render any number of recaptchas that are on a single page

gistfile1.html

In the page somewhere:

<script type="text/javascript" defer>
var gcRender = function() {
  var sitekey, gcs, i, rcks, len;
  sitekey = '';
  // Render all .g-recaptcha's on the page
  gcs = document.getElementsByClassName('g-recaptcha');
  len = gcs.length;
  for (i = 0; i < len; i++) {
    grecaptcha.render(gcs[i], {'sitekey' : sitekey});
  } 
  // Add a value for the forms' recaptcha_keys
  rcks = document.querySelectorAll('input[name=recaptcha_key]');
  len = rcks.length;
  for (i = 0; i < len; i++) {
    rcks[i].value = sitekey;
  }
};
</script>
<script src='https://www.google.com/recaptcha/api.js?onload=gcRender&render=explicit' async defer></script>

In each form:
<input type="hidden" name="recaptcha_key">
<div class="g-recaptcha"></div>