Add a Character Counter to Excerpt box in Edit Post screen (with suggested range).

wp-excerpt-char-count.php

<?php
/**
 * Add a Character Counter to Excerpt box in Edit Post screen.
 *
 * Includes suggested range.
 * Char-count number is color red when outside range and green within.
 *
 * @link http://wpsites.org/add-character-counter-excerpt-box-10503/
 */
function my_excerpt_count_js() {
    echo '<script>jQuery(document).ready(function(){

    // Classic Editor
    if ( jQuery("#excerpt").length) {
        jQuery("#postexcerpt .handlediv").after("<div style=\"position:absolute;top:5px;right:80px;color:#666;\"><input type=\"text\" value=\"0\" maxlength=\"3\" size=\"3\" id=\"excerpt_counter\" readonly=\"\" style=\"background:#fff;\"> <small>characters (best for HE: 120-240)</small></div>");

        jQuery("#excerpt_counter").val(jQuery("#excerpt").val().length);
        jQuery("#excerpt").keyup( function() {
            jQuery("#excerpt_counter").val(jQuery("#excerpt").val().length);
            if ( jQuery("#excerpt_counter").val() < 120 || jQuery("#excerpt_counter").val() > 240 ) {
                jQuery("#excerpt_counter").css("color","red");
            } else {
                jQuery("#excerpt_counter").css("color","green");
            }
        });
    }

    // Gutenberg editor
    if ( jQuery("#editor-post-excerpt-0").length) {
        jQuery("#editor-post-excerpt-0").parent().after("<div style=\"\"><input type=\"text\" value=\"0\" maxlength=\"3\" size=\"3\" id=\"excerpt_count\" readonly=\"\" style=\"background:#fff;\"> <small>characters (best for HE: 120-240)</small></div>");

        jQuery("#excerpt_count").val(jQuery("#editor-post-excerpt-0").val().length);
        jQuery("#editor-post-excerpt-0").keyup( function() {
            jQuery("#excerpt_count").val(jQuery("#editor-post-excerpt-0").val().length);
            if ( jQuery("#excerpt_count").val() < 120 || jQuery("#excerpt_count").val() > 240 ) {
                jQuery("#excerpt_count").css("color","red");
            } else {
                jQuery("#excerpt_count").css("color","green");
            }
        });
    }
});</script>';
}
add_action( 'admin_head-post.php', 'my_excerpt_count_js');
add_action( 'admin_head-post-new.php', 'my_excerpt_count_js');
?>