Multi-tab browsing is now the norm, so you can't assume the user is watching your app just because it's running. Fortunately, the new Page Visibility API lets your app discover if it's visible or not. You could use the API to cut down on unnecessary network activity and computation.
document.webkitHidden is a boolean value indicating if the current page is hidden (you can try it now in the console if you're using a recent build of Chromium).
document.webkitVisibilityState will return a string indicating the current state, one of
prerendered. And a new
webkitvisibilitychange event will fire when any of these changes, e.g. when the user opens you app's tab, or moves away from it.
If you're interested in giving this a whirl, check out visibility.js which adds a little bit of sugar on the API to make watching these interactions a bit more fun.