diff --git a/src/calibre/gui2/tweak_book/boss.py b/src/calibre/gui2/tweak_book/boss.py
index d18c2b2d01..dd7bc62a37 100644
--- a/src/calibre/gui2/tweak_book/boss.py
+++ b/src/calibre/gui2/tweak_book/boss.py
@@ -1612,7 +1612,7 @@ class Boss(QObject):
             syntax = syntax or syntax_from_mime(name, guess_type(name))
             if use_template is None:
                 data = current_container().raw_data(name)
-                if isbytestring(data) and syntax in {'html', 'css', 'text', 'xml'}:
+                if isbytestring(data) and syntax in {'html', 'css', 'text', 'xml', 'javascript'}:
                     try:
                         data = data.decode('utf-8')
                     except UnicodeDecodeError: