Detect visibility change of Browser Window in JavaScript

var defaultTitle = document.title;

// subscribe to visibility change events
document.addEventListener('visibilitychange', function () {
    // fires when user switches tabs, apps, goes to homescreen, etc.
    if (document.visibilityState === 'hidden') {
        document.title = 'Baby, Come Back!'
    }
    // fires when app transitions from prerender, user returns to the app / tab.
    if (document.visibilityState === 'visible') {
        document.title = defaultTitle;
    }
});

Taken from igvita.com.

To provide a reliable and consistent user experience, both on desktop and mobile, the application must use Page Visibility API and execute its session save and restore logic whenever visibilityChange state changes. This is the only event your application can count on. (Source)