Note: post data in node/express failing

I've updated the question based on the comments below:

I don't know where I'm going wrong.

Client side, I send the "user" information.

$.ajax({
    type: "POST",
    url: "/user",
    data: JSON.stringify({ "user": "John"}),
    contentType : 'application/json',
    dataType: "json"
})
    .done(function() {
        alert( "done");
    })
    .fail(function() {
        alert( "fail");
    })

On the server side,

app.use(bodyParser.json()); 

app.post('/user', function (req, res) {
    console.log(req.body.user);
    res.end();
});

This results in the console.log correctly recording "John", but on the client side I still get a "fail" as an alert. Can't figure out why.

I'm using latest versions of node, express and jquery.

Thanks.

Update:

This was missing:

res.contentType('json');
res.send({ some: JSON.stringify({response:'json'}) });