How to alert user before leaving the page

onunload (or onbeforeunload) will prevent user from navigating to another page.

Use the below JavaScript code

window.onbeforeunload = function()

{ 

 return 'Are you sure you want to leave?'; 

};

Or with jQuery:

$(window).bind('beforeunload', function()

{ 

 return 'Are you sure you want to leave?'; 

});