python flask how to initiate flash message from javascript code example

Example: flask flash

{% with messages = get_flashed_messages(with_categories=true) %}
    {% if messages %}
        {% for category, message in messages %}
            <div class="alert alert-{{ category }} alert-dismissible fade show" role="alert">
                <span>{{ message }}</span>
                <button type="button" class="close" data-dismiss="alert" aria-label="Close">
                <span aria-hidden="true">×</span>
                </button>
            </div>
        {% endfor %}
    {% endif %}
{% endwith %}