Extracts aggregated values from a SharePoint view using grouping

getGroupedViewData.js

function Grouping(title, value, count) {
	Object.defineProperties(this, {
		title: { value: title },
		value: { value: value },
		count: { value: count },
		groups: { writable: true }
	});
}

function getGroupedViewData(listTitle, viewTitle, success, fail) {
	var url = window._spPageContextInfo.webServerRelativeUrl +
			"/_api/Web/Lists/GetByTitle(%27" + 
			encodeURIComponent(listTitle) + 
			"%27)/Views/GetByTitle(%27" + 
			encodeURIComponent(viewTitle) + 
			"%27)/renderAsHtml()";
	var oReq = new XMLHttpRequest();
	oReq.onload = function(e) {
		var s = document.createElement("div");
		s.innerHTML = this.responseXML.documentElement.textContent;
		var pattern = /^\s*([\w\s]+)?\s:\s(.+?)[\s\W]+[(](\d+)/,
			match,
			tbody,
			mapped,
			retArr = Array.prototype.map.call(s.querySelectorAll(".ms-gb"), function (g1) {				
				match = pattern.exec(g1.textContent);
				tbody = g1.closest("tbody");
				mapped = Array.prototype.map.call(
						tbody.closest("table").querySelectorAll("tbody[groupstring^='" + tbody.getAttribute("groupstring") + "']"), 
						function (g2) {
							match = pattern.exec(g2.textContent);
							return new Grouping(match[1], match[2], Number(match[3]));
						});
				var gHead = mapped.shift(); // extract first item
				gHead.groups = mapped;
				return gHead;
			});
		success(retArr);
	};
	onerror = fail;
	oReq.open("GET", url);
	oReq.send();
}