diff --git a/vlib/builtin/js/jsfns.js.v b/vlib/builtin/js/jsfns.js.v
index 0dd89dba13..83cc2aff1d 100644
--- a/vlib/builtin/js/jsfns.js.v
+++ b/vlib/builtin/js/jsfns.js.v
@@ -122,7 +122,7 @@ fn (v JS.Map) toString() JS.String
 // Hack for "`[]JS.String` is not a struct" when returning arr.length or arr.len
 // TODO: Fix []JS.String not a struct error
 fn native_str_arr_len(arr []JS.String) int {
-	len := 0
+	mut len := 0
 	#len = arr.length
 
 	return len