jQuerz keep checking for an element appearance

<script>
var waitForEl = function(selector, callback) {
if (jQuery('.advanced-order').length) {
callback();
} else {
setTimeout(function() {
waitForEl('.advanced-order', callback);
}, 100);
}
};

waitForEl('.advanced-order', function() {
$('<p>xxx</p>').insertAfter('.advanced-order .extra.step');
});
</script>