How can I make a product field required?

Question

I have a drop-down of variants of a product. For example t-shirts:

{% for purchasable in product.variants %}
    <option value="">Select</option>
    <option {% if purchasable.stock <= 0 and purchasable.unlimitedStock == false %}disabled {% endif %} value="{{ purchasable.purchasableId }}">
        {{ purchasable.title }} &ndash; {{ purchasable.price|currency(cart.currency) }}
    </option>
{% endfor %}

As you can see I have "Select" as the default option, and it has no value. I can validate the form client side no problem, but I am curious if there is a way to designate that fields as required from the server-side? I can mark the field as required in the layout builder, but that is for the CP only I think.

If I do not choose an option, I get this error:

Craft\Exception 
Not a purchasable ID

Which makes perfect sense since I am not passing it a valid purchasable element.

Answer 1

I can mark the field as required in the layout builder, but that is for the CP only I think.

Nope... marking the field as required makes it required globally (regardless of if it's coming from the control panel or the front-end).