Описание тега onbeforeunload
The onbeforeunload event is triggered before a user navigates away. This event listener can be used to show a confirmation dialog to the user on page leave.
The onbeforeunload event is triggered before a user navigates away. This event listener can be used to show a confirmation dialog to the user on page leave.
In most browsers, the return value of the onbeforeunload event is displayed in the dialog, together with a default message. Since Firefox 4, this value is not displayed any more.
Example
window.onbeforeunload = function(e) {
return 'Dialog text here.';
};