implement ajax post by pure js

pure_js_ajax_post.js

var data = [
    {'key': 'username', 'value': 'shellbye'},
    {'key': 'password', 'value': 'password1'},
    {'key': 'age', 'value': '26'},
    {'key': 'key_x', 'value': "value_x"},
    {'key': 'key_y', 'value': "value_y"}
];
function js_ajax(url, data) {
  var myForm = new FormData();
  for (var i = 0; i < data.length; i++) {
      myForm.append(data[i].key, data[i].value);
  }
  var xmlHttp = new XMLHttpRequest();
  xmlHttp.open("post", url);
  xmlHttp.send(myForm);
  xmlHttp.onreadystatechange = function () {
      if (xmlHttp.readyState == 4 && xmlHttp.status == 200) {
          console.log("js_ajax_done");
      }
  };