Check JavaScript files content type. (#1802)

A common problem with windows set ups is that the registry is
misconfigured when resolving the content type for a JavaScript
file. This check captures this sooner to explain why the toolbar
doesn't show and attempts to tell the user what to change to make
it work.

debug_toolbar/apps.py

@@ -1,4 +1,5 @@
 import inspect
+import mimetypes
 
 from django.apps import AppConfig
 from django.conf import settings
@@ -174,3 +175,34 @@ def check_panels(app_configs, **kwargs):
             )
         )
     return errors
+
+
+@register()
+def js_mimetype_check(app_configs, **kwargs):
+    """
+    Check that JavaScript files are resolving to the correct content type.
+    """
+    # Ideally application/javascript is returned, but text/javascript is
+    # acceptable.
+    javascript_types = {"application/javascript", "text/javascript"}
+    check_failed = not set(mimetypes.guess_type("toolbar.js")).intersection(
+        javascript_types
+    )
+    if check_failed:
+        return [
+            Warning(
+                "JavaScript files are resolving to the wrong content type.",
+                hint="The Django Debug Toolbar may not load properly while mimetypes are misconfigured. "
+                "See the Django documentation for an explanation of why this occurs.\n"
+                "https://docs.djangoproject.com/en/stable/ref/contrib/staticfiles/#static-file-development-view\n"
+                "\n"
+                "This typically occurs on Windows machines. The suggested solution is to modify "
+                "HKEY_CLASSES_ROOT in the registry to specify the content type for JavaScript "
+                "files.\n"
+                "\n"
+                "[HKEY_CLASSES_ROOT\\.js]\n"
+                '"Content Type"="application/javascript"',
+                id="debug_toolbar.W007",
+            )
+        ]
+    return []

docs/changes.rst

@@ -11,6 +11,8 @@ Pending
   ``djdt.cookie.set``.
 * Converted ``StaticFilesPanel`` to no longer use a thread collector. Instead,
   it collects the used static files in a ``ContextVar``.
+* Added check ``debug_toolbar.W007`` to warn when JavaScript files are
+  resolving to the wrong content type.
 
 4.1.0 (2023-05-15)
 ------------------

docs/checks.rst

@@ -18,3 +18,6 @@ Django Debug Toolbar setup and configuration:
   configuration needs to have
   ``django.template.loaders.app_directories.Loader`` included in
   ``["OPTIONS"]["loaders"]`` or ``APP_DIRS`` set to ``True``.
+* **debug_toolbar.W007**: JavaScript files are resolving to the wrong content
+  type. Refer to :external:ref:`Django's explanation of
+  mimetypes on Windows <staticfiles-development-view>`.

tests/test_checks.py

@@ -1,5 +1,6 @@
 import os
 import unittest
+from unittest.mock import patch
 
 import django
 from django.conf import settings
@@ -227,3 +228,33 @@ def test_check_w006_valid(self):
     )
     def test_check_w006_valid_nested_loaders(self):
         self.assertEqual(run_checks(), [])
+
+    @patch("debug_toolbar.apps.mimetypes.guess_type")
+    def test_check_w007_valid(self, mocked_guess_type):
+        mocked_guess_type.return_value = ("text/javascript", None)
+        self.assertEqual(run_checks(), [])
+        mocked_guess_type.return_value = ("application/javascript", None)
+        self.assertEqual(run_checks(), [])
+
+    @patch("debug_toolbar.apps.mimetypes.guess_type")
+    def test_check_w007_invalid(self, mocked_guess_type):
+        mocked_guess_type.return_value = ("text/plain", None)
+        self.assertEqual(
+            run_checks(),
+            [
+                Warning(
+                    "JavaScript files are resolving to the wrong content type.",
+                    hint="The Django Debug Toolbar may not load properly while mimetypes are misconfigured. "
+                    "See the Django documentation for an explanation of why this occurs.\n"
+                    "https://docs.djangoproject.com/en/stable/ref/contrib/staticfiles/#static-file-development-view\n"
+                    "\n"
+                    "This typically occurs on Windows machines. The suggested solution is to modify "
+                    "HKEY_CLASSES_ROOT in the registry to specify the content type for JavaScript "
+                    "files.\n"
+                    "\n"
+                    "[HKEY_CLASSES_ROOT\\.js]\n"
+                    '"Content Type"="application/javascript"',
+                    id="debug_toolbar.W007",
+                )
+            ],
+        )