function theme_scripts() {
  wp_enqueue_script( 'buy-button', 'http://sdks.shopifycdn.com/buy-button/0.1.34/buybutton.min.js', array( ), '', true );
  wp_enqueue_script( 'shopify-product', '/assets/js/custom.js', array('buy-button' ), '', true );
} 

add_action( 'wp_enqueue_scripts', 'theme_scripts' );