Fixes undefined handler passing in gRPC module

js/modules/k6/grpc/stream.go

@@ -293,9 +293,13 @@ func (s *stream) processSendError(err error) {
 	})
 }
 
-// on registers a listener for a certain event type
-func (s *stream) on(event string, listener func(sobek.Value) (sobek.Value, error)) {
-	if err := s.eventListeners.add(event, listener); err != nil {
+// on registers a handler for a certain event type
+func (s *stream) on(event string, handler func(sobek.Value) (sobek.Value, error)) {
+	if handler == nil {
+		common.Throw(s.vu.Runtime(), fmt.Errorf("handler for %q event isn't a callable function", event))
+	}
+
+	if err := s.eventListeners.add(event, handler); err != nil {
 		s.vu.State().Logger.Warnf("can't register %s event handler: %s", event, err)
 	}
 }

js/modules/k6/grpc/stream_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/golang/protobuf/ptypes/wrappers"
 	"github.com/grafana/sobek"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/metadata"
 	"google.golang.org/grpc/status"
@@ -363,3 +364,45 @@ func TestStream_Wrappers(t *testing.T) {
 	},
 	)
 }
+
+func TestStream_UndefinedHandler(t *testing.T) {
+	t.Parallel()
+
+	ts := newTestState(t)
+
+	stub := grpc_wrappers_testing.Register(ts.httpBin.ServerGRPC)
+	stub.TestStreamImplementation = func(stream grpc_wrappers_testing.Service_TestStreamServer) error {
+		return stream.SendAndClose(&wrappers.StringValue{
+			Value: "test",
+		})
+	}
+
+	replace := func(code string) (sobek.Value, error) {
+		return ts.VU.Runtime().RunString(ts.httpBin.Replacer.Replace(code))
+	}
+
+	initString := codeBlock{
+		code: `
+		var client = new grpc.Client();
+		client.load([], "../../../../lib/testutils/httpmultibin/grpc_wrappers_testing/test.proto");`,
+	}
+	vuString := codeBlock{
+		code: `
+		client.connect("GRPCBIN_ADDR");
+		let stream = new grpc.Stream(client, "grpc.wrappers.testing.Service/TestStream");
+		stream.on('data', undefined);
+
+		stream.end();
+		`,
+	}
+
+	val, err := replace(initString.code)
+	assertResponse(t, initString, err, val, ts)
+
+	ts.ToVUContext()
+
+	_, err = replace(vuString.code)
+	ts.EventLoop.WaitOnRegistered()
+
+	require.ErrorContains(t, err, "handler for \"data\" event isn't a callable function")
+}