Your BASE_DIR by default points to the same directory that manage.py is in. If you haven't changed it then capstone.js is currently located in
os.path.join(BASE_DIR, 'capstone/static')
BUT django by default looks for a static folder inside each installed app so in your case STATCFILES_DIRS is redundant.
Make sure you have added the static files to your urls.py.
from django.conf import settings
from django.conf.urls.static import static
urlpatterns = [
# ... the rest of your URLconf goes here ...
] + static(settings.STATIC_URL, document_root=settings.STATIC_ROOT)
or
from django.contrib.staticfiles.urls import staticfiles_urlpatterns
urlpatterns = [
# ...urls
]
# static content urls
urlpatterns += staticfiles_urlpatterns()
This is my best guess as to why you are getting a 404 error.
NB: if you have setup static urls then try
http://127.0.0.1:8000/static/capstone.js instead.