Internet Explorer 9 - Changes From Previous Versions - Scripting - JavaScript Engine

JavaScript Engine

Internet Explorer 9 (32-bit) features a faster JavaScript engine than Internet Explorer 8's, internally known as Chakra. Chakra has a separate background thread for compiling JavaScript. Windows runs that thread in parallel on a separate core when one is available. Compiling in the background enables users to keep interacting with webpages while Internet Explorer 9 generates even faster code. By running separately in the background, this process can take advantage of modern multi-core machines.

In Microsoft's preliminary SunSpider benchmarks for the third 32-bit Internet Explorer 9 Platform Preview, it outperformed the Internet Explorer 8 engine by a factor of 10 and also outperformed the newest Firefox 4.0 pre-release. Microsoft provided information that its new javascript engine uses dead code elimination optimization for faster performance, which included a small section of code in the SunSpider test as dead code. Robert Sayre, a Mozilla developer investigated this further, showing that Internet Explorer 9's preview 3 dead code elimination had bugs, providing test cases exposing these bugs resulting in wrong compilation.

After its final release, 32-bit Internet Explorer 9 has been tested to be the leading mainstream browser in the Sunspider performance test.

The engine significantly improves support for ECMA-262: ECMAScript Language Specification standard, including features new to the recently finalized Fifth Edition of ECMA-262 (often abbreviated ES5). The Internet Explorer 9 browser release scored only 3 faults from 10440 tests in the Test262 Ecmascript conformace test (Ver. 0.6.2 5-Apr-2011) created by Ecma International.

The 64-bit version of Internet Explorer 9, which is not the default browser even on 64-bit systems, does not have the JIT compiler and performs up to 4 times slower.