Adding Custom JS on WP Admin

// download plugin from http://keith-wood.name/maxlength.html

// yourthemes/functions.php
function load_custom_wp_admin_style() {
        wp_register_style( 'maxlength', get_template_directory_uri() . '/js/maxlength/jquery.maxlength.css', false, '1.0.0' );
        wp_enqueue_style( 'maxlength' );
    wp_enqueue_script('maxlength-plugin', get_template_directory_uri() . '/js/maxlength/jquery.plugin.min.js', array(), '', true);
    wp_enqueue_script('maxlength-core', get_template_directory_uri() . '/js/maxlength/jquery.maxlength.min.js', array(), '', true);
    wp_enqueue_script('maxlength-options', get_template_directory_uri() . '/js/maxlength/maxlength-options.js', array(), '', true);
}
add_action( 'admin_enqueue_scripts', 'load_custom_wp_admin_style' );

// yourthemes/js/maxlength/maxlength-options.js
jQuery(document).ready(function() { jQuery('#excerpt').maxlength({max: 250}); });