How to vertically align Bootstrap v4 modal dialogs

Question

Vertically center modal dialogues in Bootstrap 4.

Note: The requirements below have been added to make it clear I am looking for a prop

Answer 1

Here's a simple Flexbox Approach.

SCSS

.modal-open .modal {
    display: flex!important;
    align-items: center!important;
    .modal-dialog {
        flex-grow: 1;
    }
}

Working Demo