How do I pass params to Rails controller method then return value using JQuery/AJAX?

I am using a click function to first get the array/position, then post it via an AJAX call to a controller method. I would then like the method to return a boolean value to enable the addClass in the jquery function. The console is consistently throwing a 500 server error.

Here is the .js file:

   $(document).ready(function(){
     $("#backboard").on("click", "div", function(e){
       $("#backboard div").removeClass("selected");
       var val = $(this).data('val');
     $.ajax ({
        type: 'POST',
        url: "https://localhost:3000/chess/:pos",
        data: {pos: {value: val}},
        success: function(d){
          if(d === true){
            $(this).addClass("selected").append("<p>"+val+"<p>");
          }
        }
      });
    });
  });

here is the controller.rb file:

 class ChessController < ApplicationController
    include ChessHelper

    def new
    @select = false
    @game = Game.new("white", "ARSEFACE!!!")
    @arr = @game.board
    @cpu = @game.cpuName
    @player = @game.playerName



   end

   def select
      pos = params[:pos]
     a = pos[0]
     b = pos[1]

     if(@arr[a][b][0]===@color)
        @select = !@select
     end

     respond_to do |format|
       format.json { render json: @select }
      end
    end
  end

Here is the config/routes.rb:

   get '/chess', to: 'chess#new'
   post '/chess/:pos' => 'chess#select' 

I know I am probably missing something incredibly obvious here, but any help would be appreciated!

PS and the val variable taking the data from the tag does actually have a value. I am trying to pass that value to the controller in order to return true or false, and will eventually be a validation function once I get this basic issue worked out.