why not

  const $ = s => document.querySelectorAll(s);
  const hide = e => e.style.display = 'none';
  $('.submenus').forEach(hide);

or, if you're okay with changing prototypes

  const $ = s => document.querySelectorAll(s);
  NodeList.prototype.hide = function () {this.forEach(e => e.style.display = 'none');};
  $('.submenus').hide();