Instead of hacking the js files under assets/js, its better to create one yourself. Say you want to modify assets/js/frontend/checkout.js for example, in your theme functions.php
/** * use our own checkout js in the proper way * @return null */ function use_custom_js() { global $woocommerce; // make sure its checkout page if (is_checkout()) { wp_dequeue_script('wc-checkout'); // create assets/js/frontend/ dir under your theme dir. wp_enqueue_script( 'wc-checkout-my', dirname(get_stylesheet_uri()).'/assets/js/frontend/checkout.js', array( 'jquery', 'woocommerce' ), $woocommerce->version, true ); } } add_action('wp_enqueue_scripts', 'use_custom_js');