swarm-ui: test which alert instances the alerts page lists

The firing-only filter and the helpers beside it move out of the page
component into a plain module, with tests for the state match, the
never-active time and the row key.

frontend/packages/swarm-ui/test/alerts.test.mjs

@@ -0,0 +1,68 @@
+// Which alert instances the alerts page lists. Run by `npm test`, which Node
+// runs with its built-in test runner and type stripping, so the `.ts` module
+// is imported as it is.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { activeSinceMs, alertKey, firingAlerts } from "../src/pages/alerts.ts";
+
+const alert = (name, state, extra = {}) => ({
+  name,
+  state,
+  active_at: null,
+  summary: null,
+  labels: { instance: name },
+  ...extra,
+});
+
+test("only instances in the Alerting state are listed", () => {
+  const all = [
+    alert("a", "Alerting"),
+    alert("b", "Pending"),
+    alert("c", "NoData"),
+    alert("d", "Error"),
+    alert("e", "Alerting"),
+  ];
+  assert.deepEqual(
+    firingAlerts(all).map((a) => a.name),
+    ["a", "e"],
+  );
+});
+
+test("nothing firing leaves the list empty, and the input is not changed", () => {
+  const all = [alert("a", "Pending"), alert("b", "Error")];
+  assert.deepEqual(firingAlerts(all), []);
+  assert.equal(all.length, 2);
+  assert.deepEqual(firingAlerts([]), []);
+});
+
+test("the state match is exact, so an unknown or lower-case state is left out", () => {
+  assert.deepEqual(firingAlerts([alert("a", "alerting"), alert("b", "")]), []);
+});
+
+test("an instance that never became active has no active-since time", () => {
+  assert.equal(activeSinceMs(alert("a", "Alerting")), null);
+  assert.equal(
+    activeSinceMs(
+      alert("a", "Alerting", { active_at: "0001-01-01T00:00:00Z" }),
+    ),
+    null,
+  );
+  assert.equal(
+    activeSinceMs(alert("a", "Alerting", { active_at: "not a date" })),
+    null,
+  );
+  assert.equal(
+    activeSinceMs(
+      alert("a", "Alerting", { active_at: "2026-10-18T08:00:00Z" }),
+    ),
+    Date.parse("2026-10-18T08:00:00Z"),
+  );
+});
+
+test("instances of one rule get different keys, whatever the label order", () => {
+  const a = alert("r", "Alerting", { labels: { x: "1", y: "2" } });
+  const b = alert("r", "Alerting", { labels: { y: "2", x: "1" } });
+  const c = alert("r", "Alerting", { labels: { x: "1", y: "3" } });
+  assert.equal(alertKey(a), alertKey(b));
+  assert.notEqual(alertKey(a), alertKey(c));
+});