function getEvent(event) {
return event || window.event // IE:window.event
}

function getEvent() {
return arguments[0] || window.event // IE:window.event
}

<button id="btn" onclick="foo()">按鈕</button>
<script>
function foo(){
var e = getEvent();
alert(e);}
</script>

function onclick(event) {
foo();
}

function foo(){
var e = getEvent();
alert(e);
}

function getEvent(event) {
var ev = event || window.event;
if (!ev) {
var c = this.getEvent.caller;
while (c) {
ev = c.arguments[0];
if (ev && (Event == ev.constructor || MouseEvent == ev.constructor)) { /懌飛注：YUI 源碼 BUG，ev.constructor 也可能是 MouseEvent，不一定是 Event
break;
}
c = c.caller;
}
}
return ev;
}

<button id="btn" onclick="foo(event)">按鈕</button>