flux - custom field validation on JS side

functions.php

/**
 * Flux checkout birthday field custom validation.
 */
add_action( 'wp_footer', function() {
	?>
	<script>
		jQuery( function($) {
			$( 'form.checkout' ).on( 'blur', '.birthday-field input', function() {
				// todo change your validation logic here.
				var age = parseInt($( this ).val());

				// If there is an error then add error class and show error message.
				if ( age < 18 ) {
					$p = $( this ).closest('.form-row');
					$p.addClass( 'woocommerce-invalid' );
					var error_msg = 'You must be at least 18 years old to place an order';

					if ( $p.find( '.error' ).length ){
						$p.find( '.error' ).html( error_msg );
					} else {
						$p.append( `<span class="error">${error_msg}.</span>` );
					}
				} else {
					// If there is no error then remove error class and hide error message.
					$p.removeClass( 'woocommerce-invalid' );
					$p.find( '.error' ).remove();
				}
			} );
		} );
	</script>
	<?php
} );