The result read-only property of the SVGFEMergeElement interface describes the assigned name of an SVG filter primitive as a SVGAnimatedString.
It reflects the result attribute of the <feMerge> element. The attribute value is a <custom-ident>. If supplied, then graphics that result from processing this filter primitive can be referenced by an in attribute on a subsequent filter primitive within the same <filter> element.
If no result attribute is defined, the filter's result.baseVal and result.animVal are empty strings, and the output of the <feMerge> filter will only be available for re-use as the implicit input into the next filter primitive if that filter primitive provides no value for its in attribute.
const feMergeElement = document.querySelector("feMerge");
const filterName = feMergeElement.result;
console.log(filterName.baseVal); // the filter's assigned nameSVGFEMergeNodeElement.in1<custom-ident> data type