Using promise():
function do_animations() {
var n = random_num(1, 5),
$rec = $('#rec');
for (var i = 0; i < n; i++) {
var width = random_num(-50, 50);
$rec.animate({'left': '+=' + width + 'px'}, 300);
}
return $rec.promise();
}
$(document).ready(function(){
do_animations().done(function(){ alert("All animations done!") });
});