How to call onclick event on confirm message alert box?

I am using a javascript

function ValueChanged()
{           
    if (document.getElementById("<%=isDirty.ClientID %>").value != document.getElementById("<%=rdbQuestiontype.ClientID %>").value)     
    {

       if (window.confirm('Your changes are not saved. Do you want 开发者_JAVA技巧to save your changes before you exit.'))
           return false;
       else
           return true;
    }
}

this java script i am calling on OnClientClick="return ValueChanged();

I have a Save button on the same page i want to save the data when client click on OK button of cinfirm alert OK btn. Means my onclick="btnsave_Click" event also call and my data get save on confirm message OK btn.

<asp:Button runat="server" ID="btnOK" Text="OK" OnClientClick="Javascript:return confirm('Your Message')" OnClick="btnOK_Click" />

I have change something in javascript

if (document.getElementById("<%=isDirty.ClientID %>").value != document.getElementById("<%=txbText.ClientID %>").value) 
{ 
  var ssave = window.confirm('Your changes are not saved. Do you want to save your changes before you exit.') 
if (ssave == true) 
{ 
  document.getElementById('<%=btnSave.ClientID%>').click(); 
  return false; 
} 
  else return true; 
}