Default MailPoet opt-in to checked on WooCommerce / Cartflows checkout pages

mailpoet-woocommerce-optin-default.php

<?php

/**
 * Defaults the MailPoet opt-in checkbox on the WooCommerce and/or
 * Cartflows checkout pages to a checked state. Also properly handles
 * the checkbox when 'update_checkout' events are fired.
 *
 * This code could either be placed in the functions.php of the active theme,
 * a custom plugin, or be injected via plugins like WP Code or Code Snippets
 */

add_action('wp_footer', function() {
	if ( function_exists('is_checkout') && is_checkout() ) {
		ob_start(); ?>

		<script type="text/javascript" id="default-checkout-optin-true">	
			(function($) {
				let isFirstRun = true;
				let wasAlreadyChecked = false;
				
				function checkTheBox() {
					document.getElementById("mailpoet_woocommerce_checkout_optin").checked = true;
				}
				
				function getTheCheckboxValue() {
					return document.getElementById("mailpoet_woocommerce_checkout_optin").checked;
				}
				
				/**
				 * this event fires after the ajax request from 'update_checkout' is complete
				 * and the checkout pages has been updated 
				 */
				$(document).on('updated_checkout', function() {
					/**
					 * this is the inital checkout load, check the box
					 */
					if (isFirstRun) { 
						checkTheBox();
						isFirstRun = false;
						wasAlreadyChecked = true;
					} 
					/**
					 * not the first 'update_checkout' cycle, persist the
					 * checkbox state if it was already checked before the
					 * event was fired 
					 */
					else if (!isFirstRun && wasAlreadyChecked) {
						checkTheBox();
					}
				});
				
				/**
				 * update_checkout event is fired when anything changes
				 * on the checkout page that would cause possible cart
				 * recalculations. 
				 */
				$(document).on('update_checkout', function() {
					/**
					 * if the event fired and it's NOT the first run,
					 * set wasAlreadyChecked to the current checkbox value.
					 * This is so that if the user manually unchecked the box
					 * our 'updated_checkout' event listener above won't re-check
					 * it 
					 */
					if (!isFirstRun) {
						wasAlreadyChecked = getTheCheckboxValue();
					}
				});
			})(jQuery);
		</script>

		<?php echo ob_get_clean();
	}
}, 999999); // high priority number to ensure this injected into the footer after jQuery