The orientType read-only property of the SVGMarkerElement interface returns an SVGAnimatedEnumeration object indicating whether the orient attribute is auto, an angle value, or something else.

This something else is most likely to be the keyword auto-start-reverse however the spec leaves it open for this to be other values. Unsupported values will generally be thrown away by the parser, leaving the value the default of auto.

An SVGAnimatedEnumeration object. This contains one of the following values:

0

SVG_MARKER_ORIENT_UNKNOWN which means that the orient attribute has a value other than auto or an angle.

1

SVG_MARKERUNITS_ORIENT_AUTO which means that the orient attribute has the keyword value auto.

2

SVG_MARKERUNITS_ORIENT_ANGLE which means that the orient attribute has an <angle> or <number> value indicating the angle.

The orientType property returns an SVGAnimatedEnumeration object. As the value of the orient attribute is an angle, returning the SVGAnimatedEnumeration.baseVal returns 2.

<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=".63deg">
      <path d="M 0 0 L 10 5 L 0 10 z" />
    </marker>
  </defs>
</svg>

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