AJAX add to cart - woocommerce

add_to_cart_ajax.html

<a id="buy" href="#">Buy this!</a>
<script>
$('#buy').click(function(e) {
    e.preventDefault();
    addToCart(19);
    return false;
});

function addToCart(p_id) {
    $.get('/wp/?post_type=product&add-to-cart=' + p_id, function() {
        // call back
    });
}
</script>

add_to_cart_ajax.php

<?php
// Add item to cart on visit 
add_action( 'template_redirect', 'add_product_to_cart' );
 
function add_product_to_cart() {         
    // select ID
    $product_id = 851;         
            //check if product already in cart
    if ( WC()->cart->get_cart_contents_count() == 0 ) {             
        // if no products in cart, add it
        WC()->cart->add_to_cart( $product_id );         
    }
}
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