Ajax request with Vanilla JS

ajax.js

function ajax(type, url) {
  var request = new XMLHttpRequest();
  request.open(type, url, true);

  request.onload = function() {
    if (this.status >= 200 && this.status < 400) {
      //succes.
      var resp = this.response;
    }
    else {
      //server error.
    }
  };

  request.onerror = function() {
    //error.
  };

  request.send();
}