这段JS是干什么的？

代码:

if(!isIE){
	window.constructor.prototype.__defineGetter__("event",function(){
		var func = arguments.callee.caller;
		while(func != null){
			var arg0 = func.arguments[0];
			if(arg0 && (arg0.constructor==Event || arg0.constructor ==MouseEvent)){
				return arg0;
			}
			func = func.caller;
		}
    	return null;
	});
	Event.prototype.__defineSetter__("returnValue",function(b){
		if(!b)this.preventDefault();
		return b;
	});
	Event.prototype.__defineGetter__("srcElement",function(){
		var node=this.target;
		while(node.nodeType != 1)node=node.parentNode;
		return node;
	});
	Event.prototype.__defineGetter__("offsetX",function(){
			return this.layerX;
	});
	Event.prototype.__defineGetter__("offsetY",function(){
		return this.layerY;
	});
	HTMLElement.prototype.attachEvent = function(sType,foo){
		this.addEventListener(sType.slice(2),foo,false);
	}
	HTMLElement.prototype.detachEvent = function(sType,foo){
		this.removeEventListener(sType.slice(2),foo,false);
	}
	HTMLDocument.prototype.attachEvent = function(sType,foo){
		this.addEventListener(sType.slice(2),foo,false);
	}
	HTMLDocument.prototype.detachEvent = function(sType,foo){
		this.removeEventListener(sType.slice(2),foo,false);
	}
	HTMLElement.prototype.__defineGetter__("innerText",function(){
		return this.textContent;
	});
	HTMLElement.prototype.__defineSetter__("innerText",function(str){
		this.textContent = str;
	});
}
else document.execCommand("BackgroundImageCache",false,true);