Javascript – How to capture browser close event

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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;
  }
};