滚动监视高亮当前区域

var heads = document.querySelectorAll('h2[id], h3[id]');
if (heads.length && 'IntersectionObserver' in window) {
  var obs = new IntersectionObserver(function (entries) {
    entries.forEach(function (en) {
      if (en.isIntersecting) {
        document.querySelectorAll('a[href^="#"]').forEach(function (a) {
          var on = a.getAttribute('href') === '#' + en.target.id;
          a.style.fontWeight = on ? '700' : '';
          a.style.background = on ? '#fff3a8' : '';
        });
      }
    });
  }, { rootMargin: '0px 0px -70% 0px' });
  heads.forEach(function (h) { obs.observe(h); });
}