如何使导航栏变得粘性？

<nav id="navbar">
  <!-- 导航栏内容 -->
</nav>

#navbar {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  z-index: 9999;
  /* 导航栏的原始样式 */
}

#navbar.sticky {
  /* 导航栏的粘性样式 */
}

window.addEventListener('scroll', function() {
  var navbar = document.getElementById('navbar');
  var sticky = navbar.offsetTop;

  if (window.pageYOffset >= sticky) {
    navbar.classList.add('sticky');
  } else {
    navbar.classList.remove('sticky');
  }
});