focus selector
Description: Selects element if it is currently focused.
- version added: 1.6jQuery( ":focus" )
As with other pseudo-class selectors (those that begin with a ":"), it is recommended to precede :focus with a tag name or some other selector; otherwise, the universal selector ( "*" ) is implied. In other words, the bare $( ":focus" ) is equivalent to $( "*:focus" ).  If you are looking for the currently focused element, $( document.activeElement ) will retrieve it without having to search the whole DOM tree.
Example:
Adds the focused class to whatever element has focus
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