Подписаться/отписаться на/от события

add-remove-event.js

var addEvent = function(el, e, callback, capture) {
    if (!!window.addEventListener) {
      el.addEventListener(e, callback, !!capture);
    } else {
      el.attachEvent("on" + e, callback);
    }
  },
  removeEvent = function(el, e, callback, capture) {
    if (!!window.addEventListener) {
      el.removeEventListener(e, callback, !!capture);
    } else {
      el.detachEvent("on" + e, callback);
    }
  };