Cannot use alert() in a jQuery method

Cannot use alert() in a jQuery method

When using jQuery to make an AJAX call, for the time being I want to just
want to have a popup box (using alert()) showing me the response text.
$(document).ready(function() {
$(".jeobutton").mouseup(function() {
var $button = $(this);
$.ajax({ url: 'getdata.php',
data: // <parameters>
type: 'get',
dataType: 'json',
success: function(output) {
// do something
},
error: function(xhr) {
alert("<some error>");
console.error(xhr.responseText);
}
});
});
});
The response text prints out fine. However, the alert() dialog is nowhere
to be found.
Please help this poor noob.