Woocommerce Checkout - Display custom inline error messages (including custom validation messages)

functions.php

<?php

/**
 * adds custom js file to handle inline error messages
 */

add_action( 'woocommerce_after_checkout_form', 'add_custom_validation_script', 20 );

function add_custom_validation_script() {
    wp_enqueue_script(
        'inline_validation_script',
        get_stylesheet_directory_uri() . '/js/inline-validation.js',
        ['jquery']
    );
}



/**
 * adds error message field element to get inline error messages working
 * 
 * @param array $fields
 * @param object $errors
 */

add_filter( 'woocommerce_form_field', 'add_inline_error_messages_element', 10, 4 );

function add_inline_error_messages_element( $field, $key, $args, $value ) {
    if ( strpos( $field, '</span>' ) !== false ) {
        $error = '<span class="js-custom-error-message" style="display:none"></span>';
        $field = substr_replace( $field, $error, strpos( $field, '</span>' ), 0);
    }

    return $field;
}



/**
 * process custom checkout validations
 *
 * @param array $fields
 * @param object $errors
 */

add_action('woocommerce_after_checkout_validation', 'custom_checkout_validations', 10, 2);

function custom_checkout_validations($data, $errors)
{
    $your_custom_checkout_field = filter_input(INPUT_POST, 'your_custom_input_field');
    
    // your custom validations goes here
    // this is an example to check for the length of the string
    if ( !empty($your_custom_checkout_field) && strlen($your_custom_checkout_field) > 50 ) {
        $errors->add('your_custom_input_field', __('This field needs to be max 50 chars', 'YourThemeName'));
    }

    // this loop adds a data array to all error messages which will be applied as a "data-error-for" HTML attribute
    // to read out the corresponding field ids with javascript and display the error messages inline
    foreach( $errors->errors as $original_key => $error ) {
        $field_key = $original_key;

        // filter and rewrite the field id for native woocommerce error messages with a key containing _required
        if(strpos($original_key, '_required') !== false) {
            $field_key = str_replace('_required','', $original_key);
            $error[0] = __('This is a required field', 'YourThemeName');
        }

        // switch out the old error messages with the ones including a spiced up data array
        // to display with javascript
        $errors->remove($original_key);
        $errors->add($original_key, trim($error[0]), ['error-for' => $field_key . '_field']);
    }
}

inline-validation.js

jQuery(function ($) {
    'use strict';

    addInlineMessages();

    // Implementation

    // Listen to js event
    $(document.body).on('updated_checkout', function() {
        addInlineMessages();
    });

    function addInlineMessages() {
        var woocommerceErrorsEl = $('.woocommerce-error');
        var woocommerceInlineErrorsEl = $('li[data-error-for]', woocommerceErrorsEl);
        var inlineErrorMessagesEl = $('.js-custom-error-message');

        // as we use ajax submitting hide old validation messages
        if(inlineErrorMessagesEl.length) {
            inlineErrorMessagesEl.hide();
        }

        if(woocommerceInlineErrorsEl.length) {
            woocommerceInlineErrorsEl.each(function () {
                var errorEl = $(this);
                var errorText = $.trim(errorEl.text());

                var targetFieldId = errorEl.data('error-for');

                if(errorText && targetFieldId) {
                    var targetFieldEl = $('#' + targetFieldId);
                    var errorMessageField = $('.js-custom-error-message', targetFieldEl);

                    if(targetFieldEl.length && errorMessageField.length) {
                        targetFieldEl.removeClass('woocommerce-validated');
                        targetFieldEl.addClass('woocommerce-invalid');

                        errorMessageField.text(errorText);
                        errorMessageField.show();
                        errorEl.hide();
                    }
                }
            });

            if(woocommerceInlineErrorsEl.filter(':visible').length === 0) {
                woocommerceErrorsEl.hide();

                if(inlineErrorMessagesEl.filter(':visible').length > 0) {
                    scrollToElement(inlineErrorMessagesEl.filter(':visible').first());
                }
            } else {
                $('li:not([data-error-for])', woocommerceErrorsEl).hide();
                scrollToElement(woocommerceErrorsEl);
            }

        }
    }

    function scrollToElement(el) {
        if(el.length) {
            $([document.documentElement, document.body]).animate({
                scrollTop: el.offset().top - 100
            }, 2000);
        }
    }

    // event listeners
    $(document.body).on('checkout_error', function (event) {
        jQuery('html, body').stop();

        addInlineMessages();
    });
});

@Z1pas this is totally possible. This is done in my code in the custom_checkout_validations function.

Basically you can add any validation you want and it will add it to the corresponding fields if it is created with the woocommerce_form_field method.

Anyway, I think I have to have a little overhaul of the code above as some people have some problems with it (probably due to other themes/plugins) enabled etc. but the basic I idea stays.

Cheers.

@lucasjahn thank you so much for your help and quick response. I finally validated my checkbox field! The code works perfectly!

All the best to you!