我不理解这个Rails模板呈现的行为

I have this controller:

我有这个控制器:

class FormCreatorController
  def build_form
    if param[:ftype] == 2
       @partial_file = "extra"
    else
       @partial_file = "basic"
    end
    respond_to do |format|
      format.js
    end
  end
end
class Form