Are there elements are activated or how to judge the document gets focus

This issue involves two points: 

1. document.activeElement; acquired element is activated, no null is returned;

2. document.hasFocus (); determining whether there has been activated or acquire focus, returns a Boolean value;

 

There are the focus of the document must be activated (active), not vice versa.

 

In other words:

1. document.activeElement not empty, document.hasFocus () is not necessarily true;

2. document.hasFocus () is true, document.activeElement certainly not empty;