Minusator

<header>
    <p>Хедер</p>
</header>
<main>
    <p>Пример контента</p>
    <a id="page_up_down_button" href="#">Наверх</a>
</main>
<footer>
    <p>Футер</p>
</footer>

#page_up_down_button {
    position: sticky;
    bottom: 0;
}

<script type="text/javascript">
    var scroll_position = null;

    $('#page_up_down_button').click(function(){
        if ($(document).scrollTop() > 0) {
            scroll_position = $(document).scrollTop();
            $('#page_up_down_button').html('Обратно');
            return true;
        } else if (scroll_position > $(document).scrollTop()) {
            $(document).scrollTop(scroll_position);
            $('#page_up_down_button').html('Наверх');
            return false;
        };
    });

    function check_status_button(){
        if ($(document).scrollTop() > 0) {
            scroll_position = null;
            $('#page_up_down_button').html('Наверх');
        };
    };

    $(window).resize(function() {
        check_status_button();
    });

    $(window).scroll(function() {
        check_status_button();
    });
</script>

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>

function sum(data, key = n => n) {
  const getVal = key instanceof Function ? key : n => n[key];
  let result = 0;

  for (const n of data) {
    result += getVal(n);
  }

  return result;
}

const form = document.querySelector('form');
const result = form.querySelector('.result');
form.addEventListener('change', onChange);
form.dispatchEvent(new Event('change'));

function onChange() {
  result.textContent = sum(
    this.querySelectorAll(':checked'),
    n => +n.getAttribute('rel')
  );
}

// или

function onChange(e) {
  result.innerText = sum(
    e.currentTarget.elements,
    n => n.attributes.rel.value * n.checked
  );
}

<srcset="image-small.png 320w, image-medium.png 800w, image-large.png 1200w">

class ShapeOverlays {
 toggle() {
    this.isAnimating = true;
    for (var i = 0; i < this.numPoints; i++) {
      this.delayPointsArray[i] = 0;
    }
    if (this.isOpened === false) {
      this.open();
    } else {
      this.close();
    }
  }
  open() {
    this.isOpened = true;
    this.elm.classList.add('is-opened');
    this.timeStart = Date.now();
    this.renderLoop();
  }
  close() {
    this.isOpened = false;
    this.elm.classList.remove('is-opened');
    this.timeStart = Date.now();
    this.renderLoop();
  }
}
(function() {
  const elmHamburger = document.querySelector('.hamburger');
  const gNavItems = document.querySelectorAll('.global-menu__item');
  const elmOverlay = document.querySelector('.shape-overlays');
  const overlay = new ShapeOverlays(elmOverlay);

  elmHamburger.addEventListener('click', () => {
    if (overlay.isAnimating) {
      return false;
    }
    overlay.toggle();
    if (overlay.isOpened === true) {
      elmHamburger.classList.add('is-opened-navi');
      for (var i = 0; i < gNavItems.length; i++) {
        gNavItems[i].classList.add('is-opened');
      }
    } else {
      elmHamburger.classList.remove('is-opened-navi');
      for (var i = 0; i < gNavItems.length; i++) {
        gNavItems[i].classList.remove('is-opened');
      }
    }
  });
  // Add code below
  gNavItems.forEach((item, index) => (
    item.addEventListener('click', () => {
      overlay.close();
    })
  ));
}());