Replace the MenuItem.render_js mechanism with the 'media' mechanism from django.forms, which does basically the same thing but better (e.g. handles de-duping where multiple items import the same js file)

wagtail/wagtailadmin/menu.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 
-from six import text_type
+from six import text_type, with_metaclass
 
 try:
     # renamed util -> utils in Django 1.7; try the new name first
@@ -9,13 +9,14 @@ except ImportError:
     from django.forms.util import flatatt
 
 from django.conf import settings
+from django.forms import MediaDefiningClass
 from django.utils.text import slugify
 from django.utils.html import format_html, format_html_join
 
 from wagtail.wagtailcore import hooks
 
 
-class MenuItem(object):
+class MenuItem(with_metaclass(MediaDefiningClass)):
     def __init__(self, label, url, name=None, classnames='', attrs=None, order=1000):
         self.label = label
         self.url = url
@@ -28,19 +29,6 @@ class MenuItem(object):
         else:
             self.attr_string = ""
 
-    js_files = []
-    def render_js(self):
-        """
-        Return a string of any Javascript declarations required by this menu item.
-        These will be included on all admin pages, regardless of whether the menu item
-        is actually shown or not - this allows us to cache/compress the JS globally.
-        By default this returns a script tag for every file listed in self.js_files.
-        """
-        if self.js_files:
-            return format_html_join('\n', '<script src="{0}{1}"></script>',
-                    ((settings.STATIC_URL, filename) for filename in self.js_files)
-                )
-
     def is_shown(self, request):
         """
         Whether this menu item should be shown for the given request; permission

wagtail/wagtailadmin/templatetags/wagtailadmin_tags.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 from django.conf import settings
 from django import template
+from django.forms import Media
 
 from wagtail.wagtailcore import hooks
 from wagtail.wagtailcore.models import get_navigation_menu_items, UserPagePermissionsProxy, PageViewRestriction
@@ -41,8 +42,11 @@ def main_nav(context):
 
 @register.simple_tag
 def main_nav_js():
-    js_snippets = [item.render_js() or '' for item in get_master_menu_item_list()]
-    return ''.join(js_snippets)
+    media = Media()
+    for item in get_master_menu_item_list():
+        media += item.media
+
+    return media['js']
 
 
 @register.filter("ellipsistrim")

wagtail/wagtailadmin/wagtail_hooks.py

@@ -6,9 +6,8 @@ from wagtail.wagtailadmin.menu import MenuItem
 
 
 class ExplorerMenuItem(MenuItem):
-    js_files = [
-        'wagtailadmin/js/explorer-menu.js',
-    ]
+    class Media:
+        js = ['wagtailadmin/js/explorer-menu.js']
 
 @hooks.register('register_admin_menu_item')
 def register_explorer_menu_item():