The axis read-only property of the ScrollTimeline interface returns an enumerated value representing the scroll axis that is driving the progress of the timeline. 
 
An enumerated value. Possible values are:
  "block"  -  
The scrollbar on the block axis of the scroll container, which is the axis in the direction perpendicular to the flow of text within a line. For horizontal writing modes, such as standard English, this is the same as "y", while for vertical writing modes, it is the same as "x".
   "inline"  -  
The scrollbar on the inline axis of the scroll container, which is the axis in the direction parallel to the flow of text in a line. For horizontal writing modes, this is the same as "x", while for vertical writing modes, this is the same as "y".
   "y"  -  
The scrollbar on the vertical axis of the scroll container.
   "x"  -  
The scrollbar on the horizontal axis of the scroll container.
   
 
See the main ScrollTimeline page for an example.