Show the stock counter in more than one place on the same page

This script works great as a stock countdown, but I couldn't show it in more than 1 place on the same page.

For example, on the same page there are 4 or 5 products/plans and each product shows a different stock quantity and changing the quantity at different times from each other. Only shows on 1 product.

<span class="qty" id="qty"></span>

<script>
    const setQty = (qty) => {
        qtySpan.innerHTML = qty;

        if (qty == 0) return;

        let parts = Math.floor((Math.random() * 3) + 1);
        if (parts > qty) parts = qty;

        const msec =  Math.floor(((Math.random() * 15) + 15) * 1000);
        qty -= parts;

        // Save the updated quantity in localStorage
        localStorage.setItem('saved_countdown', qty);

        setTimeout(() => setQty(qty), msec);
    }

    // Get the saved countdown value from localStorage, or use default value of 57 if not found
    const defaultQty = localStorage.getItem('saved_countdown') ?? 57;
    
    const qtySpan = document.getElementById('qty');

    // Set the initial value of the quantity
    setQty(defaultQty);
</script>

I duplicated the script 4x changing the ID "qty" by qty1, qty2, qty3 and qty4, but it didn't work, it keeps showing only in 1 product... :/

Would anyone here be able to help me out? Thank you!

I've modified your script into a Custom Element. This means that you can now create an element called <stock-counter>.

This element has 2 attributes, quantity and storage-key.

1. quantity is the amount to start counting from.
2. storage-key is the name of the Local Storage key that is used to store the last quantity of this specific counter. If a storage key is set and a stored value has been found, then this value will supersede the quanitity value, unless the stored value is 0.

So the element would look like this:

<stock-counter quantity="40" storage-key="countdown-one">40</stock-counter>

You can place as many of these elements on your page and modify the quantity and storage-key for each one.

customElements.define('stock-counter', class extends HTMLElement {
  get quantity() {
    // Check if value has been stored.
    if (this.storageKey !== null) {
      const value = Number(localStorage.getItem(this.storageKey));

      // Use that value if it is a valid number and not 0.
      if (!Number.isNaN(value) && value !== 0) {
        return value;
      }
    }

    // Otherwise get the value from the quantity attribute.
    const value = Number(this.getAttribute('quantity'));

    if (Number.isNaN(value)) {
      return 0;
    }

    return value;
  }

  set quantity(value) {
    if (!isNaN(value)) {
    
      // Store the new value if it's possible.
      if (this.storageKey !== null) {
        localStorage.setItem(this.storageKey, value);
      }

      // Set the new attribute value.
      this.setAttribute('quantity', value);
    }
  }

  get storageKey() {
    return this.getAttribute('storage-key');
  }

  connectedCallback() {
    this.count();
  }

  count = () => {
    const qty = this.quantity;
    this.textContent = qty;

    if (qty === 0) {
      return;
    }

    let parts = Math.floor((Math.random() * 3) + 1);

    if (parts > qty) {
      parts = qty;
    }

    this.quantity -= parts;

    const msec = Math.floor(((Math.random() * 15) + 15) * 1000);
    setTimeout(this.count, msec);
  };
});

<stock-counter quantity="40" storage-key="countdown-one">40</stock-counter>
<stock-counter quantity="50">50</stock-counter>
<stock-counter quantity="80" storage-key="countdown-three">80</stock-counter>