I've recently faced the problem: I need to generate javascript that would insert some HTML block unless it is already there. So, the code should be something like this:

  if( !($('block_id')) ) {
     // insert block
  }

All that should go to RJS template.

First revision:

  page << "if( !($('block_id')) ) {"
  page.insert_html :before, 'container_id', '
'
  page << "}"

Next, it would be nice to make use of JavaScriptProxy objects (that are spawn, e.g., by page#[] method). The problem is that HTML is emited when those proxy created or evaluated, thus it's not possible to wrap that javascript code into e.g. "if( ... )"..
Solution, first try:

class << page
  def if
    self << "if("
  end
  def then
    self << ") {"
  end
  def end
    self << "}"
  end
end

page.if
  page['block_id']
page.then
  page.insert_html .......
page.end

This is better then the first one, but too ugly though. A better solution would include some black magick:

  module AcitiveView
    module Helpers
      class JavaScriptProxy
        def respond_to?(name)
          name.to_sym == :to_json
        end

        def to_json
          @generator.instance_variable_get('@lines').pop.chomp(';')
        end
      end
    end
  end

Then:

  class << page
    def if(expr)
      page << "if( #{expr.respond_to?(:to_json) ? expr.to_json : expr} ) {"
      yield if block_given?
      page << "}"
    end
  end

  page.if page['block_id'] do
    page.insert_html .....
  end

And finally, the more peaceful variant:

module ApplicationHelper
  def if_exists(element_id)
    page << "if( !($('#{element_id}')) ) {"
    yield if block_given?
    page << "}"
  end
end

page.if_exists 'block_id' do
  page.insert_html .....
end