Async AJAX call to node server returns empty response

I tried searching around the web but couldn't fix my problem with the answers.

I have two node js servers running on my localhost.

Backend: localhost:9000

Frontend: localhost:3000

I have a jquery Ajax call in my frontend server which looks like this:

$(document).ready(function() {
  $.ajax({
  url: "http://localhost:9000/merchant",
  type: "post",
  data: {content: "{"+
          '"0": {"name":"kfc", "desc":"great chicken", "web":"www.kfc.com"},'+
          '"1": {"name":"pizzaexpress", "desc":"your favourite pizza express", "web":"www.pizzaexpress.com"},'+
          '"2": {"name":"indianmasala", "desc":"great indian food", "web":"www.masala.com"},'+
          '"3": {"name":"starbucks", "desc":"all coffeee", "web":"www.starks.com"}'+
      "}"}
}).done(function(data) {
  alert(data);
});
});

Yes, I post a JSON in content parameter. Which is getting stored in my mongoDB and should be returning the inserted ids.

My backend app.post function looks something like this:

var content = JSON.stringify(req.body.content);
    content = JSON.parse(req.body.content);
    var ids = new Array();
    for (var key in content) {
        if (content.hasOwnProperty(key)) {

            var merchant = new Merchant();
            merchant.name = content[key].name;
            merchant.desc = content[key].desc;
            merchant.web = content[key].web;
            merchant.save();
            ids[key] = merchant._id;

        }
    }    
    var response = {"status":"Request processed successfully", "id": ids};
console.log(JSON.stringify(response));
    res.contentType('json');
    res.send(JSON.stringify(response));

When I load the frontend localhost:3000 on my browser the AJAX calls the localhost:9000/merchant which then stores the merchant information in mongodb but neither

res.send();

nor

res.write();

send the response. I checked the console.log for the JSON.stringify(response) which looks fine.

My response is empty when I see it in firebug.

Not sure what going wrong. Much appreciate if you lot give me an answer.

Many thanks.