fetch('http://localhost:8080/login', {
        method: 'POST',
        mode:'cors',
        headers: {
            'Accept': 'application/json',
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({
            username: this.state.username,
            password: this.state.password
        })
    });

var responseHeaders = {
    "access-control-allow-origin": "*",
    "access-control-allow-methods": "GET, POST, PUT, DELETE, OPTIONS",
    "access-control-allow-headers": "content-type, accept",
    "access-control-max-age": 10,
    "Content-Type": "application/json"
};

app.post('/login', function(req, res, next) {
    if (req.method == "OPTIONS") {
        res.writeHead(statusCode, responseHeaders);
        res.end();
    }
    console.log("hello");
   ...

OPTIONS /login 200 8.570 ms - 4