<button id="backToTop" aria-label="Back to top">&#8679;</button>
<div style="height: 1500px; padding: 20px;">Scroll down to see the button appear.</div>

#backToTop {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 50px;
  height: 50px;
  background: #2563eb;
  color: white;
  border: none;
  border-radius: 50%;
  font-size: 1.5rem;
  cursor: pointer;
  box-shadow: 0 4px 12px rgba(37, 99, 235, 0.4);
  opacity: 0;
  visibility: hidden;
  transition: opacity 0.3s, transform 0.3s, background 0.2s;
  z-index: 999;
  display: flex;
  align-items: center;
  justify-content: center;
}
#backToTop.show {
  opacity: 1;
  visibility: visible;
}
#backToTop:hover {
  transform: translateY(-5px);
  background: #1d4ed8;
}

const btn = document.getElementById("backToTop");

window.onscroll = function() {
  if (document.body.scrollTop > 300 || document.documentElement.scrollTop > 300) {
    btn.classList.add("show");
  } else {
    btn.classList.remove("show");
  }
};

btn.addEventListener("click", function() {
  window.scrollTo({
    top: 0,
    behavior: "smooth"
  });
});