gecko-dev/dom/workers/test/script_createFile.js

Cu.importGlobalProperties(["File"]);

addMessageListener("file.open", function (e) {
  var tmpFile = Cc["@mozilla.org/file/directory_service;1"]
                  .getService(Ci.nsIDirectoryService)
                  .QueryInterface(Ci.nsIProperties)
                  .get('TmpD', Ci.nsIFile)
  tmpFile.append('file.txt');
  tmpFile.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o600);

  File.createFromNsIFile(tmpFile).then(function(file) {
    sendAsyncMessage("file.opened", { data: file });
  });
});

addMessageListener("nonEmptyFile.open", function (e) {
  var tmpFile = Cc["@mozilla.org/file/directory_service;1"]
                  .getService(Ci.nsIDirectoryService)
                  .QueryInterface(Ci.nsIProperties)
                  .get('TmpD', Ci.nsIFile)
  tmpFile.append('file.txt');
  tmpFile.createUnique(Ci.nsIFile.NORMAL_FILE_TYPE, 0o600);

  var outStream = Cc["@mozilla.org/network/file-output-stream;1"].createInstance(Ci.nsIFileOutputStream);
  outStream.init(tmpFile, 0x02 | 0x08 | 0x20, // write, create, truncate
                 0o666, 0);
  var fileData = "Hello world!";
  outStream.write(fileData, fileData.length);
  outStream.close();


  File.createFromNsIFile(tmpFile).then(function(file) {
    sendAsyncMessage("nonEmptyFile.opened", { data: file });
  });
});