How to wirite a Node.js express server to receive curl POST

My curl post message is as follow:

curl -X POST -H "application/json" -d '{"u_username":"a"}' http://localhost:3000/save

My node.js server function are as follow:

router.post('/save', function (req, res) {
     console.log(req.body);
});

what I expect console log is {u_username:a}, but I actually get { '\'{u_username:a}\'': '' }

Is anybody I tell me why?

I am a starter to learn node.js