LibJS: Add the WeakRef.prototype.deref method

This is the only exposed method of the WeakRef built-in.

Userland/Libraries/LibJS/Runtime/WeakRefPrototype.cpp

@@ -18,6 +18,8 @@ void WeakRefPrototype::initialize(GlobalObject& global_object)
     auto& vm = this->vm();
     Object::initialize(global_object);
 
+    define_native_function(vm.names.deref, deref, 0, Attribute::Writable | Attribute::Configurable);
+
     define_property(vm.well_known_symbol_to_string_tag(), js_string(global_object.heap(), vm.names.WeakRef), Attribute::Configurable);
 }
 
@@ -25,4 +27,19 @@ WeakRefPrototype::~WeakRefPrototype()
 {
 }
 
+// 26.1.3.2 WeakRef.prototype.deref ( ), https://tc39.es/ecma262/#sec-weak-ref.prototype.deref
+JS_DEFINE_NATIVE_FUNCTION(WeakRefPrototype::deref)
+{
+    auto* this_object = vm.this_value(global_object).to_object(global_object);
+    if (!this_object)
+        return {};
+    if (!is<WeakRef>(this_object)) {
+        vm.throw_exception<TypeError>(global_object, ErrorType::NotA, "WeakRef");
+        return {};
+    }
+    auto& weak_ref = static_cast<WeakRef&>(*this_object);
+    weak_ref.update_execution_generation();
+    return weak_ref.value() ?: js_undefined();
+}
+
 }

Userland/Libraries/LibJS/Runtime/WeakRefPrototype.h

@@ -17,6 +17,9 @@ public:
     WeakRefPrototype(GlobalObject&);
     virtual void initialize(GlobalObject&) override;
     virtual ~WeakRefPrototype() override;
+
+private:
+    JS_DECLARE_NATIVE_FUNCTION(deref);
 };
 
 }

Userland/Libraries/LibJS/Tests/builtins/WeakRef/WeakRef.prototype.deref.js

@@ -0,0 +1,21 @@
+test("length is 0", () => {
+    expect(WeakRef.prototype.deref).toHaveLength(0);
+});
+
+test("basic functionality", () => {
+    var original = { a: 1 };
+    var weakRef = new WeakRef(original);
+
+    expect(weakRef.deref()).toBe(original);
+});
+
+test("kept alive for current synchronous execution sequence", () => {
+    var weakRef;
+    {
+        weakRef = new WeakRef({ a: 1 });
+    }
+    weakRef.deref();
+    gc();
+    // This is fine 🔥
+    expect(weakRef.deref()).not.toBe(undefined);
+});