diff --git a/vlib/strings/builder_js.v b/vlib/strings/builder_js.v
index d5a4b7b947..dc8a97b87e 100644
--- a/vlib/strings/builder_js.v
+++ b/vlib/strings/builder_js.v
@@ -7,7 +7,7 @@ module strings
 pub struct Builder {
 mut:
 	buf []byte
-pub:
+pub mut:
 	len int
 	initial_size int = 1
 }