When implementing single-page applications, sooner or later the question of offline use will come up. Given how hard it is to get this right when retrofitting an offline mode into an existing application, there is a trend towards implementing single-page applications with an “offline-first” mindset. An important implementation technique that we have used successfully is local storage sync. With this technique, the user facing code never makes requests to the backend. It retrieves data solely from local storage. A background worker synchronises the data in local storage with the backend systems, usually employing calls to some form of REST API.