Render js if variable length is 0 (rails)

I am defining a variable in my controller method. I want to render js if the size of the variable is zero.

My code is:

def department
  @department = Department.all
end

I want or do something like this:

def department
  @department = Department.all
  if @department.count > 0
    render :html => html-template-name
  else
    render :js => "alert('No department available');"
  end
end

What should be the syntax of render html part?