How can I use java script as a arguments of ruby function?
For example i want
<%= label :placehoder => result of javascript %>
You can't do this because the ruby part will run on the server before the javascript gets a chance to run in the browser. You can use a server side language to build javascript dynamically, but not the other way around.
A workaround would be to run an ajax request from javascript to pass data to the server.