Surface shopify product tag for on page JS

include-pre-release-check.liquid

//Add this file to /snippets
{% assign is_presale_product = false %}
{% if product.tags contains 'pre-release' %}
    {% assign is_presale_product = true %}
{% endif %}
<script>
var is_prerelease_product = "{{is_presale_product | json}}";
</script>

product-page-addtion.liquid

//Add this line just before {% render 'product-form'....
{% render 'include-pre-release-check', product: product %}

some.js

//Assuming you have ajax functionality and are using the jQuery wrapper library, or however you are 
//set up, we want to add this line to into the add to cart js success function somewhere...
if(is_prerelease_product){
   jQuery.post('/cart/update.js', "attributes[_is_preorder]=true");
}