How to call a function before leaving page with Javascript

You can always call your function before leaving the page.

function myfun(){
     // Write your business logic here
     console.log('hello');
}

onbeforeunload:

window.onbeforeunload = function(){
  myfun();
  return 'Are you sure you want to leave?';
};

Or with jQuery:

$(window).bind('beforeunload', function(){
  myfun();
  return 'Are you sure you want to leave?';
});

This will just ask the user if they want to leave the page or not, you cannot redirect them if they select to stay on the page. If they select to leave, the browser will go where they told it to go.

You can use onunload to do stuff before the page is unloaded, but you cannot redirect from there (Chrome 14+ blocks alerts inside onunload):

window.onunload = function() {
    myfun();
    alert('Bye.');
}

Or with jQuery:

$(window).unload(function(){
  myfun();
  alert('Bye.');
});