Cross-Browser event attach handler

addEventListener.js

// Crossbrowser event attacher.
var addEventListener = function (elem, evento, handler) {
  console.assert(elem.addEventListener || elem.attachEvent, "Can't find an event handler.");
  if (elem.addEventListener) {
    elem.addEventListener(evento, function (e) {
      if (handler.call(elem, e) === false)
        e.preventDefault();
    });
  } else if (elem.attachEvent) {
    elem.attachEvent('on' + evento, function (e) {
      if (handler.call(elem, e) === false)
        e.returnValue = false;
    });
  }
};