Klaviyo subscribe checkbox on WooCommerce Checkout Block

Checks the Klaviyo email subscribe checkbox on the WooCommerce Checkout Block or the classic WooCommerce checkout template.

add_action( 'template_redirect', function() {

	if( ! class_exists( 'WooCommerce' ) || ! is_checkout() ) {
		return;
	}

	add_action( 'wp_print_footer_scripts', function() {

		?>
		<script type="text/javascript">
		document.addEventListener( 'DOMContentLoaded', function() {

			// Classic Checkout
			if( document.getElementById( 'kl_newsletter_checkbox' ) ) {
				document.getElementById( 'kl_newsletter_checkbox' )
					.checked = true;
				return;
			}

			// Block Checkout
			let checkoutBlockInterval = setInterval( function() {
				let checkboxClass = '.wp-block-woocommerce-checkout-contact-information-block .wc-block-components-checkbox__input';
				if( document.querySelector( checkboxClass ) === null ) {
					return;
				}
				document.querySelectorAll( checkboxClass )
					.forEach( function( checkbox ) {
					checkbox.click();
				} );
				clearInterval( checkoutBlockInterval );
			}, 500 );

		} );
		</script>
		<?php

	} );

} );