Confirm message before leaving site cross browser (JS)

Question

Want to make a confirm box appear before someone leave my site. They have information that will go lost if they leave and you don´t want that right. Pleas be specific with were I should put the code and so since I´m not the best of coders...

Answer 1

window.onbeforeunload = function(){
    return "Are you sure you wanna leave my site?";
}