How to create add to compare in Wordpress?

You could use you $_SESSION which might be a cleaner way of handling the logic. functions to add / remove items would be the following (untested code):

$_SESSION['barata_products'] = [];function add_products(varid) {    array_push($_SESSION['barata_products'],varid);}function remove_product(varid) {    $product_index = array_search(varid,$_SESSION['barata_products']);    unset($_SESSION['barata_products'][$product_index]);}function remove_all_products() {    $_SESSION['barata_products'] = [];}//helper function to check if the session array is greater than your desired amount, where x is number of products within the array.function more_than_x(session,x) {    $_SESSION['barata_products'].count() > x ? true : false}

If you need fancier logic with wordpress sessions, check out this blog

You can simply use ajax to create a session for each product like :

$(document).ready(function(){  $('.product').click(function(){    var product_id = $(this).attr('id');     $.ajax({     method: 'POST',     url : "YOUR PHP FILE NAME",     data : {product_id:product_id},     success : function(resp){     alert("Product is added to be compared");     }     });  });});

PHP FILE CODE :

 <?php   session_start();  $product_id = $_POST['product_id'];  if(!is_array($_SESSION['ids']))  {   $_SESSION['ids'] = array();  }  else  {    array_push($_SESSION['ids'], $product_id);  } ?>

Then you can use that $_SESSION variable to get all the product ids to your compare template.