Digital world

A good starting point is the Hugo footnotes

But I have done some improvements (according to my needs), as the following:

1. link (at the end of a html page) the file footnotes.js, with this content:

function getFootnoteContent(index) {
  const id = "fn:" + index;
  const fn = document.getElementById(id);
  return fn.innerHTML.trim();
}

function footnotePopup(showIndex, showCloseBtn) {
  const popupWrapper = document.querySelector("#popup-wrapper");

  // Set whether to display index and/or close button. Default is true for both
  if (showIndex === undefined) {
    showIndex = true;
  }

  if (showCloseBtn === undefined) {
    showCloseBtn = false;
  }

  // Create main container that will hold footnote content
  const popupContent = popupWrapper.appendChild(document.createElement("div"));
  popupContent.id = "popup-content";

  let popupIndex = null;
  if (showIndex) {
    popupIndex = popupWrapper.insertBefore(
      document.createElement("div"),
      popupContent
    );
    popupIndex.id = "popup-index";
  }

  let popupCloseButton = null;
  if (showCloseBtn) {
    popupCloseButton = popupWrapper.appendChild(document.createElement("div"));
    popupCloseButton.innerHTML = "[x]";
    popupCloseButton.id = "popup-close";
  }

  // Remove redundant [return] links from footnote list (optional)
  const fnReturns = document.querySelectorAll("a.footnote-return");
  fnReturns.forEach(function (fnReturn) {
    const parent = fnReturn.parentNode;
    parent.removeChild(fnReturn);
  });

  const fnRefs = document.querySelectorAll("a[id^='fnref:']");
  fnRefs.forEach(function (fnRef) {
    fnRef.addEventListener("click", handler("refs", fnRef));
  });

  //window.addEventListener("scroll", handler("close"));
  
  document.addEventListener(
    "click",
    function (event) {
      if (
        event.target.matches("a[id^='fnref']") ||
        event.target.matches("#popup-content")
      ) {
        return;
      }
      popupWrapper.style.display = "none";
    },
    false
  );

  if (showCloseBtn) {
    popupCloseButton.addEventListener("click", handler("close"));
  }

  function handler(type, node) {
    console.log("test");
    return function (event) {
      if (type === "close") {
        popupWrapper.style.display = "none";
      }

      if (type === "refs") {
        event.preventDefault();

        const index = node.id.substring(6);

        if (showIndex) {
          popupIndex.innerHTML = index + ".";
        }

        popupContent.innerHTML = getFootnoteContent(index);
        popupWrapper.style.display = "flex";
      }
    };
  }
}

footnotePopup(false, true);
 

2) Put, before that link, the div where the note will be shown

<div class=\"bottom-panel\" id=\"bottom-panel\">
 <div class=\"popup-wrapper\" id=\"popup-wrapper\"></div>
</div> 

3) Modify the anchor tag (within your main text), with something like

<a href="#n1" id="fnref:1">[1]</a> 

4) and the footnote should be like the following:

<p id="fn:1"><a href="#fnref:1">[1]</a> your footnote here.</p>