LibJS: Add the WeakSet built-in object

Userland/Libraries/LibJS/Tests/builtins/WeakSet/WeakSet.js

@@ -0,0 +1,30 @@
+test("constructor properties", () => {
+    expect(WeakSet).toHaveLength(0);
+    expect(WeakSet.name).toBe("WeakSet");
+});
+
+describe("errors", () => {
+    test("invalid array iterators", () => {
+        [-100, Infinity, NaN, {}, 152n].forEach(value => {
+            expect(() => {
+                new WeakSet(value);
+            }).toThrowWithMessage(TypeError, "is not iterable");
+        });
+    });
+    test("called without new", () => {
+        expect(() => {
+            WeakSet();
+        }).toThrowWithMessage(TypeError, "WeakSet constructor must be called with 'new'");
+    });
+});
+
+describe("normal behavior", () => {
+    test("typeof", () => {
+        expect(typeof new WeakSet()).toBe("object");
+    });
+
+    test("constructor with single array argument", () => {
+        var a = new WeakSet([{ a: 1 }, { a: 2 }, { a: 3 }]);
+        expect(a instanceof WeakSet).toBeTrue();
+    });
+});