Mobile HTML5

Chapter 6. Other HTML5 APIs

Offline Web Applications

Until now, users of web applications have only been able to use applications while connected to the Internet. When offline, web-based email, calendars, and other online tools have been unavailable, and for the most part, continue to be.

While offline, users may still access some portions of sites they have visited by accessing what is in the browser cache, but that is limited and difficult to manage. If a user gets bumped offline in the middle of a process, like writing an email or filling in a form, hitting submit can lead to a loss of all the data entered.

The HTML5 specification provides a few solutions, including local and session storage for storing data locally, and an offline application HTTP cache for ensuring applications are available even when the user is offline. HTML5 contains several features that address the challenge of building web applications that don’t lose all functionality while offline, including indexDB, offline application caching APIs, connection events, status, as well as the localStorage and sessionStorage APIs.

Am I Even Connected to the Internet?

One thing you may want to know when implementing offline features is if the user is indeed connected to the Internet. HTML5 defines an onLine property on the Navigator object so you can determine whether the user is currently online:

var isOnline = navigator.onLine;

This will return true or false. Note that if it returns true, it could mean the user is on an intranet ...