Execute Javascript Method After Completing Code Behind Method?

I want execute below callback() method after completion of document.getElementById('btnDownload').click(); method . Now callback()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");
}