MongoDB Documentation, Release 3.0.4 Chapter 12. Release Notes 886 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. • standardized JSON$^{795}$, • strict mode$^{796}$, • function.bind()$^{797}$, • array extensions$^{798}$, and • getters and setters. • 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. • XML() • Namespace() • QName() You need to use alternative XML processing if you used any of the following constructors/methods: E4X Extensions V8 does not support the non-standard E4X 804 extensions. E4X provides a native XML 805 object to the JavaScript language and adds the syntax for embedding literal XML documents in JavaScript code. For objects that have not been returned from MongoDB queries, the features work as expected. See SERVER-8216$^{799}$, SERVER-8223$^{800}$, SERVER-8215$^{801}$, and SERVER-8214 802 for more information. Removed Non-Standard SpiderMonkey Features V8 does not support the following non-standard SpiderMon- key 803 JavaScript extensions, previously supported by MongoDB’s use of SpiderMonkey as its JavaScript engine. Note: The following features do not work as expected on documents returned from MongoDB queries : With V8, MongoDB supports the ES5 implementation of Javascript with the following exceptions. Modernized JavaScript Implementation (ES5) The 5th edition of ECMAscript$^{794}$, abbreviated as ES5, adds many new language features, including: $^{794}$http://www.ecma-international.org/publications/standards/Ecma-262.htm $^{795}$http://www.ecma-international.org/ecma-262/5.1/#sec-15.12.1 $^{796}$http://www.ecma-international.org/ecma-262/5.1/#sec-4.2.2 $^{797}$http://www.ecma-international.org/ecma-262/5.1/#sec-15.3.4.5 $^{798}$http://www.ecma-international.org/ecma-262/5.1/#sec-15.4.4.16 $^{799}$https://jira.mongodb.org/browse/SERVER-8216 $^{800}$https://jira.mongodb.org/browse/SERVER-8223 $^{801}$https://jira.mongodb.org/browse/SERVER-8215 $^{802}$https://jira.mongodb.org/browse/SERVER-8214 $^{803}$https://developer.mozilla.org/en-US/docs/SpiderMonkey $^{804}$https://developer.mozilla.org/en-US/docs/E4X $^{805}$https://developer.mozilla.org/en-US/docs/E4X/Processing_XML_with_E4X