The markerUnits read-only property of the SVGMarkerElement interface returns an SVGAnimatedEnumeration object. This object returns an integer which represents the keyword values that the markerUnits attribute accepts.

An SVGAnimatedEnumeration object. The baseVal property of this object contains one of the following values:

0

SVG_MARKERUNITS_UNKNOWN which means that the markerUnits attribute has a value other than the two predefined keywords.

1

SVG_MARKERUNITS_USERSPACEONUSE which means that the markerUnits attribute has the keyword value userSpaceOnUse.

2

SVG_MARKERUNITS_STROKEWIDTH which means that the markerUnits attribute has the keyword value strokeWidth.

The markerUnits property returns an SVGAnimatedEnumeration object that contains the value of the markerUnits 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"
      markerUnits="strokeWidth">
      <path d="M 0 0 L 10 5 L 0 10 z" />
    </marker>
  </defs>
</svg>

let marker = document.getElementById("arrow");
console.log(marker.markerUnits.baseVal); // 2