How-to display a dropdown field with optgroups?

Question

The display of a dropdown without optgroups is defined:

{% set field = craft.app.fields.getFieldByHandle('<FieldHandle>') %}

<select name="fields[<FieldHandle>]">
    {% for option in field.options %}

        {% set selected = entry is defined
            ? entry.<FieldHandle>.value == option.value
            : option.default %}

        <option value="{{ option.value }}"
                {% if selected %}selected{% endif %}>
            {{ option.label }}
        </option>
    {% endfor %}
</select>

The following error occurs:

Key "default" for array with keys "optgroup" does not exist.

Answer 1

This would work for both with or without optgroup (replace aDropdown with your fieldname):

{% set field = craft.app.fields.getFieldByHandle('aDropdown') %}

{# If we have optgroups in this field, set a counter to 0 else return false #}

{% set optgroupCount = field.optgroups ? 0 : false %}
<select name="fields[aDropdown]">
    {% for option in field.options %}

        {% if option.optgroup is not defined %}
            {# This is a regular option #}
            {% set selected = entry is defined ? entry.aDropdown.value == option.value : option.default %}
            <option value="{{ option.value }}" {% if selected %}selected{% endif %}>
                {{ option.label }}
            </option>
        {% else %}
            {# This is an optgroup #}
            {# Check if we should close a group before opening a new one.#}
            {{ optgroupCount > 0 ? '</optgroup>' }}
            <optgroup label="{{ option.optgroup }}">
            {% set optgroupCount = optgroupCount + 1 %}
        {% endif %}

    {% endfor %}
    {# If we had optgroups better close the last one #}
    {{ optgroupCount ? '</optgroup>' }}
</select>