reference generator using counters

gen_refs.js

/* Build a references section that lists URLs of hyperlinks
 * with specified class "ref". Put list in div element with
 * specified id "references". Use CSS class called references
 * that uses counters and ::before psuedo-element to add
 * brackets around reference list numbers.
 * links in body look like this:
 * blah <a href="URL>reference</a><a class="ref"><a> */
document.body.onload = gen_refs("a", "references", "ref");
function gen_refs(tagname, refsname, classname) {
  var alltags = document.getElementsByTagName(tagname);
  var refs = document.getElementById(refsname);
  var content = '<h2 class="references">References</h2>';
  var ctr = 0
  for (var i = 0; i < alltags.length; i++) {
    var tag_class = alltags[i].getAttribute("class");
    if (tag_class == classname) {
      console.log(alltags[i]);
      content += '<p class="references"><a href="';
      content += alltags[i].previousSibling.getAttribute("href");
      content += '">' + alltags[i].previousSibling.getAttribute("href") + '</a></p>';
      alltags[i].setAttribute("href", "#references");
      alltags[i].innerHTML = '<b>[' + ++ctr + ']</b>';
    }
  }
  console.log(content);
  refs.innerHTML = content;
}