If you have a File object (say, one stored using the FileSystem API), it’s possible to seek into it and read chunks without reading the entire file into memory:

var url = "filesystem:http://example.com/temporary/myfile.zip";

window.webkitResolveLocalFileSystemURL(url, function(fileEntry) {
  fileEntry.file(function(file) {
    var reader = new FileReader();

    reader.onload = function(e) {
      var ab = e.target.result; // arrayBuffer containing bytes 0-10 of file.
      var uInt8Arr = new Uint8Array(ab);
      ...
    };

    var blob = file.webkitSlice(0, 10, "application/zip");  // mimetype is optional
    reader.readAsArrayBuffer(blob);
  }, errorHandler);
}, errorHandler);
file access offline code filesystem

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