live querySelector tool w/callback and observation

foreverEach.js

// A live querySelector tool with a callback that runs now and auto-applies to newly added elements.

function foreverEach(selector, fnCallback, root = document.documentElement){
  var used = new WeakSet();
  function isNew(elm){ if(!used.has(elm)) return used.add(elm); }
  
  [...root.querySelectorAll(selector)].filter(isNew).forEach(fnCallback);

  return {
  ob: new MutationObserver(function _addWatcher(mutationsList) {
	  for (var x of mutationsList)  
		  if (x.type === 'childList')  [...x.addedNodes]
			  .filter(x=>x.matches && x.matches(selector))
			  .filter(isNew)
			  .forEach( fnCallback );
	  }).observe(root, { 
		  childList: true,  subtree: true 
	  }),
	elms: used, 
	selector, 
	fnCallback, 
	root, 
	$: x=>[...root.querySelectorAll(selector)] 
 };
} // end foreverEach()


/* // uninspired example: current and newly-added <P> tags will get a random number title attribute
var Ps = foreverEach("p", x=>{
  x.title = Math.random();
});
*/