diff -r 000000000000 -r 6474c204b198 toolkit/components/osfile/tests/xpcshell/test_osfile_async_bytes.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/osfile/tests/xpcshell/test_osfile_async_bytes.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,44 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Task.jsm");
+
+function run_test() {
+  do_test_pending();
+  run_next_test();
+}
+
+/**
+ * Test to ensure that {bytes:} in options to |readTo| and |write| are correctly
+ * preserved.
+ */
+add_task(function* test_bytes() {
+  let path = OS.Path.join(OS.Constants.Path.tmpDir,
+                          "test_osfile_async_bytes.tmp");
+  let file = yield OS.File.open(path, {trunc: true, read: true, write: true});
+  try {
+    try {
+      // 1. Test write, by supplying {bytes:} options smaller than the actual
+      // buffer.
+      yield file.write(new Uint8Array(2048), {bytes: 1024});
+      do_check_eq((yield file.stat()).size, 1024);
+
+      // 2. Test same for |readTo|.
+      yield file.setPosition(0, OS.File.POS_START);
+      let read = yield file.readTo(new Uint8Array(1024), {bytes: 512});
+      do_check_eq(read, 512);
+
+      // 3. Test that passing nullish values for |options| still works.
+      yield file.setPosition(0, OS.File.POS_END);
+      yield file.write(new Uint8Array(1024), null);
+      yield file.write(new Uint8Array(1024), undefined);
+      do_check_eq((yield file.stat()).size, 3072);
+    } finally {
+      yield file.close();
+    }
+  } finally {
+    yield OS.File.remove(path);
+  }
+});
+
+add_task(do_test_finished);