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HTML

              
                <h1 class="intro-copy">
  Scroll down to use this simple back-to-top button made with modern vanilla javascript.
</h1>

<a class="top-link hide" href="" id="js-top">
  <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 12 6"><path d="M12 6H0l6-6z"/></svg>
  <span class="screen-reader-text">Back to top</span>
</a>
              
            
!

CSS

              
                body {
  height: 2000px;
  position: relative;
}

.intro-copy {
  padding: 1em;
  margin: 50vh auto;
  max-width: 15em;
  font-family: Helvetica;
  font-weight: lighter;
  font-size: 2em;
  line-height: 1.2;
  text-align: center;
}

.top-link {
  transition: all .25s ease-in-out;
  position: fixed;
  bottom: 0;
  right: 0;
  display: inline-flex;
  
	cursor: pointer;
	align-items: center;
	justify-content: center;
	margin: 0 3em 3em 0;
	border-radius: 50%;
	padding: .25em;
	width: 80px;
	height: 80px;
  background-color: #F8F8F8;
  
  &.show {
    visibility: visible;
    opacity: 1;
  }
  
  &.hide {
    visibility: hidden;
    opacity: 0;
  }

	svg {
		fill: #000;
		width: 24px;
		height: 12px;
	}

	&:hover {
		background-color: #E8E8E8;

		svg {
			fill: #000000;
		}
	}
}

// Text meant only for screen readers.
.screen-reader-text {
	position: absolute;
	clip-path: inset(50%);
	margin: -1px;
	border: 0;
	padding: 0;
	width: 1px;
	height: 1px;
	overflow: hidden;
	word-wrap: normal !important;
	clip: rect(1px, 1px, 1px, 1px);

	&:focus {
		display: block;
		top: 5px;
		left: 5px;
		z-index: 100000; // Above WP toolbar
		clip-path: none;
		background-color: #eee;
		padding: 15px 23px 14px;
		width: auto;
		height: auto;
		text-decoration: none;
		line-height: normal;
		color: #444;
		font-size: 1em;
		clip: auto !important;
	}
}
              
            
!

JS

              
                // Set a variable for our button element.
const scrollToTopButton = document.getElementById('js-top');

// Let's set up a function that shows our scroll-to-top button if we scroll beyond the height of the initial window.
const scrollFunc = () => {
  // Get the current scroll value
  let y = window.scrollY;
  
  // If the scroll value is greater than the window height, let's add a class to the scroll-to-top button to show it!
  if (y > 0) {
    scrollToTopButton.className = "top-link show";
  } else {
    scrollToTopButton.className = "top-link hide";
  }
};

window.addEventListener("scroll", scrollFunc);

const scrollToTop = () => {
  // Let's set a variable for the number of pixels we are from the top of the document.
  const c = document.documentElement.scrollTop || document.body.scrollTop;
  
  // If that number is greater than 0, we'll scroll back to 0, or the top of the document.
  // We'll also animate that scroll with requestAnimationFrame:
  // https://developer.mozilla.org/en-US/docs/Web/API/window/requestAnimationFrame
  if (c > 0) {
    window.requestAnimationFrame(scrollToTop);
    // ScrollTo takes an x and a y coordinate.
    // Increase the '10' value to get a smoother/slower scroll!
    window.scrollTo(0, c - c / 10);
  }
};

// When the button is clicked, run our ScrolltoTop function above!
scrollToTopButton.onclick = function(e) {
  e.preventDefault();
  scrollToTop();
}
              
            
!
999px

Console