custom.js

   function addMultiToCart(productIds, cartId){
      /* Sets the URL to an existing cart id + /items, if not use the cart endpoint to create a new cart */
      let url = cartId ?
      `/api/storefront/carts/${cartId}/items`:
      `/api/storefront/cart`
      /* Set a data variable to our lineItems object with the product ids mapped with a quantity of 1 */
      let data = {
          lineItems: productIds.map(id => ({
              quantity: 1,
              productId: id
          }))
      }
      console.log(url);
      let options = {
          method: 'POST',
          body: JSON.stringify(data),
          credentials: 'include',
          headers: {
              "Content-Type": "application/json"
          }
      }
      /* Finally we fetch the cart URL with the generated URL and supplied options */
      return fetch(url, options)
      .then(res => res.json())
      .then(json => location.reload()); // Reloads the page
      
    }
    const multiButton = document.querySelector('#multiButton');
    // Selects the #id that we'll attach the addMultiToCart function to
    multiButton.addEventListener('click', event  => {
      addMultiToCart(arr, cartId) // run the addMultiToCart function with the array we created and a cart ID if ones available in the context.
    });