Disable async function as group callback

The general inconsistencies between group as an idea and
asynchronous code makes it not great fit and such we will try to
discourage users to mix them.

Updates #2728

js/modules/k6/k6.go

@@ -22,6 +22,8 @@ var (
 	ErrCheckInInitContext = common.NewInitContextError("Using check() in the init context is not supported")
 )
 
+const asyncFunctionNotSupportedMsg = "Async Function not supported: Come up with a message here"
+
 type (
 	// RootModule is the global module instance that will create module
 	// instances for each VU.
@@ -85,16 +87,24 @@ func (mi *K6) RandomSeed(seed int64) {
 }
 
 // Group wraps a function call and executes it within the provided group name.
-func (mi *K6) Group(name string, fn goja.Callable) (goja.Value, error) {
+func (mi *K6) Group(name string, val goja.Value) (goja.Value, error) {
 	state := mi.vu.State()
 	if state == nil {
 		return nil, ErrGroupInInitContext
 	}
 
-	if fn == nil {
+	if val == nil || goja.IsNull(val) {
 		return nil, errors.New("group() requires a callback as a second argument")
 	}
-
+	fn, ok := goja.AssertFunction(val)
+	if !ok {
+		return nil, errors.New("group() requires a callback as a second argument")
+	}
+	rt := mi.vu.Runtime()
+	o := val.ToObject(rt)
+	if o.ClassName() == "AsyncFunction" {
+		return goja.Undefined(), errors.New(asyncFunctionNotSupportedMsg)
+	}
 	g, err := state.Group.Group(name)
 	if err != nil {
 		return goja.Undefined(), err
@@ -138,6 +148,7 @@ func (mi *K6) Group(name string, fn goja.Callable) (goja.Value, error) {
 }
 
 // Check will emit check metrics for the provided checks.
+//
 //nolint:cyclop
 func (mi *K6) Check(arg0, checks goja.Value, extras ...goja.Value) (bool, error) {
 	state := mi.vu.State()

js/modules/k6/k6_test.go

@@ -186,6 +186,20 @@ func TestGroup(t *testing.T) {
 		_, err := rt.RunString(`k6.group("::", function() { throw new Error("nooo") })`)
 		assert.Contains(t, err.Error(), "group and check names may not contain '::'")
 	})
+
+	t.Run("async function", func(t *testing.T) {
+		t.Parallel()
+		rt, _, _ := setupGroupTest()
+		_, err := rt.RunString(`k6.group("something", async function() { })`)
+		assert.ErrorContains(t, err, asyncFunctionNotSupportedMsg)
+	})
+
+	t.Run("async lambda", func(t *testing.T) {
+		t.Parallel()
+		rt, _, _ := setupGroupTest()
+		_, err := rt.RunString(`k6.group("something", async () => { })`)
+		assert.ErrorContains(t, err, asyncFunctionNotSupportedMsg)
+	})
 }
 
 func checkTestRuntime(t testing.TB) (*goja.Runtime, chan metrics.SampleContainer, *metrics.BuiltinMetrics) {