Drupal 8 - Commerce 2.x - Transform quantity field in add to cart form into select list

commerce-quantity-select-add-to-cart-form.php

<?php

 /**
  * hook_form_alter()
  *
  * Switch quantity field from input text to select in add_to_cart forms
  * Supports both standard add_to_cart, as shipped in commerce framework
  * as well as ajax add_to_cart forms implemented by dc_ajax_add_cart module
  * 
  * It also supports quantity fields as they appear in view cart form, both with
  * and without ajax functionality provided by dc_ajax_add_cart module
  *
  * @see https://www.drupal.org/project/commerce
  * @see https://www.drupal.org/project/dc_ajax_add_cart
  */
function your_module_form_alter(&$form, $form_state, $form_id) {
  
  $total_units_allowed = 9; // Customize this value according to site requirements
  $select_options = [];
  for ($i = 1; $i <= $total_units_allowed; $i++) {
    $select_options[$i] = $i;
  }
  
  $add_to_cart_is_classic = strpos($form_id, 'commerce_order_item_add_to_cart_form') === 0;
  $add_to_cart_is_dc_ajax = strpos($form_id, 'commerce_order_item_dc_ajax_add_cart_form') === 0;
  $add_to_cart_is_views = strpos($form_id, 'views_form_mix_cart_form') === 0;

  if ($add_to_cart_is_classic || $add_to_cart_is_dc_ajax) {
    $form['quantity']['widget'][0]['value']['#type'] = 'select';
    $form['quantity']['widget'][0]['value']['#options'] = $select_options;
  }
  elseif ($add_to_cart_is_views) {
    $qty_key_name = 'edit_quantity';
    if (isset($form['dc_ajax_add_cart_views_edit_quantity'])) {
      $qty_key_name = 'dc_ajax_add_cart_views_edit_quantity';
    }
    $form[$qty_key_name][0]['#type'] = 'select';
    $form[$qty_key_name][0]['#options'] = $select_options;
  }
}