Как сохранить данные на локальном hd с помощью javascript?

function saveData() {
    var xhr = new XMLHttpRequest();
    //set a timeout, in ms, to see if we're still connected
    xhr.timeout = 2000;
    xhr.addEventListener('timeout', localStore, false);
    xhr.onreadystatechange = function() {
      if (xhr.readyState == 4) {
        // not a good news
        if (xhr.status !== 200) {
          localStore();
        }
        else{
          document.querySelector('#local_alert').style.display = 'none';
        }
      }
    }
    //I assume you already have the part where you set the credentials
    xhr.open('POST', 'your/url.php');
    xhr.send();
  }

  //Show the link + Store the text in localStorage
  function localStore() {
    document.querySelector('#local_alert').style.display = 'block';
    var userText = document.querySelector('textArea').value;
    localStorage.setItem("myAwesomeTextEditor", userText);
  }

  //provide a link to download a file with txt content
  window.URL = window.URL || window.webkitURL;
  function clickSave(e) {
    var userText = document.querySelector('textArea').value;
    var blob = new Blob([userText], {type: 'plain/text'});
    var a = document.createElement('a');
    a.download = "myAwesomeTextEditor" + (new Date).getTime() + '.txt';
    a.href = URL.createObjectURL(blob);
    a.style.display = "none";
    document.body.appendChild(a);
    a.click();
    document.body.removeChild(a);
    }

  setInterval(saveData, 3000);

#local_alert {
  display: none;
  position: static;
  width: 100%;
  height: 3em;
  background-color: #AAA;
  color: #FFF;
  padding: 0.5em;
}
body,
html {
  margin: 0
}

<div id="local_alert">You're actually offline, please beware your draft is not saved on our server
  <button onclick="clickSave()">Save Now</button>
</div>

<textarea></textarea>