LibJS: Implement Object.freeze() and Object.seal()

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

@@ -0,0 +1,50 @@
+test("length is 1", () => {
+    expect(Object.seal).toHaveLength(1);
+});
+
+describe("normal behavior", () => {
+    test("returns given argument", () => {
+        const o = {};
+        expect(Object.seal(42)).toBe(42);
+        expect(Object.seal("foobar")).toBe("foobar");
+        expect(Object.seal(o)).toBe(o);
+    });
+
+    test("prevents addition of new properties", () => {
+        const o = {};
+        expect(o.foo).toBeUndefined();
+        Object.seal(o);
+        o.foo = "bar";
+        expect(o.foo).toBeUndefined();
+    });
+
+    test("prevents deletion of existing properties", () => {
+        const o = { foo: "bar" };
+        expect(o.foo).toBe("bar");
+        Object.seal(o);
+        delete o.foo;
+        expect(o.foo).toBe("bar");
+    });
+
+    test("prevents changing attributes of existing properties", () => {
+        const o = { foo: "bar" };
+        Object.seal(o);
+        // FIXME: These don't change anything and should not throw!
+        // expect(Object.defineProperty(o, "foo", {})).toBe(o);
+        // expect(Object.defineProperty(o, "foo", { configurable: false })).toBe(o);
+        expect(() => {
+            Object.defineProperty(o, "foo", { configurable: true });
+        }).toThrowWithMessage(
+            TypeError,
+            "Cannot change attributes of non-configurable property 'foo'"
+        );
+    });
+
+    test("doesn't prevent changing value of existing properties", () => {
+        const o = { foo: "bar" };
+        expect(o.foo).toBe("bar");
+        Object.seal(o);
+        o.foo = "baz";
+        expect(o.foo).toBe("baz");
+    });
+});