefirst.org/sync/) there are two relatively easy options to achieve this: 1) Using Firebase, whic

As documented here (http://offlinefirst.org/sync/ http://offlinefirst.org/sync/) there are two relatively easy options to achieve this: 1) Using Firebase, which is closed source (and owned by Google) 2) Combining CouchDB with PouchDB (which can be considered "CouchDB for the browser"), which are both open source. Both allow saving data in the browser, and both can be configured to sync data saved in the browser with a server backend when needed.