Write to existing file file api?

Question

JavaScript

There is such code in the example

chrome.fileSystem.getWritableEntry(chosenFileEntry, function(writableFileEntry) {
    writableFileEntry.createWriter(function(writer) {
      writer.onerror = errorHandler;
      writer.onwriteend = callback;

    chosenFileEntry.file(function(file) {
      writer.write(file);
    });
  }, errorHandler);
});

here is the link https://developer.chrome.com/apps/app_storage
the code should seem to write new information to a file, but how can I not understand where to write new information?