在div滚动上动态检查div的offsetTop

Question

我正在尝试在uswr在div中滚动50px时提醒消息，但它不起作用：

function handleScroll() {
  console.log('scrolling... ', this.offsetTop);
  if(this.offsetTop > 50) alert('scrolled more than 50px!');
}
document.getElementById('a').addEventListener('scroll', handleScroll);

这是怎么回事？ codepen demo

Answer 1

function handleScroll() {
 alert('scrolling... '+ this.scrollTop);
  if(this.scrollTop > 50) alert('scrolled more than 50px!');
 }
document.getElementById('a').addEventListener('scroll', handleScroll);