how to use ruby in a js assets file? (undefined method 'body')

I'm following this tutorial and having a problem where using ruby in an ActionCable file causes an undefined method 'body' error. How can I use ruby in this file? I want the data that's submitted over Action Cable to be properly formatted when it is appended, and I cannot do that with plain js. This is my app/assets/js/channels/messages.js.erb file:

App.messages = App.cable.subscriptions.create('MessagesChannel', {
 received: function(data) {
  $("#response").val("");
  $('#messages').append(this.render_message(data));
  $("#conversation-main").scrollTop($("#conversation-main")[0].scrollHeight);
 },

 render_message: function(data) {
  return "<div class='message'><div><strong>" + data.user + ":</strong> <%= simple_format(@message.body) %></div><div class='date'><%= readable_datetime(@message.created_at) %></div></div>"
 }
});

I can't do simple_format(data.body) since that variable is js. Here's the #create action in my messages controller:

def create
    @message = @conversation.messages.new(message_params)
    @message.user = current_user

    respond_to do |format|
        if @message.save
            ActionCable.server.broadcast 'messages',
                body: @message.body,
                user: @message.user.username,
                time: @message.created_at
            head :ok
        end
    end
end

I have @message defined in the action, and it should be passed onto the js.erb file. But it's not. Why?

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