问使用浮动侧边栏创建目录

EN

var scrollPos = window.scrollY,
    fixedEle = document.querySelector('aside'),
    fixedElePos = fixedEle.offsetTop;

window.addEventListener('scroll', function() {
  scrollPos = window.scrollY;

  if (scrollPos >= fixedElePos) {
    fixedEle.classList.add('fixed');
  } else {
    fixedEle.classList.remove('fixed');
  }
});

.container {
  height:3000px;
  padding-top:150px;
}
aside {
  position:relative;
  top:0;
}
  aside.fixed {
    position:fixed;
  }

<div class="container">
  <aside>
    This element should be fixed on scroll
  </aside>
</div>