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.

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