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3.2-javascript.txt
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3.2-javascript.txt
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=================================
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/Mozilla/Projects/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.