WooCommerce auto-update cart when quantity changed

functions.php

/**
 * Auto update cart after quantity change
 *
 * @return  string
 **/
add_action( 'wp_footer', 'cart_update_qty_script' );
function cart_update_qty_script() {
    if (is_cart()) :
    ?>
    <script>
        jQuery('div.woocommerce').on('change', '.qty', function(){
            jQuery("[name='update_cart']").trigger("click"); 
        });
    </script>
    <?php
    endif;
}

Hi there. If it helps anyone, to solve the 'it works every second time' issue I had to...

Use a more specific selector:
jQuery('div.woocommerce').on('click', 'input.qty', function(){

remove the 'disabled' attribute from the Update Cart button before applying the click trigger:
jQuery("[name='update_cart']").removeAttr("disabled").trigger("click");