awesome-uses/src/components/BackToTop.js

import React, { useState, useEffect } from 'react';
import styled from 'styled-components';

const BackToTopLink = styled.a`
  position: fixed;
  bottom: 1%;
  right: 1%;
  background: var(--pink);
  color: white;
  background: rgba(0, 0, 0, 0.5);
  cursor: pointer;
  border-radius: 3px;
  padding: 1rem;
  transition: opacity 0.2s;
  opacity: 0;
  text-decoration: none;
  ${props =>
    props.percent > 0.25 &&
    `
    opacity: 1;
  `}
  @media screen and (max-width: 500px) {
    display: none;
  }
`;

function scrollTopMax() {
  let ref;
  // scrollTopMax is only on Firefox right now! https://caniuse.com/#search=scrolltopmax
  return (ref = document.scrollingElement.scrollTopMax) != null
    ? ref
    : document.scrollingElement.scrollHeight - document.documentElement.clientHeight
}

function useScrollPosition() {
  const [percent, setPercent] = useState(0);

  function handleScroll(event) {
    console.log(document.documentElement.scrollTop);
    const howFar =
      document.documentElement.scrollTop /
      scrollTopMax();
    setPercent(howFar);
  }

  useEffect(() => {
    // listen for window scroll event
    document.addEventListener('scroll', handleScroll);
    return () => {
      document.removeEventListener('scroll', handleScroll);
    };
  });

  return percent;
}

export default function BackToTop() {
  const percent = useScrollPosition();
  return (
    <BackToTopLink href="#top" title="Back To Top" percent={percent}>
      &uarr;
    </BackToTopLink>
  );
}