The width read-only property of the SVGFEMergeElement interface describes the horizontal size of an SVG filter primitive as a SVGAnimatedLength.
It reflects the width attribute of the <feMerge> element. The attribute is a <length> or a <percentage> relative to the width of the filter region. The default value is 100%. The property value is a length in user coordinate system units.
const feMerge = document.querySelector("feMerge");
const horizontalSize = feMerge.width;
console.log(horizontalSize.baseVal.value); // the `width` value