What's 'this' in IE's attachEvent

Question

I'm tired of writing jQuery, so I decide to learn some raw JavaScript. Something in IE's attachEvent confused me. Here's the code:

var btn = document.getElementById('myBtn');
    
btn.onclick = function(){
   alert(window.event.srcElement === this); //true, I know why.
};
        
btn.attachEvent('onclick', function(event){
   alert(event.srcElement === this); //fasle, but why?
});

I try to use IE's built-in debug tools, but it just told me that 'this' is an object, but nothing more... so what's 'this' in IE's attachEvent?

Answer 1

Within an event handler bound by the IE-specific attachEvent method, this refers to the global window object:

btn.attachEvent('onclick', function(event) {
    alert(this === window); // true
}

In contrast, within an event handler bound by the standard addEventListener method, this refers to the DOM element from which the event handler was triggered.