Why IE alert 'unknow'

by Lien Z

HTML

<button id='btn1'>Click 1 !!</button>
<button id="btn2">Click 2 !!</button>

JavaScript

var btn1EventObj = null;
document.getElementById('btn1').onclick = function() {
    btn1EventObj = window.event;
    alert(btn1EventObj.srcElement.id);
}
document.getElementById('btn2').onclick = function() {
    alert(btn1EventObj === window.event); // false
    alert(btn1EventObj.srcElement === window.event.srcElement); // true : 
    //it should alert false but true ,
    //so in MSIE,every time an event has been fired ,it will be only one Event Object, and all event share this Event Object;
    alert(btn1EventObj.srcElement.id); // btn2 :
    // as we think it should be output 'btn2', but it alert 'btn2 ' ,and this is make sure that the theory is correct:
    // all the Event are all share one Evnet Object at all.
}