diff --git a/src/Pure/General/js.scala b/src/Pure/General/js.scala
--- a/src/Pure/General/js.scala
+++ b/src/Pure/General/js.scala
@@ -1,39 +1,39 @@
 /*  Title:      Pure/General/json.scala
     Author:     Makarius
 
 Support for JavaScript syntax.
 */
 
 package isabelle
 
 
 object JS {
   /* basic syntax */
 
   type Source = String
 
   def arguments(args: Source*): Source = args.mkString("(", ", ", ")")
   def function(f: Source, args: Source*): Source = f + arguments(args: _*)
   def selection(a: Source, arg: Source): Source = a + "[" + arg + "]"
 
   def commands(args: Source*): Source = args.mkString("; ")
   def command_list(args: List[Source]): Source = args.mkString("; ")
 
 
   /* JSON values */
 
   def value(t: JSON.T): Source = JSON.Format(t)
   def string(s: String): Source = value(s)
 
   def json_parse(arg: Source): Source = function("JSON.parse", arg)
   def json_print(arg: Source): Source = function("JSON.stringify", arg)
 
 
   /* file-system operations */
 
   def standard_path(p: Path, dir: Boolean = false): Source =
     string(File.standard_path(p) + (if (dir) "/" else ""))
 
   def platform_path(p: Path, dir: Boolean = false): Source =
-    string(File.platform_path(p) + (if (dir) File.platform_path(Path.root) else ""))
+    string(File.platform_path(p) + (if (dir) "/" else ""))
 }