primer.js

!function() {
  var doc     = document,
      htm     = doc.documentElement,
      lct     = null,   // last click target                                              
      nearest = function(elm, tag) {
        while (elm && elm.nodeName != tag) {
          elm = elm.parentNode;
        }
        return elm;
  };

  // Listeners for our most common interations                                            
  htm.onclick = function(e) {
    e = e || window.event;
    lct = e.target || e.srcElement;

    var elem = nearest(lct, 'A') || htm,
        href = elem.getAttribute('ajaxify') || elem.href;

    switch (elem.rel) {
    case 'dialog':
    case 'dialog-post':
      Bootloader.loadComponents('dialog', function() {
        Dialog.bootstrap(href, null, elem.rel == 'dialog');
      });
      break;
    case 'async':
    case 'async-post':
      Bootloader.loadComponents('async', function() {
        AsyncRequest.bootstrap(href, elem);
      });
      break;
    default:
      return;
    }

    return false;
  };

  htm.onsubmit = function(e) {
    e = e || window.event;
    var elem = e.target || e.srcElement;

    if (!elem || elem.nodeName != 'FORM' || !elem.getAttribute('ajaxify')) {
      return;
    }

    Bootloader.loadComponents('dom-form', function() {
      bootstrap_form(elem, lct);
    });

    return false;
  };

  // Remove the no JS class, if it is here                                                
  htm.className = htm.className.replace('no_js', '');
}();

@josuebrunel - That's the one! Thanks for the link.

I work at Meta and Primer is still in the codebase, but isn't in use in any new development. It's mostly only used on legacy pages that are rendered server-side with XHP and Hack (XHP being the precursor to BoltJS components, which were the precursor to React's JSX).

I kinda miss using it since it was so simple to get basic interactions working pretty well. All the new stuff is React and Relay, which are great for highly interactive apps, but the barrier to entry and amount of boilerplate to build a basic page is a lot higher compared to just server-rendering a chunk of HTML, especially in small internal tools that just show some data and aren't very interactive (and where the team building tool may consist entirely of backend engineers with limited frontend experience). React definitely has its place, but I do still really like the Primer / htmx model.