Появляющаяся кнопка “ВВЕРХ” внизу экрана

var totop = document.getElementById('totop'); 
 
totop.addEventListener("click", function() { 
  document.body.scrollTop = document.documentElement.scrollTop = 0; 
}, false); 
 
window.onscroll = function() { 
  var scrolled = window.pageYOffset || document.documentElement.scrollTop; 
  if (scrolled > 1000) { 
    totop.classList.add('visible'); 
  } else { 
    totop.classList.remove('visible'); 
  } 
}

body { 
  min-height: 3000px; 
  background: linear-gradient(to top, rgba(240,249,52,1) 30%,rgba(161,219,255,1) 70%); 
} 
 
#totop { 
  opacity: 0; 
  pointer-events: none; 
  position: fixed; 
  bottom: 10%; 
  left: 10%; 
  transition: opacity .2s; 
  cursor: pointer; 
} 
 
#totop.visible { 
  opacity: 1; 
  pointer-events: all; 
}

<button id="totop">Наверх</button>

var showBtn = 500; 
$(window).scroll(function() { 
  var scrolled = $(window).scrollTop(); 
  if (scrolled > showBtn) { 
    $('button').addClass('visible'); 
  } else { 
    $('button').removeClass('visible'); 
  } 
});

body { 
  min-height: 2000px; 
} 
 
button { 
  position: fixed; 
  right: 50px; 
  bottom: 50px; 
  transition: all 0.4s; 
  opacity: 0; 
  visibility: hidden; 
} 
 
button.visible { 
  opacity: 1; 
  visibility: visible; 
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> 
<button>Вверх</button>