xhr.js

function postJson(method, url, params) {
  let xhr = new XMLHttpRequest();
  xhr.open(method, url, true);
  xhr.setRequestHeader('Content-Type', 'application/json; charset=UTF-8');
  xhr.send(JSON.stringify(params));
  xhr.onreadystatechange = function() {
    let status;
    let data;
    if (xhr.readyState === 4) {
      status = xhr.status;
      if (status === 200) {
        data = JSON.parse(xhr.responseText);
        console.log("success", data);
      } else {
        console.log("failed", status);
      }
    }
  };
}