Menu-accordion. How to collapse inactive items?

Question

css

How is the folding of inactive menu items done in the accordion menu? So that you open one, and the previously open one closes.
HTML

<ul class="accordeon">

  <li><a class="btn-menu" href="#">Верхний пункт</a>
    <ul class="sub-menu">
      <li><a class="sub-menu__item" href="#">Первое меню 1</a></li>
      <li><a class="sub-menu__item" href="#">Первое меню 2</a></li>
      <li><a class="sub-menu__item" href="#">Первое меню 3</a></li>
      <li><a class="sub-menu__item" href="#">Первое меню 4</a></li>
    </ul>
  </li>

  <li><a class="btn-menu" href="#">Средний пункт</a>
    <ul class="sub-menu">
      <li><a class="sub-menu__item" href="#">Второе меню 1</a></li>
      <li><a class="sub-menu__item" href="#">Второе меню 2</a></li>
      <li><a class="sub-menu__item" href="#">Второе меню 3</a></li>
      <li><a class="sub-menu__item" href="#">Второе меню 4</a></li>
    </ul>
  </li>

  <li><a class="btn-menu" href="#">Нижний пункт</a>
    <ul class="sub-menu">
      <li><a class="sub-menu__item" href="#">Третье меню 1</a></li>
      <li><a class="sub-menu__item" href="#">Третье меню 2</a></li>
      <li><a class="sub-menu__item" href="#">Третье меню 3</a></li>
      <li><a class="sub-menu__item" href="#">Третье меню 4</a></li>
    </ul>
  </li>

</ul>

JS

var btnMenu = document.querySelectorAll('.btn-menu'),
    subMenu = document.querySelectorAll('.sub-menu'),
    i, btn, sub;

for (i = 0; i < btnMenu.length; i++) {
    btn = btnMenu[i];
    sub = subMenu[i];

    btn.addEventListener('click', menuOpen(btn, sub));
};

function menuOpen(btn, sub) {
    return function (event) {
        event.preventDefault();
        btn.classList.toggle('btn-menu--active');
        sub.classList.toggle('sub-menu--show');
    };
};

Answer 1

Hide everything and open the current one

.sub-menu {
  display: none;
}

jQuery

$('.btn-menu').on('click', function() {
    $('.sub-menu').hide();
    $(this).siblings('.sub-menu').show();
});

Answer 2

Dreamweaver CC Insert-jQuery UI-Accordion All that's
left is to color it