Saving a Typed Array ( ArrayBuffer ) to a file in Firefox Extension / Addon. What sucks: --1) FOS.write converts to a string (toString()) whatever buffer you pass and uses the returned primitive as the memory location of the buffer. --2) NetUtil.asyncCopy only accepts nsIInputStreams --3) Can't implement a nsIInputStream in javascript. --4) data…

buffer2File.js

function read2Buffer(from, to, maxN, offset){
	if(!from || !to || !maxN) throw "Invalid Arguments";
	from = from.buffer || from;
	if(from.byteLength) from = new Uint8Array(from);
	var tot = Math.min((from.length && (from.length - offset)) || (from.available && from.available()), maxN) + offset;
	for(var i = offset; i < tot; i++){
		to[i] = String.fromCharCode(from[i]);
	}
	return tot - offset;
};
//setup
var file = FileUtils.getFile("ProfD", ["myfile.data"]);
var v = new Uint8Array(THE_DATA);
var fos = FileUtils.openSafeFileOutputStream(file, FileUtils.MODE_WRONLY|FileUtils.MODE_CREATE|FileUtils.MODE_TRUNCATE);
var done = 2048, offset=0, buff = new Array(done);

//action - synchronous
while(done == 2048){
	offset+= (done = read2Buffer(v,buff,done,offset));
	fos.write(buff.join(""),done);
}
////fos.close() doesn't seem to work - actually, it deletes the file ~~??
//fos.flush(); //one can access the file via an inputstream at this point, but file will get canned by FF
FileUtils.closeSafeFileOutputStream(fos); //apparently, when you open with FU.oSFOS, you have to close with FU.cSFOS


//action - aynchronous
var fun = function(){
	if(done == 2048){
		offset+= (done = read2Buffer(v,buff,done,offset));
		fos.write(buff.join(""),done);
		setTimeout(fun,10); //arguments.callee, if you like
	}else fos.close();
};
fun();

Discovered a third write option: BinaryOutputStream. I knew about BIS so guess I should have figured there was a match for output. Anyways, BOS DOES accept arrays (or array accessible objects like Typed Arrays). So all the (non-setup code) above could be reduced to:

var bos = Components.classes["@mozilla.org/binaryoutputstream;1"].createInstance(Components.interfaces.nsIBinaryOutputStream);
bos.setOutputStream(fos);
bos.writeByteArray(v,v.length);
bos.close();