Listen for onfocus events on document

If you want to set an event listener on document for an input's onFocus event, you'll find there is no "onFocus" event for document. Instead you can use the focusin event listener:

document.addEventListener("focusin", function(e) {
  console.log("an input received focus");
});

So if you had dynamic events you can achieve this:

const on = (eventName, elementSelector, handler) => {
  document.addEventListener(eventName, function (e) {
    for (var target = e.target; target && target != this; target = target.parentNode) {
      if (target.matches(elementSelector)) {
        handler.call(target, e);
        break;
      }
    }
  }, false);
}

on("focusin", "#email_address", function(e) {
  const currentTarget = this;
  console.log("Email address just received focus");
});