How to use the bitfield.prototype function in bitfield

function renderLoadingBar (state) {
  var torrentSummary = getPlayingTorrentSummary(state)
  if (!torrentSummary.progress) {
    return []
  }

  // Find all contiguous parts of the torrent which are loaded
  var prog = torrentSummary.progress
  var fileProg = prog.files[state.playing.fileIndex]
  var parts = []
  var lastPiecePresent = false
  for (var i = fileProg.startPiece; i <= fileProg.endPiece; i++) {
    var partPresent = Bitfield.prototype.get.call(prog.bitfield, i)
    if (partPresent && !lastPiecePresent) {
      parts.push({start: i - fileProg.startPiece, count: 1})
    } else if (partPresent) {
      parts[parts.length - 1].count++
    }
    lastPiecePresent = partPresent
  }

  // Output some bars to show which parts of the file are loaded
  return hx`
    <div class="loading-bar">
      ${parts.map(function (part) {
        var style = {
          left: (100 * part.start / fileProg.numPieces) + '%',
          width: (100 * part.count / fileProg.numPieces) + '%'
        }</div>