var uploadReq = new Request(LOGIN_URL, {
        method: 'post',
        headers: new Headers({
            'Content-Type': 'application/json',
        }),
        body: JSON.stringify({
            username: this.state.username,
            password: this.state.password
        })

    });

    fetch(uploadReq)
        .then(this._requestHandler)
        .catch(function (err) {
            console.log('Fetch Error :-S', err);
        });

Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. The response had HTTP status code 400. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.