Why does my http://localhost CORS origin not work?

I am stuck with this CORS problem, even though I set the server (nginx/node.js) with the appropriate headers.

I can see in Chrome Network pane -> Response Headers:

Access-Control-Allow-Origin:http://localhost

which should do the trick.

Here’s the code that I now use to test:

var xhr = new XMLHttpRequest();
xhr.onload = function() {
   console.log('xhr loaded');
};
xhr.open('GET', 'http://stackoverflow.com/');
xhr.send();

I get

XMLHttpRequest cannot load http://stackoverflow.com/. Origin http://localhost is not allowed by Access-Control-Allow-Origin.

I suspect it’s a problem in the client script and not server configuration…