I can execute below callback() method after completion of document.getElementById('btnDownload').click(); .This Click is Code behind method. Now it is executing immediatly. I want wait "Click()" process done then Execute callback(); method.

function LoadPopup() {
        //  find the popup behavior
        this._popup = $find('mdlPopup');
        // show the popup
        this._popup.show();

        // synchronously run the server side validation ...
        document.getElementById('btnDownload').click();
       callback();         
    }

 function callback() {
        this._popup = $find('mdlPopup');
        //  hide the popup
        this._popup.hide();
        alert("hi");

}