LibJS: Implement Temporal.PlainDate.prototype.era

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

@@ -0,0 +1,24 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const plainDate = new Temporal.PlainDate(2021, 7, 6);
+        expect(plainDate.era).toBeUndefined();
+    });
+
+    test("calendar with custom era function", () => {
+        const calendar = {
+            era() {
+                return "foo";
+            },
+        };
+        const plainDate = new Temporal.PlainDate(2021, 7, 6, calendar);
+        expect(plainDate.era).toBe("foo");
+    });
+});
+
+describe("errors", () => {
+    test("this value must be a Temporal.PlainDate object", () => {
+        expect(() => {
+            Reflect.get(Temporal.PlainDate.prototype, "era", "foo");
+        }).toThrowWithMessage(TypeError, "Not a Temporal.PlainDate");
+    });
+});