diff --git a/js/31-classes-ids.js b/js/31-classes-ids.js
new file mode 100644
index 0000000..4844fb2
--- /dev/null
+++ b/js/31-classes-ids.js
@@ -0,0 +1,25 @@
+export default function compute() {
+
+let ret = {
+	class: {},
+	id: {},
+	attribute: {}
+};
+
+walkSelectors(ast, selector => {
+	let sast = parsel.parse(selector, {list: false});
+
+	parsel.walk(sast, node => {
+		if (node.type === "id" || node.type === "class" || node.type === "attribute") {
+			incrementByKey(ret[node.type], node.name);
+		}
+	}, {subtree: true});
+});
+
+for (let type in ret) {
+	ret[type] = sortObject(ret[type]);
+}
+
+return ret;
+
+}