var user ={
    username: "some",
    password: "a"
}

router.post('/login', authenticate, function (req, res) {
    //if it passes the middleware, send back the user

    var token = jwt.sign({
        username: user.username
    }, jwtSecret);
    res.send({
         token: token,
        user: user
    });
});

function authenticate(req, res, next) {
    var body = req.body;
    var username = body.username, password = body.password;

    //if nothing is sent
    if(!username || !password){
        res.status(400).end('Must send a user and pass');
    }

    //if incorrect credentials are sent
    if(username !== user.username || password !== user.password){
        res.status(401).end("Incorrect credentials");
    }

    //if it reaches here, it means credentials are correct
    next();
}

POST /apis/auth/login 401 0.841 ms - -
Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:346:11)
at ServerResponse.header     (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/response.js:718:10)
at ServerResponse.json (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/response.js:246:10)
at ServerResponse.send (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/response.js:151:21)
at /home/vivek/dev/qwiksplit/jsback/app.js:81:9
at Layer.handle_error (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/router/layer.js:71:5)
at trim_prefix (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/router/index.js:310:13)
at /home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/router/index.js:280:7
at Function.process_params (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/router/index.js:330:12)
at next (/home/vivek/dev/qwiksplit/jsback/node_modules/express/lib/router/index.js:271:10)