Custom alert not working like normal alert

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Javascript

jQuery

var ALERT_TITLE = "Message";
        var ALERT_BUTTON_TEXT = "Ok";

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

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

            mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
            mObj.id = "modalContainer";
            //mObj.style.height = d.documentElement.scrollHeight + "px";

            alertObj = mObj.appendChild(d.createElement("div"));
            alertObj.id = "alertBox";
            if (d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
            //alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth) / 2 + "px";
            alertObj.style.visiblity = "visible";

            h1 = alertObj.appendChild(d.createElement("h1"));
            h1.appendChild(d.createTextNode(ALERT_TITLE));

            msg = alertObj.appendChild(d.createElement("div"));
            msg.id = "alertMsgContainer";
            //msg.appendChild(d.createTextNode(txt));
            msg.innerHTML = txt;

            btn = alertObj.appendChild(d.createElement("a"));
            btn.id = "closeBtn";
            btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
            btn.href = "#";
            btn.focus();
            btn.onclick = function () { removeCustomAlert(); return false; }
           
            alertObj.style.display = "block";

        }

        function removeCustomAlert() {
           
            document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
        }

What I have tried:

I Make A Custom alert box but custom alert not hold script ,normal alert hold script till user not clicked ok .In My Case only show alert and execute full function not wait.
I Want my custom alert open and when i clicked ok then rest of function execute till then function delay.like normal alert working.
Thanks,
Shiavm