Check if the person will close the site

Question

Is there any way to check if the person’s mouse is going to close the site? With Javascript, etc... In the simplest case:

if(estaindofechar){
  alert("Tem certeza?");
}

Answer 1

This is the method called when the user tries to exit the page

window.onbeforeunload = function(e) {
    return confirm("Você tem certeza que deseja fechar a página?");
};