calculate next y line of a text tspan

gistfile1.txt

var replaceTextNode = function($parent, txt) {
		// has a textPath?
		var $textPath = $parent.getElementsByTagName('textPath');
		if ($textPath) $textPath = $textPath[0];
		// clean off
		while ($parent.hasChildNodes())
			$parent.removeChild($parent.lastChild);
		// white space off
		txt = txt.trim();
		// get array of lines
		var arr = $textPath ? [txt] : txt.split("\n");
		var inx = 0;
		// proper positioning the lines
		while (arr.length){
			var fontHeight = parseFloat($parent.style.getPropertyValue('font-size'));
			let lh = $parent.style.getPropertyValue('line-height');
			let div = lh.slice(-1) == "%" ? 0.01 : 1;
			var lineHeight = parseFloat(lh)*div;
			var dy = fontHeight * lineHeight * inx;
			var $elem = document.createElementNS(xmlns,"tspan");
			$elem.textContent = arr.shift();
			if ($textPath) {
				while($textPath.hasChildNodes()) $textPath.removeChild($textPath.lastChild);
				$textPath.appendChild($elem);
				$parent.appendChild($textPath);
			} else {
				$elem.setAttribute('x', $parent.getAttribute('x'));
				$elem.setAttribute('y', 1 * $parent.getAttribute('y') + dy );
				$parent.appendChild($elem);
			}
			inx++;
		}
	};