src/HashLink.jsx

import React from 'react';
import PropTypes from 'prop-types';
import { Link, NavLink } from 'react-router-dom';

let hashFragment = '';
let observer = null;
let asyncTimerId = null;
let scrollFunction = null;

function reset() {
  hashFragment = '';
  if (observer !== null) observer.disconnect();
  if (asyncTimerId !== null) {
    window.clearTimeout(asyncTimerId);
    asyncTimerId = null;
  }
}

function isInteractiveElement(element) {
  const formTags = ['BUTTON', 'INPUT', 'SELECT', 'TEXTAREA'];
  const linkTags = ['A', 'AREA'];
  return (
    (formTags.includes(element.tagName) && !element.hasAttribute('disabled')) ||
    (linkTags.includes(element.tagName) && element.hasAttribute('href'))
  );
}

function getElAndScroll() {
  let element = null;
  if (hashFragment === '#') {
    // use document.body instead of document.documentElement because of a bug in smoothscroll-polyfill in safari
    // see https://github.com/iamdustan/smoothscroll/issues/138
    // while smoothscroll-polyfill is not included, it is the recommended way to implement smoothscroll
    // in browsers that don't natively support el.scrollIntoView({ behavior: 'smooth' })
    element = document.body;
  } else {
    // check for element with matching id before assume '#top' is the top of the document
    // see https://html.spec.whatwg.org/multipage/browsing-the-web.html#target-element
    const id = hashFragment.replace('#', '');
    element = document.getElementById(id);
    if (element === null && hashFragment === '#top') {
      // see above comment for why document.body instead of document.documentElement
      element = document.body;
    }
  }

  if (element !== null) {
    scrollFunction(element);

    // update focus to where the page is scrolled to
    // unfortunately this doesn't work in safari (desktop and iOS) when blur() is called
    let originalTabIndex = element.getAttribute('tabindex');
    if (originalTabIndex === null && !isInteractiveElement(element)) {
      element.setAttribute('tabindex', -1);
    }
    element.focus({ preventScroll: true });
    if (originalTabIndex === null && !isInteractiveElement(element)) {
      // for some reason calling blur() in safari resets the focus region to where it was previously,
      // if blur() is not called it works in safari, but then are stuck with default focus styles
      // on an element that otherwise might never had focus styles applied, so not an option
      element.blur();
      element.removeAttribute('tabindex');
    }

    reset();
    return true;
  }
  return false;
}

function hashLinkScroll(timeout) {
  // Push onto callback queue so it runs after the DOM is updated
  window.setTimeout(() => {
    if (getElAndScroll() === false) {
      if (observer === null) {
        observer = new MutationObserver(getElAndScroll);
      }
      observer.observe(document, {
        attributes: true,
        childList: true,
        subtree: true,
      });
      // if the element doesn't show up in specified timeout or 10 seconds, stop checking
      asyncTimerId = window.setTimeout(() => {
        reset();
      }, timeout || 10000);
    }
  }, 0);
}

export function genericHashLink(As) {
  return React.forwardRef((props, ref) => {
    let linkHash = '';
    if (typeof props.to === 'string' && props.to.includes('#')) {
      linkHash = `#${props.to.split('#').slice(1).join('#')}`;
    } else if (
      typeof props.to === 'object' &&
      typeof props.to.hash === 'string'
    ) {
      linkHash = props.to.hash;
    }

    const passDownProps = {};
    if (As === NavLink) {
      passDownProps.isActive = (match, location) =>
        match && match.isExact && location.hash === linkHash;
    }

    function handleClick(e) {
      reset();
      hashFragment = props.elementId ? `#${props.elementId}` : linkHash;
      if (props.onClick) props.onClick(e);
      if (
        hashFragment !== '' &&
        // ignore non-vanilla click events, same as react-router
        // below logic adapted from react-router: https://github.com/ReactTraining/react-router/blob/fc91700e08df8147bd2bb1be19a299cbb14dbcaa/packages/react-router-dom/modules/Link.js#L43-L48
        !e.defaultPrevented && // onClick prevented default
        e.button === 0 && // ignore everything but left clicks
        (!props.target || props.target === '_self') && // let browser handle "target=_blank" etc
        !(e.metaKey || e.altKey || e.ctrlKey || e.shiftKey) // ignore clicks with modifier keys
      ) {
        scrollFunction =
          props.scroll ||
          ((el) =>
            props.smooth
              ? el.scrollIntoView({ behavior: 'smooth' })
              : el.scrollIntoView());
        hashLinkScroll(props.timeout);
      }
    }
    const { scroll, smooth, timeout, elementId, ...filteredProps } = props;
    return (
      <As {...passDownProps} {...filteredProps} onClick={handleClick} ref={ref}>
        {props.children}
      </As>
    );
  });
}

export const HashLink = genericHashLink(Link);

export const NavHashLink = genericHashLink(NavLink);

if (process.env.NODE_ENV !== 'production') {
  HashLink.displayName = 'HashLink';
  NavHashLink.displayName = 'NavHashLink';

  const propTypes = {
    onClick: PropTypes.func,
    children: PropTypes.node,
    scroll: PropTypes.func,
    timeout: PropTypes.number,
    elementId: PropTypes.string,
    to: PropTypes.oneOfType([PropTypes.string, PropTypes.object]),
  };

  HashLink.propTypes = propTypes;
  NavHashLink.propTypes = propTypes;
}