Downloading a large file with window XHR so that we can follow redirects/proxies/etc. How to let the user cancel the download?

cancelableDownload.coffee

downloadChunk = (url, start, end) ->
  xhr = new window.XMLHttpRequest()
  deferred = Q.defer()
  
  xhr.open "GET", url, true
  xhr.setRequestHeader "range", "bytes=" + start + '-' + end
  
  xhr.onload = ->
    if @status == 200
      defer.resolve @response
    else
      defer.reject new Error "Status code: " + @status
      
  xhr.onerror = (err) ->
    defer.reject err
    
  do xhr.send
  
  # Maybe something like this?
  deferred.promise.cancel = ->
    do xhr.abort
    deferred.reject new Error "Download Cancelled"
  
  deferred.promise
  
util.inherits(XHRBinaryStream, Readable)
XHRBinaryStream = (url) ->
  Readable.call this
  @index = 0
  @chunkSize = 1024 * 1024 * 5 # 5 MB
  @_url = url
  
XHRBinaryStream::_read = (size) ->
  chunkSize = size? or @chunkSize
  @currentChunkPromise = downloadChunk(@targetUrl, @bytesIndex, @bytesIndex + chunkSize)
  @currentChunkPromise.then (data) ->
    @bytesIndex += chunkSize
    @push data
    
XHRBinaryStream::cancel = ->
  do @currentChunkPromise.cancel
  @push null
    
getToDisk = (url, dest) ->
  binaryStream = new XHRBinaryStream url
  writeStream = fs.createWriteStream dest
  
  binaryStream.pipe(writeStream)
  
  cancel = ->
    do binaryStream.unpipe
    do binaryStream.cancel
    do writeStream.end
    
    fs.unlinkSync dest
    
canceller = getToDisk('http://www.google.com/robots.txt', 'foo')
setTimeout(canceller, 100);