if (-1 !== handler) 

will always run code

That's a great explanation, thank you! I've been trying to examine it very closely and everything seems to have become clear, except for the "unexpected" part.

fn.apply(fn); // 1002 -> don't expected behaviour

It may be "unexpected", but I would still like to understand it. Why does obj.fn.val becomes changed at all.....
Wait, I think I've finally got it right now while I am writing this. Since fn is now 'made' (i think it's a bad word) of obj.fn, we can actually use fn.val, which is by now equal to 1001. When we apply the function itself as this, it of course changes the value which used to be obj.fn.val and is now fn.val. I think I had a wrong understanding that this by default refers to the function itself. But it's not the function, it's the context where the function is triggered.

Your example is very helpful, thanks again.

Oh, yes. Should be if (-1 !== index)
Thank you(=

In most cases you don't want to do that. If function relay on this in its body (which is equal to window unless function defined in object or "use strict"; is used) you probably will encounter some side effects :) It's safer to pass null and use bind if you need to keep function's current context.

Perhaps an example will make it clear:

var obj = {
  val: 0,
  fn: function func() {
    this.val++;
    return this.val;
  }
}
window.val = 100;
obj.fn.val = 1000;

obj.fn(); // 1 -> this === obj
obj.fn.apply(obj); // 2
obj.fn.apply(null); // 101
obj.fn.apply(obj.fn); // 1001

var fn = obj.fn;
// this now refer to window instance
fn(); // 102
fn.apply(null); // 103 -> same as fn()
fn.apply(fn); // 1002 -> don't expected behaviour
fn.apply(obj); // 3

var bindedFn = obj.fn.bind(obj);
// this now is always refer to obj
bindedFn.apply(obj); // 4
bindedFn.apply(null); // 5
bindedFn.apply(obj.fn); // 6

I have a question. What if in the Event.prototype.emit we apply not null as this, but the function itself? Like this:

handler.apply(handler, args);

Can someone explain if this can be better or worse in some cases?

if (-1 !== handler) will always run code