find_with_mutations.js

// browser support: chrome, safari, firefox, opera, ie9+, ieMobile10+ (Opera Mini not supported)
// No external libraries needed.

// calls fn on all current and future elements matching selector within the first element matching container_selector.
// find_with_mutations('.container', '.element', function(el) {  })
window.find_with_mutations = function(container_selector, selector, fn) {
  var observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
      [].slice.call(mutation.addedNodes).forEach(function(el) {
        if (el.nodeType !== 1) return;
        if (matchesSelector(el, selector)) // Node itself matches
          fn(el);
        [].slice.call(el.querySelectorAll(selector)).forEach(function(el2) { // subnodes match
          fn(el2);
        })
      })
    });
  });

  var target = document.querySelector(container_selector);
  [].slice.call(target.querySelectorAll(selector)).forEach(function(el) {    // handle existing elements
    fn(el)
  })
  observer.observe(target, { childList: true, subtree: true });              // handle future elements
}

// crossbrowser matchesSelector. 
window.matchesSelector = function(element, selector) {
  ms = element.matchesSelector  || element.webkitMatchesSelector || 
    element.mozMatchesSelector  || element.oMatchesSelector || element.msMatchesSelector;
  if (ms) return ms.call(element, selector)
}