I haven't used SRS for js specifically, but I've been using it lately to cement certain Ruby concepts. Best advice I can give is that it's totally worth it, but don't take shortcuts when authoring cards. Take the time to separate concepts into bite-sized, more simple (Rich Hickey simple, not "easy" simple) concepts, and write questions that elicit that knowledge.
Also, Anki is free for both desktop (every OS) and Android.
Also, Anki is free for both desktop (every OS) and Android.