How to read a local file by chunks in JavaScript?

Say, I have a file that is 500 bytes in size on my local hard drive and I want to read first 100 bytes from it without loading the whole f开发者_如何学Cile into memory. How to accomplish that in JavaScript with the help of UniversalXPConnect? In Firefox only, of course.

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Assuming that you want to read ASCII text data (no character set conversion):

var file = Components.classes["@mozilla.org/file/local;1"]
                     .createInstance(Components.interfaces.nsILocalFile);
file.initWithPath("/foo/bar");
var fstream = Components.classes["@mozilla.org/network/file-input-stream;1"]
                        .createInstance(Components.interfaces.nsIFileInputStream);
fstream.init(file, -1, 0, 0);
var sstream = Components.classes["@mozilla.org/scriptableinputstream;1"]
                        .createInstance(Components.interfaces.nsIScriptableInputStream);
sstream.init(fstream);
var data = sstream.read(100);
sstream.close();

Further information: https://developer.mozilla.org/en/Code_snippets/File_I%2F%2FO