app.js

var titleAlert = "It Pops!!!";
var closeModal = "Ok";

if(document.getElementById) {
    window.alert = function(txt) {
        createCustomAlert(txt);
    }
}

//Function To Create Custom Alert
function createCustomAlert(txt) {
    doc = document;

    if(doc.getElementById("modalContainer")) return;

  //Create Div For Modal Container Body
    modalObject = doc.getElementsByTagName("body")[0].appendChild(doc.createElement("div"));
    modalObject.id = "modalContainer";
    modalObject.style.height = doc.documentElement.scrollHeight + "px";
  
   //Create Div For Alert Box Container Body
    alertObj = modalObject.appendChild(doc.createElement("div"));
    alertObj.id = "alertBox";
    if(doc.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
    alertObj.style.left = (doc.documentElement.scrollWidth - alertObj.offsetWidth)/2 + "px";
    alertObj.style.visiblity="visible";

   //Create Element For Title Alert
    h1 = alertObj.appendChild(doc.createElement("h1"));
  h1.appendChild(doc.createTextNode(titleAlert));
  
   //Create Tag For Alert Body Content
      msg = alertObj.appendChild(doc.createElement("p"));
      msg.appendChild(doc.createTextNode(txt));
      msg.innerHTML = txt;

     //Create Tag To Close Modal Button
    btn = alertObj.appendChild(doc.createElement("a"));
    btn.id = "closeBtn";
    btn.appendChild(doc.createTextNode(closeModal));
    btn.href = "#";
    btn.focus();
    btn.onclick = function() { removeCustomAlert();return false; }

    alertObj.style.display = "block";
    
}

  //Function To Remove Custom Alert
function removeCustomAlert() {
    document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}