How to image progress display % in javascript

<style>
  #progress_bar {
    margin: 10px 0;
    padding: 3px;
    border: 1px solid #000;
    font-size: 14px;
    clear: both;
    opacity: 0;
    -moz-transition: opacity 1s linear;
    -o-transition: opacity 1s linear;
    -webkit-transition: opacity 1s linear;
  }
  #progress_bar.loading {
    opacity: 1.0;
  }
  #progress_bar .percent {
    background-color: #99ccff;
    height: auto;
    width: 0;
  }
</style>

<input type="file" id="files" name="file" />
<button onclick="abortRead();">Cancel read</button>
<div id="progress_bar" style="width:100%;"><div class="percent">0%</div></div>

<script>
  var reader;
  var progress = document.querySelector('.percent');

  function abortRead() {
    reader.abort();
  }

  function errorHandler(evt) {
    switch(evt.target.error.code) {
      case evt.target.error.NOT_FOUND_ERR:
        alert('File Not Found!');
        break;
      case evt.target.error.NOT_READABLE_ERR:
        alert('File is not readable');
        break;
      case evt.target.error.ABORT_ERR:
        break; // noop
      default:
        alert('An error occurred reading this file.');
    };
  }

  function updateProgress(evt) {
    // evt is an ProgressEvent.
    if (evt.lengthComputable) {
      var percentLoaded = Math.round((evt.loaded / evt.total) * 100);
      // Increase the progress bar length.
      if (percentLoaded < 100) {
        progress.style.width = percentLoaded + '%';
        progress.textContent = percentLoaded + '%';
      }
    }
  }

  function handleFileSelect(evt) {
    // Reset progress indicator on new file selection.
    progress.style.width = '0%';
    progress.textContent = '0%';

    reader = new FileReader();
    reader.onerror = errorHandler;
    reader.onprogress = updateProgress;
    reader.onabort = function(e) {
      alert('File read cancelled');
    };
    reader.onloadstart = function(e) {
      document.getElementById('progress_bar').className = 'loading';
    };
    reader.onload = function(e) {
      // Ensure that the progress bar displays 100% at the end.
      progress.style.width = '30%';
      progress.textContent = '100%';
      setTimeout("document.getElementById('progress_bar').className='';", 2000);
    }

    // Read in the image file as a binary string.
    reader.readAsBinaryString(evt.target.files[0]);
  }

  document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>