Ajaxify simple javascript msg

I’m trying to Ajaxify a javascript error message in an existing WordPress plugin. The original code in the plugin looks like this:

if (msg.length > 0) {
			msg = "The following fields need to be completed before you can submit.\n\n" + msg;
			alert(msg);
			return false;
		}

	return true;

This doesn’t work unfortunately:

if (msg.length > 0) {
			msg = "The following fields need to be completed before you can submit.\n\n" + msg;

			if (xmlHttp==null)
			{
			alert(msg);
			return false;
			}  

			document.getElementById("message").innerHTML = 'Please fill out all required information';
			return false;
		}

	return true;   

}

What am I missing?