Disable add to cart on product page if product already in the cart

booking-form.liquid

{% comment %}
BookThatApp auto installed snippet. Changes made to this file will be lost if installed again.
{% endcomment %}

{% if product.metafields.bookthatapp.config %}
{% unless bta_configured %}
<div class="booking-form">
  <p id="bta-loading" class="loading">Loading availability...</p>
  
  <div id="variant-in-cart" style="display:none">
    <p>Item already in cart.</p>
    <a href="#" onclick="removeBookingItem(); return false;">Remove</a> to change dates. 
    <a id="to-cart" href="/cart" class="btn">Go to cart</a>
  </div>

  <div class="hide-if-already-in-cart" style="display:none">
    <div class="selector-wrapper">
      {% capture attribute %}booking-start{% endcapture %}
      <label for="{{ attribute }}-{{ product.handle }}">Select a Date:</label>
      <input id="{{ attribute }}-{{ product.handle }}" type="text" name="properties[{{ attribute }}]" size="12" class="datepicker bta required bta-load-enable" data-handle="{{ product.handle }}" data-variant="{{ product.variants.first.id}}" data-bta-product-config="{{ product.metafields.bookthatapp.config }}" data-bta-variant-config="{% for variant in product.variants %}{{ variant.id }}:{{ variant.metafields.bookthatapp.config }}{% unless forloop.last %},{% endunless %}{% endfor %}" disabled="disabled" />
      <div class="bta-scheduled-times-widget" data-label="Select a Time:" data-prompt="Please choose a date..." data-datepicker="{{ attribute }}">
        <p class="loading">Loading class schedule...</p>
        <div class="booked-out-message" style="display:none">Sorry, that time is not available.</div>
      </div>
    </div>
  </div>
</div>

<script>
var bta = {
    productId: {{ product.id }},
    cart: {{ cart | json }},
    callbacks: {
      available: function() {
		$('#bta-loading').hide();
        checkCart();
      }
  	}
}

$('form[action="/cart/add"]').on('change', '.single-option-selector', function() {
  if ($('#{{ attribute }}-{{ product.handle }}').hasClass('bta-initialized')) {
    checkCart();
  }
});

function checkCart() {
  var variant = parseInt(bta.findSelectedVariantId(), 10),
      carted = false;

  for (var i = 0; i < bta.cart.items.length; i++) {
    if (bta.cart.items[i].product_id == {{ product.id}}) {
      carted = true;
      break;
    }
  }

  if (carted) {
    $('#variant-in-cart').show();
    $('.hide-if-already-in-cart').hide();
  } else {
    $('#variant-in-cart').hide();
    $('.hide-if-already-in-cart').show();
  }
}

function removeBookingItem() {
  Shopify.removeItem(bta.findSelectedVariantId(), function() {
    window.location.href = window.location.pathname;
  })
}
</script>
{% assign bta_configured = true %}
{% endunless %}
{% endif %}