How to collect return value of onbeforeunload

I am displaying a warning message if user try to close window without saving the form.

window.onbeforeunload = askConfirm;

    function askConfirm()
    {
        // needToConfirm is set to true if any changes are there in the form
        if (needToConfirm)
        {
            return "Your unsaved data will be lost.";
        }       
    } 

    function call_this_if_user_clicks_on_cancel()
    {
       // Bla bla bla
       // After this user should remain on the same page.
    }  

Now I want to call another function when user clicks on Okay. And rest of the functionality should remain same. So how can I collect onbeforeunload return value and call another function ?

SOLUTION:

    needToConfirm = false; 
    window.onbeforeunload = askConfirm;
    window.onunload = unloadPage;
    isDelete = true;

    function unloadPage()
    {
        if(isDelete)
        {
           call_this_if_user_clicks_on_cancel() 
        }   
    }

    function askConfirm()
    {
        alert("onbeforeunload");
        if (needToConfirm)
        {
            // Message to be displayed in Warning.
            return "Your data will be lost.";
        } 
        else
        {
            isDelete = false;
        }   
    }