Fix locator required field validation

internal/js/modules/k6/browser/browser/locator_mapping.go

@@ -212,9 +212,14 @@ func mapLocator(vu moduleVU, lo *common.Locator) mapping {
 			ml := mapLocator(vu, lo.GetByTitle(ptitle, popts))
 			return rt.ToValue(ml).ToObject(rt), nil
 		},
-		"locator": func(selector string) *sobek.Object {
-			ml := mapLocator(vu, lo.Locator(selector))
-			return rt.ToValue(ml).ToObject(rt)
+		"locator": func(selector sobek.Value) (*sobek.Object, error) {
+			s, err := validateRequiredString(selector, "selector")
+			if err != nil {
+				return nil, err
+			}
+
+			ml := mapLocator(vu, lo.Locator(s))
+			return rt.ToValue(ml).ToObject(rt), nil
 		},
 		"innerHTML": func(opts sobek.Value) *sobek.Promise {
 			return k6ext.Promise(vu.Context(), func() (any, error) {

internal/js/modules/k6/browser/browser/page_mapping.go

@@ -338,9 +338,14 @@ func mapPage(vu moduleVU, p *common.Page) mapping { //nolint:gocognit,cyclop
 			}), nil
 		},
 		"keyboard": mapKeyboard(vu, p.GetKeyboard()),
-		"locator": func(selector string, opts sobek.Value) *sobek.Object {
-			ml := mapLocator(vu, p.Locator(selector, parseLocatorOptions(rt, opts)))
-			return rt.ToValue(ml).ToObject(rt)
+		"locator": func(selector sobek.Value, opts sobek.Value) (*sobek.Object, error) {
+			s, err := validateRequiredString(selector, "selector")
+			if err != nil {
+				return nil, err
+			}
+
+			ml := mapLocator(vu, p.Locator(s, parseLocatorOptions(rt, opts)))
+			return rt.ToValue(ml).ToObject(rt), nil
 		},
 		"mainFrame": func() *sobek.Object {
 			mf := mapFrame(vu, p.MainFrame())
@@ -1044,3 +1049,26 @@ func waitForNavigationBodyImpl(vu moduleVU, target interface {
 		return mapResponse(vu, resp), nil
 	}), nil
 }
+
+func validateRequiredString(v sobek.Value, name string) (string, error) {
+	var s string
+
+	if k6common.IsNullish(v) {
+		return s, errors.New("missing required argument '" + name + "'")
+	}
+
+	const stringType = string("")
+
+	switch v.ExportType() {
+	case reflect.TypeOf(stringType):
+		s = v.String()
+	default:
+		return s, errors.New("'" + name + "' must be a string")
+	}
+
+	if s == "" {
+		return s, errors.New("'" + name + "' must be a non-empty string")
+	}
+
+	return s, nil
+}

internal/js/modules/k6/browser/tests/locator_test.go

@@ -1270,3 +1270,53 @@ func TestVisibilityWithCORS(t *testing.T) {
 		})
 	}
 }
+
+func TestLocatorLocatorEmpty(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		code     string
+		expected string
+	}{
+		{
+			name:     "undefined",
+			code:     "undefined",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "null",
+			code:     "null",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "empty",
+			code:     "",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "empty_string",
+			code:     "''",
+			expected: "'selector' must be a non-empty string",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Setup
+			tb := newTestBrowser(t, withFileServer())
+			tb.vu.ActivateVU()
+			tb.vu.StartIteration(t)
+
+			// test logic
+			code := fmt.Sprintf(`
+				page = await browser.newPage();
+				page.locator('test').locator(%s);`, tt.code)
+
+			_, err := tb.vu.RunAsync(t, code)
+			require.ErrorContains(t, err, tt.expected)
+		})
+	}
+}

internal/js/modules/k6/browser/tests/page_test.go

@@ -3436,3 +3436,57 @@ func TestClickInNestedFramesCORS(t *testing.T) {
 		assert.Equal(t, expectedCount, countD)
 	})
 }
+
+func TestPageLocatorEmpty(t *testing.T) {
+	t.Parallel()
+
+	if runtime.GOOS == "windows" {
+		t.Skip("Skipped due to https://github.com/grafana/k6/issues/4937")
+	}
+
+	tests := []struct {
+		name     string
+		code     string
+		expected string
+	}{
+		{
+			name:     "undefined",
+			code:     "undefined",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "null",
+			code:     "null",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "empty",
+			code:     "",
+			expected: "missing required argument 'selector'",
+		},
+		{
+			name:     "empty_string",
+			code:     "''",
+			expected: "'selector' must be a non-empty string",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			// Setup
+			tb := newTestBrowser(t, withFileServer())
+			tb.vu.ActivateVU()
+			tb.vu.StartIteration(t)
+
+			// test logic
+			code := fmt.Sprintf(`
+				page = await browser.newPage();
+				page.locator(%s);`, tt.code)
+
+			_, err := tb.vu.RunAsync(t, code)
+			require.ErrorContains(t, err, tt.expected)
+		})
+	}
+}