Hello,
was going to ask the question, but found the solutions in a last minute 
Here it is:
Also some gists:
javascript asset helper 1
javascript asset helper 2
Is there other options?
p.s.
I’m aware of prepending *.erb, but I don’t like.
Have a look at gon gem. Haven’t used it but it should do the job.
The gist I found and particularly js_assets gem is much easiest solution 
With “gon” you still have to index assets and add corresponding JS helper, i.e. way too much job