addEventListener.js

function addEvent(elem, event, fn) {
    function listenHandler(e) {
        var ret = fn.apply(this, arguments);
        if (ret === false) {
            e.stopPropagation();
            e.preventDefault();
        }
        return(ret);
    }
    function attachHandler() {
        var ret = fn.call(elem, window.event);   
        if (ret === false) {
            window.event.returnValue = false;
            window.event.cancelBubble = true;
        }
        return(ret);
    }
    if (elem.addEventListener) {
        elem.addEventListener(event, listenHandler, false);
    } else {
        elem.attachEvent("on" + event, attachHandler);
    }
}



// Simpler version without the propagation and default prevention options but with this and event normalization: 

function addEvent(elem, event, fn) {
    if (elem.addEventListener) {
        elem.addEventListener(event, fn, false);
    } else {
        elem.attachEvent("on" + event, function() {
            return(fn.call(elem, window.event));   
        });
    }
}