Cross Browser Event Handling

/**
 * addEvent() controls event-handling for cross-browser issues
 * 
 * @param <Array> elem
 * @param <Array> event
 * @param <Function> fn
 *
 */
var app = app || {};

app = {
    utils : {
        addEvent : function (elem, event, fn) {
            function listenHandler(e) {
                var ret = fn.apply(this, arguments);
                if (ret === false) {
                    e.stopPropagation();
                    e.preventDefault();
                }
                return(ret);
            }
            if (elem.addEventListener) {
                elem.addEventListener(event, listenHandler, false);
            } else if (elem.attachEvent) {
                elem.attachEvent ("on" + type, fn);
            } else {
                elem.attachEvent("on" + event, attachHandler);
            }
        }
    },
    fn : {
        handlerClick : function () {
            alert('Button clicked...');
        }
    },
    init : function () {
        app.utils.addEvent(document.getElementById('newButton'), 'click', app.fn.handlerClick);
    }
}

app.init();

Cross-browser event handling for any purpose to understand clearly how libraries like jQuery works for it.