HTML：

<button id="scrollDown" onclick="bottom()">Scroll to bottom</button>

这将创建一个按钮，单击该按钮时使用bottom()函数。
要实现按钮的功能，我们可以使用以下Javascript：

// Access button
var mybutton = document.getElementById("scrollDown");
window.onscroll = function() {scrollFunction()};

// Scroll to bottom function
function bottom() {
  if (document.body.scrollTop > 20 || document.documentElement.scrollTop > 20) { 
    // If scrolled down, make button appear.
    mybutton.style.display = "block"; // Show button
  } else {
    mybutton.style.display = "none"; / Hide button
  }
}

我们可以使用CSS将按钮固定在屏幕的中心：

#scrollDown {
  display: none; // Originally hide the button. Users always begin at the top of the page
  position: fixed; // Make the button fixed.
  bottom: 20px; // Fix to bottom of screen
  right: 30px; // Fix to right of screen
  z-index: 9999; // Make sure it is at the VERY top of the screen
  font-size: 18px; // Font-size. Not required
  border: none; // Remove Border. Not required
  outline: none; // Remove Outline. Not required
  background-color: red; Red Background. Not required
  color: white; // White text. Not required
  cursor: pointer; // Show that the button is a button, by making the cursor a pointer hand.
  padding: 15px; // Padding for CSS. Not required.
  border-radius: 4px; Border-radius for looks. Not required
}