Dark Hole

// ES6/Polyfill
Array.from(document.getElementtsByClassName('...')).slice(1, 2);
// Crossbrowser
[].slice.call(document.getElementsByClassName('...'), 1, 2);

$('.block').on('click', function() {
  var _this = $(this);
  var parent = _this.parent();
  var el = parent.find('.block:not(.hidden)').toggleClass('hidden');
  if(el[0] == parent.find('.block').last()[0]) parent.find('.block').first().toggleClass('hidden');
  else el.next('.block').toggleClass('hidden');
})

$('#nav').on('click', function() {
 $(this).slideToggle()
}

let els = document.getElementsByClassName('...');
Array.from(els).forEach((el) => if(!el.value) el.style.border = '1px solid red');
// Или
for(let i = 0; i < els.length; ++i) if(!el.value) els[i].style.border = '1px solid red')