<style>
.mysvg { border: 1px solid gray; fill: none; stroke: steelblue; }
</style>

<svg class="mysvg" width="200" height="200">
	<polygon id="Poly" points="100 10, 40 190, 190 120, 180 60"></polygon>
</svg>
<svg class="mysvg" width="200" height="200">
	<path id="cPath" d="M10 100 H190"></path>
</svg>
<div id="pathMsg"></div>

<script>

let createPath = (ele) => {
	let path = 'M';
	let points = Poly.getAttribute('points');
	let ar = points.replace(/[^\d.]/g, ',').split(',').filter(Number);
	for(j = 0; j < ar.length; j ++) {
		if(j <= 1) {
			path += ar[j] + ' ';
		}else{
			path += j % 2 === 0 ? 'L' + ar[j] + ' ' : ar[j];
		}
	}
	path += ele.tagName.toLowerCase() === 'polygon' ? 'z' : '';
	return path;
};

cPath.setAttribute('d',createPath(Poly));
pathMsg.innerText = createPath(Poly);

</script>