How to detect if user is away from web page using Page Visibility API JavaScript

In this simple tutorial we are going to learn how to detect if user is away from your website and if true then alert our user. Remember what i understand that here Away and idle are two different things,

Add WhatsApp Share Button On Your Website

As we all know most of users now uses their smartphone to access our website and usually our website can easily have Facebook, Google+ and Twitter button but now WhatsApp messenger is also a very popular social media platform and if we know most of our users are using mobile to access our site then why should not we put a WhatsApp share button on our web?