LibJS: Implement Temporal.Calendar.prototype.era()

Userland/Libraries/LibJS/Tests/builtins/Temporal/Calendar/Calendar.prototype.era.js

@@ -0,0 +1,26 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Calendar.prototype.era).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const plainDate = new Temporal.PlainDate(2021, 7, 6);
+        const calendar = new Temporal.Calendar("iso8601");
+        expect(calendar.era(plainDate)).toBeUndefined();
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.Calendar object", () => {
+        expect(() => {
+            Temporal.Calendar.prototype.era.call("foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.Calendar");
+    });
+
+    test("argument must be date-like", () => {
+        const calendar = new Temporal.Calendar("iso8601");
+        expect(() => {
+            calendar.era({});
+        }).toThrowWithMessage(TypeError, "Required property year is missing or undefined");
+    });
+});