relatedTarget

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

Event name	target	relatedTarget
Element/blur_event	The EventTarget losing focus	The EventTarget receiving focus (if any).
Element/focus_event	The EventTarget receiving focus	The EventTarget losing focus (if any)
Element/focusin_event	The EventTarget receiving focus	The EventTarget losing focus (if any)
Element/focusout_event	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.

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