Dinotropolis Cart Issues

fix.js

/*
Below are updated functions which fix the issues noted in issues.js below.
I have tested these on my local machine and was able to make a booking successfully with the updated functions.
*/
function addQuantity(){
  var quantity = parseInt($('#productSelectQuantity').val());
  quantity++;

  if(quantity < 0) // Handle quntity lower than 0
    quantity = 1

  if(quantity > 0) // If we have some items, enable add to cart
    $("#addToCartItem").removeAttr("disabled");

  $('#productSelectQuantity').val(quantity); // Update the quantity shown
}

function lessQuantity(){
  var quantity = parseInt($('#productSelectQuantity').val());
  quantity--;
  
  if(quantity < 0) // Stop quantity from going below 0
     quantity = 0;
  
  if(quantity <= 0) // Disable cart if we have 0 or fewer items to add
    $("#addToCartItem").attr("disabled","disabled");
  
  $('#productSelectQuantity').val(quantity); // Update the quantity shown
}

issues.js

/* 
Below are the addQuantity and lessQuantity JavaScript functions taken from dinotropolis.co.uk (https://dinotropolis.secure.force.com specifically) on 2019-07-29 at 14:02:33
annotated to show the issues preventing people from making an online booking.
*/
function addQuantity(){
  var quantity = parseInt($('#productSelectQuantity').val());
  
  if(quantity === 0){ // This check should look for larger than 0
    $("#addToCartItem").removeAttr("disabled");
  }

  if(quantity < -2) // This if statement seems random and changes between refreshes, some time it's -1, sometimes -2
    $('#productSelectQuantity').val(quantity + 1); // <------ The cause of your problem is you only update the value here
}

function lessQuantity(){
  var quantity = parseInt($('#productSelectQuantity').val());
  if(quantity>0){ // This is covered by the rest of your statements so not needed
    if(quantity === 1) // This check should be for less than 1
      $("#addToCartItem").attr("disabled","disabled");
    if(quantity > 1) // This check should be > 0, not > 1 - this stops you from going below 1
      $('#productSelectQuantity').val(quantity - 1); // <----- You only update the number is it is 2 or higher, meaning you can never go down to 0
  }
}