jQuery style event handler that attaches to the document so it works on dynamically added elements

on.js

function on (eventName, selector, callback) {
  document.addEventListener(eventName, function (event) {
    var elem = event.srcElement
    var matches = elem.matches || elem.matchesSelector
    if (matches.call(elem, selector)) {
      callback.call(elem)
    }
  })
}

usage.js

on('click', 'button.say-hello', function () {
  alert('hello')
  console.log(this) /* <button class="say-hello"></button>" */
})