Get cart total after page is laoded

I need to get the amount of cart total after the page is already loaded.

For example, I use the below code to display the total on cart page (I’m replacing the default woocommerce cart page if you are curious why I am doing this, so I’m basically making the cart page from scratch):

echo WC () -> cart -> get_total ()

But the amount won’t be updated after the coupon is added or removed, so I planned to refresh/update the amount with Ajax if the button is click.

Below is the JS code:

const getData = fetch('https://test.local/wp-content/themes/test/process.php')
   .then(response => response.text())
   .then(response => console.log(response));

test.addEventListener('click', function() {
   text.innerText = getData;
});

For process.php, it looks like this:

<?php echo WC () -> cart -> get_total () ?>

Apparent it won’t wotk because the page is already loaded, I cannot access WC ()