LibJS: Implement Object.isFrozen() and Object.isSealed()

Author: https://github.com/linusg
Commit: f3264b0dbd
Pull-request: https://github.com/SerenityOS/serenity/pull/6163
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibJS/Tests/builtins/Object/Object.isSealed.js

@@ -0,0 +1,27 @@
+test("length is 1", () => {
+    expect(Object.isSealed).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("returns true for non-object argument", () => {
+        expect(Object.isSealed(42)).toBeTrue();
+        expect(Object.isSealed("foobar")).toBeTrue();
+    });
+
+    test("returns false for regular object", () => {
+        const o = { foo: "bar" };
+        expect(Object.isSealed(o)).toBeFalse();
+    });
+
+    test("returns true for sealed object", () => {
+        const o = { foo: "bar" };
+        Object.seal(o);
+        expect(Object.isSealed(o)).toBeTrue();
+    });
+
+    test("returns true for non-extensible empty object", () => {
+        const o = {};
+        Object.preventExtensions(o);
+        expect(Object.isSealed(o)).toBeTrue();
+    });
+});