tldr; jsFiddle Demo
This functionality is not possible with an alert. However, you could use a div
function tempAlert(msg,duration)
{
var el = document.createElement("div");
el.setAttribute("style","position:absolute;top:40%;left:20%;background-color:white;");
el.innerHTML = msg;
setTimeout(function(){
el.parentNode.removeChild(el);
},duration);
document.body.appendChild(el);
}
Use this like this:
tempAlert("close",5000);