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');
