User-Agent returning null value

I'm using node.js with React and am trying to use fetch function to call a data from a server with CORS support.

However when I'm trying to set custom User-Agent it's returning a null value.

fetchData(){
    let url = 'http://localhost:8080/testData';

    var headers = new Headers();
    headers.set('user-agent', 'test_UA');

    var options = {
        method: 'GET',
        mode: 'cors',
        headers: headers
    };

    var request = new Request(url, options);
    console.log(request.headers.get('User-Agent'));
}

Is there something I'm doing wrong?

via Aesis