Drag custom alert box using javascript

// constants to define the title of the alert and button text.
var ALERT_TITLE = "Oops!";
var ALERT_BUTTON_TEXT = "Ok";

// over-ride the alert method only if this a newer browser.
// Older browser will see standard alerts
if(document.getElementById) {
	window.alert = function(txt) {
		createCustomAlert(txt);
	}
}

function createCustomAlert(txt) {
	// shortcut reference to the document object
	d = document;

	// if the modalContainer object already exists in the DOM, bail out.
	if(d.getElementById("modalContainer")) return;

	// create the modalContainer div as a child of the BODY element
	mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
	mObj.id = "modalContainer";
	 // make sure its as tall as it needs to be to overlay all the content on the page
	mObj.style.height = document.documentElement.scrollHeight + "px";

	// create the DIV that will be the alert 
	alertObj = mObj.appendChild(d.createElement("div"));
	alertObj.id = "alertBox";
	// MSIE doesnt treat position:fixed correctly, so this compensates for positioning the alert
	if(d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
	// center the alert box
	alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth)/2 + "px";

	// create an H1 element as the title bar
	h1 = alertObj.appendChild(d.createElement("h1"));
	h1.appendChild(d.createTextNode(ALERT_TITLE));

	// create a paragraph element to contain the txt argument
	msg = alertObj.appendChild(d.createElement("p"));
	msg.innerHTML = txt;
	
	// create an anchor element to use as the confirmation button.
	btn = alertObj.appendChild(d.createElement("a"));
	btn.id = "closeBtn";
	btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
	btn.href = "#";
	// set up the onclick event to remove the alert when the anchor is clicked
	btn.onclick = function() { removeCustomAlert();return false; }

	
}

// removes the custom alert from the DOM
function removeCustomAlert() {
	document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}