tiny xhr constructor

xhr.js

const xhr = new window['XMLHttpRequest' || 'ActiveXObject']('MSXML2.XMLHTTP.3.0');
let res;

xhr.open('GET', 'some/api/endpoint');
xhr.responseType = 'json';
xhr.onreadystatechange = function () {
  if (this.status === 200 && this.readyState === 4) {
    resp = this.response;
  }
}