JavaScript Changes in MongoDB 2.4
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Consider the following impacts of V8 JavaScript Engine in MongoDB 2.4:
Tip
Use the new interpreterVersion()
method in the mongo
shell and the javascriptEngine
field in the output of db.serverBuildInfo()
to determine which JavaScript engine a MongoDB binary uses.
Improved Concurrency
Previously, MongoDB operations that required the JavaScript interpreter had to acquire a lock, and a single mongod
could only run a single JavaScript operation at a time. The switch to V8 improves concurrency by permitting multiple JavaScript operations to run at the same time.
Modernized JavaScript Implementation (ES5)
The 5th edition of ECMAscript , abbreviated as ES5, adds many new language features, including:
- standardized JSON ,
- strict mode ,
- function.bind() ,
- array extensions , and
- getters and setters.
With V8, MongoDB supports the ES5 implementation of Javascript with the following exceptions.
Note
The following features do not work as expected on documents returned from MongoDB queries:
Object.seal()
throws an exception on documents returned from MongoDB queries.Object.freeze()
throws an exception on documents returned from MongoDB queries.Object.preventExtensions()
incorrectly allows the addition of new properties on documents returned from MongoDB queries.enumerable
properties, when added to documents returned from MongoDB queries, are not saved during write operations.
See SERVER-8216 , SERVER-8223 , SERVER-8215 , and SERVER-8214 for more information.
For objects that have not been returned from MongoDB queries, the features work as expected.
Removed Non-Standard SpiderMonkey Features
V8 does not support the following non-standard SpiderMonkey JavaScript extensions, previously supported by MongoDB’s use of SpiderMonkey as its JavaScript engine.
E4X Extensions
V8 does not support the non-standard E4X extensions. E4X provides a native XML object to the JavaScript language and adds the syntax for embedding literal XML documents in JavaScript code.
You need to use alternative XML processing if you used any of the following constructors/methods:
XML()
Namespace()
QName()
XMLList()
isXMLName()
Destructuring Assignment
V8 does not support the non-standard destructuring assignments. Destructuring assignment “extract[s] data from arrays or objects using a syntax that mirrors the construction of array and object literals.” - Mozilla docs
Example
The following destructuring assignment is invalid with V8 and throws a SyntaxError
:
Iterator()
, StopIteration()
, and Generators
V8 does not support Iterator(), StopIteration(), and generators .
for each...in
Construct
V8 does not support the use of for each…in construct. Use for (var x in y)
construct instead.
Example
The following for each (var x in y)
construct is invalid with V8:
Instead, in version 2.4, you can use the for (var x in y)
construct:
You can also use the array instance method forEach()
with the ES5 method Object.keys()
:
Array Comprehension
V8 does not support Array comprehensions .
Use other methods such as the Array
instance methods map()
, filter()
, or forEach()
.
Example
With V8, the following array comprehension is invalid:
Instead, you can implement using the Array
instance method forEach()
and the ES5 method Object.keys()
:
Note
The new logic uses the Array
instance method forEach()
and not the generic method Array.forEach()
; V8 does not support Array
generic methods. See Array Generic Methods for more information.
Multiple Catch Blocks
V8 does not support multiple catch
blocks and will throw a SyntaxError
.
Example
The following multiple catch blocks is invalid with V8 and will throw "SyntaxError: Unexpected token if"
:
Conditional Function Definition
V8 will produce different outcomes than SpiderMonkey with conditional function definitions .
Example
The following conditional function definition produces different outcomes in SpiderMonkey versus V8:
With SpiderMonkey, the conditional function outputs undefined
, whereas with V8, the conditional function outputs function
.
If your code defines functions this way, it is highly recommended that you refactor the code. The following example refactors the conditional function definition to work in both SpiderMonkey and V8.