Best way to create an autosave feature

What is the best way to create an autosave feature in a web application? Kind of like Google Docs.

I can already see couple of ways this could be done.

For example I could do a set interval to update the database every 30 seconds. But that would just be a waste of server resources...

What is the best way to check if an autosave is necessary? Have a duplicate of the entire database result ready and compare it every time? Is there another better way that I'm not thinking about?