FocusEvent: relatedTarget property

The FocusEvent.relatedTarget read-only property is the secondary target, depending on the type of event:

Event name	target	relatedTarget
blur	The EventTarget losing focus	The EventTarget receiving focus (if any).
focus	The EventTarget receiving focus	The EventTarget losing focus (if any)
focusin	The EventTarget receiving focus	The EventTarget losing focus (if any)
focusout	The EventTarget losing focus	The EventTarget receiving focus (if any)

Note that many elements can't have focus, which is a common reason for relatedTarget to be null. relatedTarget may also be set to null for security reasons, like when tabbing in or out of a page.

MouseEvent.relatedTarget is a similar property for mouse events.