How to write ruby erb tag in slim javascript?

javascript:
  var flash_message = '';

  - if flash.any?


  $(document).ready(function(){
    $(".button-collapse").sideNav();
  });

I want use normal erb tag in javascript area generate by slim. If have flash message, set flash_message = html string generate by rails helper. But I can't use - if flash.any? in javascript: of slim.