The markerHeight read-only property of the SVGMarkerElement interface returns an SVGAnimatedLength object containing the height of the <marker> viewport as defined by the markerHeight attribute.
An SVGAnimatedLength object. The baseVal property of this object returns an SVGLength, the value of which returns the height.
The markerHeight property returns an SVGAnimatedLength which contains an SVGLength with the value of the markerHeight 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.markerHeight.baseVal.value); // 6