ManageScroll.js

import React from "react";
import { Location } from "@reach/router";

let scrollPositions = {};

class ManageScrollImpl extends React.Component {
  componentDidMount() {
    try {
      // session storage will throw for a few reasons
      // - user settings
      // - in-cognito/private browsing
      // - who knows...
      let storage = JSON.parse(sessionStorage.getItem("scrollPositions"));
      if (storage) {
        scrollPositions = JSON.parse(storage) || {};
        let { key } = this.props.location;
        if (scrollPositions[key]) {
          window.scrollTo(0, scrollPositions[key]);
        }
      }
    } catch (e) {}

    window.addEventListener("scroll", this.listener);
  }

  componentWillUnmount() {
    window.removeEventListener("scroll", this.listener);
  }

  componentDidUpdate() {
    const { key } = this.props.location;
    if (!scrollPositions[key]) {
      // never seen this location before
      window.scrollTo(0, 0);
    } else {
      // seen it
      window.scrollTo(0, scrollPositions[key]);
    }
  }

  listener = () => {
    scrollPositions[this.props.location.key] = window.scrollY;
    try {
      sessionStorage.setItem(
        "scrollPositions",
        JSON.stringify(scrollPositions)
      );
    } catch (e) {}
  };

  render() {
    return null;
  }
}

export default () => (
  <Location>
    {({ location }) => <ManageScrollImpl location={location} />}
  </Location>
);

Why not refactor componentDidUpdate block to:

const vistedLocationPosition = scrollPositions[this.props.location.key]
window.scrollTo(0, vistedLocationPosition || 0)

For anyone looking for a functional component version of managing scroll position. I just import this component below my router <Router>...routes</Router><ManageScroll />

import { Location, WindowLocation } from "@reach/router";
import React from "react";

let scrollPositions: { [key: string]: number } = {};

type Props = {
	location: WindowLocation;
}

const ManageScrollImpl = ({ location }: Props) => {

	React.useEffect(() => {
		if (location.href) {
			window.scrollTo(0, scrollPositions[location.href || 0]);
		}
		window.addEventListener('scroll', listener);
		return () => window.removeEventListener('scroll', listener);
	}, [location?.href])

	const listener = () => {
		if (location && location.href) {
			scrollPositions[location.href] = window.scrollY;
		}
	};

	return null;
}

export const ManageScroll = () => (
	<Location>
		{({ location }) => <ManageScrollImpl location={location} />}
	</Location>
);