如何合并onclick事件和Scroll to Top

<button id="scrollToTopBtn">Scroll to Top</button>

var scrollToTopBtn = document.getElementById("scrollToTopBtn");
scrollToTopBtn.onclick = function() {
  window.scrollTo({
    top: 0,
    behavior: "smooth"
  });
};

window.onscroll = function() {
  var scrollPos = window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop;
  if (scrollPos > 100) {
    scrollToTopBtn.style.display = "block";
  } else {
    scrollToTopBtn.style.display = "none";
  }
};