as_modal.js.haml

:plain
  var existingModal = document.querySelector("[data-controller='modal']");
  if (existingModal) {
    document.body.removeChild(existingModal);
  }

  document.body.insertAdjacentHTML("beforeend", "#{j render partial: template.to_s, locals: local_assigns }");

foo_controller.rb

class FooController < ApplicationController
  def edit
    render :as_modal, locals: { template: :form }
  end

  def update
    if @model.update(model_params)
      redirect_to @model, notice: "Success!"
    else
      render :as_modal, locals: { template: :form }
    end
  end
  
  ...
    
end