Its really annoying for a site visitor to come across errors whether be it php or javascript error. Drupal by default shows an error using javascript alert() function if there is a problem during Drupal ajax request.
The alert text is something like
An AJAX HTTP request terminated abnormally. Debugging information follows. Path: /system/ajax
Good thing is we can disable it by using Drupal javascript behavior and instead of showing an alert, it will write the error message to the browser console.
(function($) { Drupal.behaviors.disableAlerts = { attach: function(context, settings) { window.alert = function(text) { //override alert() function if(typeof console != "undefined") { console.error('Error: ' + text); } return true; }; } }; })(jQuery);