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Use FileStreamer when you need to work on sequential slices of a file with the HTML5 File API.
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Use FileStreamer when you need to work on sequential slices of a file with the HTML5 File API. By working on slices, you can work on a large file without loading the entire file into memory at once.


Here is a contrived example of calculating the length of a file:

var someFile = document.getElementById('fileInput').files[0];

var fileLength = 0;
var myWorkFunc = function (event, eof) {
  binaryString =;
  fileLength += binaryString.length;

  if (eof) {
    alert('The length of this file in bytes is: ' + fileLength);

var fs = new FileStreamer(4 * 1024 * 1024);
fs.streamAsBinaryString(someFile, myWorkFunc);

This is a contrived example since you could just use someFile.size, but it demonstrates the basic usage.

You can find more examples, such as calculating a SHA1 hash on a local file, in the examples directory.


To construct a FileStreamer, specify the desired slice size in bytes. For example:

var fs = new FileStreamer(4 * 1024 * 1024); // 4MB slice size

Since the File API is relatively new and implementations vary across browsers, I can't tell you what slice size to use. Large slice sizes will need more memory, while smaller slice sizes will use more CPU.

Stream functions

The available stream functions correspond to each of the read functions in the FileReader API. Each function accepts the same arguments as its corresponding FileReader function, with the addition of a callback.

  • streamAsArrayBuffer(file, callback)
  • streamAsBinaryString(file, callback)
  • streamAsDataURL(file, callback)
  • streamAsText(file, encoding, callback)

Work function

The work function (callback) receives two arguments:

  1. event: the ProgressEvent from FileReader.onloadend.
  2. eof: true if this is the last slice in the file, otherwise false.

The callback is guaranteed to be called sequentially for each file slice. If the callback returns false, streaming will stop. Otherwise, streaming will continue to the next slice.

Using FileReaderSync

By default, FileStreamer uses a FileReader under the covers. In some cases, you may want to use a FileReaderSync instead. To do this, pass true for the second argument to the constructor:

var fs = new FileStreamer(4 * 1024 * 1024, true);

When using a FileReaderSync, your work function will receive these arguments:

  1. result: the data read from the FileReaderSync.
  2. eof: same as with normal usage.

None of the other API's change. Though a FileReaderSync will be used under the covers, FileStreamer itself will not become synchronous; you must still supply a callback.

The usual use case for this is when using FileStreamer in a web worker. As of Firefox 9, the FileReaderSync interface is available to a web worker, but the FileReader is not.

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