The refX read-only property of the SVGMarkerElement interface returns an SVGAnimatedLength object containing the value of the refX attribute of the <marker>.
An SVGAnimatedLength object. The baseVal property of this object returns an SVGLength, the value of which returns the refX.
The markerWidth property returns an SVGAnimatedLength which contains an SVGLength with the value of the refX attribute.
<svg id="svg" viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg">
<defs>
<marker
id="arrow"
viewBox="0 0 10 10"
refX="5"
refY="5"
markerWidth="6"
markerHeight="6"
orient="auto-start-reverse">
<path d="M 0 0 L 10 5 L 0 10 z" />
</marker>
</defs>
</svg>let marker = document.getElementById("arrow");
console.log(marker.refX.baseVal.value); // 5