ajax.js

const ajax = (url, {method="GET", body=null, headers={}, mode}, onProgress=()=>{}, onStartProgress=()=>{}, onStopProgress=()=>{}) => {
  return new Promise((result, reject) => {
      const ajax = new XMLHttpRequest();
      ajax.open(method, url, true);

      for(let [key, value] of headers)
          ajax.setRequestHeader(key, value);

      if(mode==='cors')
          ajax.withCredentials = false;

      ajax.onload = () => result(ajax.responseText);
      ajax.onerror = reject;
      ajax.onloadstart = onStartProgress;
      ajax.onloadend = onStopProgress;
      ajax.onprogress = onProgress;
      ajax.send(body);
  });
},