The Selection.anchorOffset read-only property returns the number of characters that the selection's anchor is offset within the Selection.anchorNode if said node is of type Text, CDATASection or Comment.
In the case of Selection.anchorNode being another type of node, Selection.anchorOffset returns the number of Node.childNodes the selection's anchor is offset within the Selection.anchorNode.
This number is zero-based. If the selection begins with the first character in the Selection.anchorNode, 0 is returned.
A number.
Selection, the interface it belongs to.