JavaScript is officially managed by Mozilla Foundation, and new language features are added periodically. However, only some non-Mozilla JavaScript engines support these new features:
property getter and setter functions (also supported by WebKit, Opera,[24] ActionScript, and Rhino)[25]
conditional catch clauses
iterator protocol adopted from Python
shallow generators/coroutines also adopted from Python
array comprehensions and generator expressions also adopted from Python
proper block scope via new let keyword
array and object destructuring (limited form of pattern matching)
concise function expressions (function(args) expr)
ECMAScript for XML (E4X), an extension that adds native XML support to ECMAScript
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