=================================
JavaScript Changes in MongoDB 3.2
=================================

.. default-domain:: mongodb

.. contents:: On this page
   :local:
   :backlinks: none
   :depth: 1
   :class: singlecol

In MongoDB 3.2, the javascript engine used for both the :binary:`~bin.mongo` shell
and for server-side javascript in :binary:`~bin.mongod` changed from V8 to
`SpiderMonkey <https://developer.mozilla.org/en-US/docs/SpiderMonkey>`_.

To confirm which JavaScript engine you are using, you can use either
:js:func:`interpreterVersion()` method in the :binary:`~bin.mongo` shell and the
:data:`~buildInfo.javascriptEngine` field in the output of
:method:`db.serverBuildInfo()`

In MongoDB 3.2, this will appear as ``MozJS-38`` and ``mozjs``, respectively.

Modernized JavaScript Implementation (ES6)
------------------------------------------

SpiderMonkey brings with it increased support for features defined in the 6th
edition of `ECMAScript
<http://www.ecma-international.org/ecma-262/6.0/index.html>`_,
abbreviated as ES6. ES6 adds many new language features, including:

- `arrow functions
  <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-arrow-function-definitions>`_,

- `destructuring assignment
  <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-destructuring-assignment>`_,

- `for-of loops
  <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-for-in-and-for-of-statements>`_, and

- `generators
  <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-generator-function-definitions>`_.

Changes to the ``mongo`` Shell
------------------------------

MongoDB 3.2 will return JavaScript and BSON
:bsontype:`undefined <data_undefined>` values intact if saved into a collection.
Previously, the :binary:`~bin.mongo` shell would convert ``undefined`` values into
``null``.

MongoDB 3.2 also adds the :parameter:`disableJavaScriptJIT` parameter to
:binary:`~bin.mongod`, which allows you to disable the JavaScript engine's JIT
acceleration. The :binary:`~bin.mongo` shell has a corresponding
:option:`--disableJavaScriptJIT <mongo --disableJavaScriptJIT>` flag.

Removed Non-Standard V8 Features
--------------------------------

SpiderMonkey does **not** allow the non-standard ``Error.captureStackTrace()``
function that prior versions of MongoDB supported. If you must record stack
traces, you can capture the ``Error().stack`` property as a workaround.