Do async work in OnClientClick before OnClick PostBack event

page.aspx

<asp:LinkButton ID="btnSave" OnClientClick="return prepareForSave(this);" OnClick="btnSave_Click" runat="server" Text="Save" />

script.js

function prepareForSave(sender) {
  if (!sender) return;

  var hrefToEvalWhenFinished = sender.attributes['href'].value; // Ext.get(sender).getAttribute('href');
 
  if (hrefToEvalWhenFinished) {
    // todo: do async work here
   
    // Fire the links HREF event which will do the postback.
    eval(hrefToEvalWhenFinished);
  }
 
  return false;
}