var data = null;

var xhr = new XMLHttpRequest();
xhr.withCredentials = true;

xhr.addEventListener('readystatechange', function() {
  if (this.readyState === this.DONE) {
    console.log(this.responseText);
  }
});

xhr.open('GET', 'http://mockbin.com/har');
xhr.setRequestHeader('accept', 'application/json');
xhr.setRequestHeader('x-foo', 'Bar');

xhr.send(data);