The Javascript for 538 footnotes

example.html

<!-- https://fivethirtyeight.com/features/who-wants-to-be-a-senator-not-these-high-profile-democrats/ -->
<p>Remember that in 2016, <a href="http://crystalball.centerforpolitics.org/crystalball/articles/senate-observations-placing-2018-in-the-context-of-upper-chamber-elections-since-1913/">for the</a> <a href="https://fivethirtyeight.com/features/there-were-no-purple-states-on-tuesday/">first time</a>,<a class="espn-footnote-link espn-footnote-item-expanded" data-footnote-id="1" href="#fn-1" data-footnote-content="<p>Or at least since popular Senate elections began <a href=&quot;https://constitutioncenter.org/interactive-constitution/amendments/amendment-xvii&quot;>after the ratification</a> of the 17th Amendment in 1913.</p>
"><sup id="ss-1">x</sup></a><span class="espn-footnote-item espn-footnote-item-displayed espn-footnote-item-expanded"><span class="espn-footnote-item-inner"><span><p>Or at least since popular Senate elections began <a href="https://constitutioncenter.org/interactive-constitution/amendments/amendment-xvii" target="_blank" rel="noopener noreferrer">after the ratification</a> of the 17th Amendment in 1913.</p>
</span></span></span> <i>every</i> Senate race was won by the same party that won the presidential vote in that state. In a <a href="https://www.people-press.org/interactives/political-polarization-1994-2017/">polarized era</a> full of <a href="https://fivethirtyeight.com/features/split-ticket-voting-hit-a-new-low-in-2018-senate-and-governor-races/">straight-ticket voters</a>, this is no doubt a prime consideration when potential candidates are deciding whether to run. And as the table below shows, by our count, there’s at least one major Democratic player who has publicly said he or she won’t seek a Republican-held Senate seat in six states that President Trump won in 2016.</p>

footnote.js

function n(e) {
    var t,n,i,s,o,r,a=h(e),
        l=parseInt(a.data("footnote-id"),10),
        c=parseInt(a.find("sup").text(),10);
    a.toggleClass("espn-footnote-item-expanded"),
    c===l ?
        (a.find("sup").text("x"),
         (t=a.next(".espn-footnote-item")).length || (
             n=a.data("wrap"),
             i=h("<span />", {
                 class:"espn-footnote-item "+(n?"footnote-wrap":"")
             }),
             s=h("<span />",{class:"espn-footnote-item-inner"}),
             o=h("<span />",{html:a.attr("data-footnote-content")}),
             s.append(o),
             r=i.append(s),
             a.after(r),
             t=a.next(".espn-footnote-item")),
         t.length && (
             t.addClass("espn-footnote-item-displayed"),
             d.setTimeout(function() {t.addClass("espn-footnote-item-expanded")},50)),
         espn && espn.track && espn.trackAndClear({
             link:"#"+l,
             linkPos:d.s_omni.prop4,
             linkId:"footnote:open:"+l,
             site:"fivethirtyeight",
             pageName:d.s_omni.pageName,
             section:d.s_omni.channel}))  :
        (a.find("sup").text(l),
         a.next(".espn-footnote-item").removeClass("espn-footnote-item-expanded"),
         d.setTimeout(function() {
             a.next(".espn-footnote-item").removeClass("espn-footnote-item-displayed")},300),
         espn && espn.track && espn.trackAndClear({
             link:"#"+l,
             linkPos:d.s_omni.prop4,
             linkId:"footnote:close:"+l,
             site:"fivethirtyeight",
             pageName:d.s_omni.pageName,
             section:d.s_omni.channel}))
}

function footnoteOnClickHandler(e) {
  e.preventDefault(), e.altKey ? jQuery(".espn-footnote-link").each(function(e, t) {
    n(t)
  }) : n(this)
}

I have been wanting to incorporate this feature into an RMarkdown document for a while. Thank you for this! Now I just need to figure out how to call it in a markdown doc 👍