Use vanilla JavaScript to fix in-page anchors when using <base>

fix-base-anchor-links-es5.js

(function () {
  var hashAnchors = document.querySelectorAll('a[href^="#"]');

  /*
    Legacy browsers don't support NodeList.prototype.forEach,
    so we have to call Array.prototype.forEach setting the
    context to the NodeList.
  */
  [].forEach.call(hashAnchors, function (anchor) {
    anchor.addEventListener('click', function (event) {
      event.preventDefault();
      /*
        Browsers support setting location.hash as a string
        with or without the starting "#".
        
        This works best when the <a> element only contains text. 
        Otherwise, "event.target" may not be the <a> element.
      */
      document.location.hash = event.target.getAttribute('href');
    });
  });
})();

fix-base-anchor-links-es6.js

( _ => {
  let hashAnchors = document.querySelectorAll('a[href^="#"]');

  /*
    If your browser supports ES6, it likely supports NodeList.prototype.forEach (no IE).
    https://developer.mozilla.org/en-US/docs/Web/API/NodeList/forEach#Browser_Compatibility
  */
  hashAnchors.forEach( anchor => {
    anchor.addEventListener('click', evt => {
      evt.preventDefault();
      document.location.hash = evt.target.getAttribute('href');
    });
  });
})();