title: How to get progress from Fetch API?
body: <p>I'm using  <code>$.ajax</code>,i can do that:</p>
  
  <pre><code>var xhr = new window.XMLHttpRequest();
  //Upload progress
  xhr.upload.addEventListener("progress", function(evt){
    if (evt.lengthComputable) {
      var percentComplete = evt.loaded / evt.total;
      //Do something with upload progress
      console.log(percentComplete);
    }
  }, false);
  </code></pre>
  
  <p>But,when i use fetch api,what should i do?</p>
  
  <pre><code>var fData = new FormData();
  fData.append("userfile",document.getElementById("file").files[0]);
  fetch("/", {method:"POST",body:fData}).then(function(res){});
  </code></pre>