Javascript – How to capture browser close event


I want to capture the browser close event in my application and show a confirm box to user.
I am using JSF 2.0 and richfaces 4.0.

Best Solution

window.onbeforeunload = function () 
  var shallIAlertUser = Do_Whatever(); //get boolen value
  if (shallIAlertUser) {
    //this will alert user
    return 'Are you sure?';
  else {
    //this wont
    window.onbeforeunload = undefined;