How to watch .sss files and re-run my npm script if they change

Is there a way to watch my .sss files and re-run this if they change?

"css": "postcss -u autoprefixer -u postcss-easy-import -p sugarss src/index.sss -o src/index.css"

I can't find a way, I've looked on the net all over. I don't see any kind of watch available from postcss. There's a -w or --watch with the postcss-cli but I tried adding that to the end of the script above. Unless I'm just doing it wrong, my css script does not re-run itself if my .sss files change.

maybe there's some generic node watcher lib I can use?

via PositiveGuy