The JavaScript Engine in IE7 Vs. IE8
Every new browser release, like a new operating system, garners both criticism and praise. Internet Explorer 8, released in 2008, is no exception. The increased performance of its JavaScript interpretation engine was notable, boosting many JavaScript operations by as much as 400 percent, but compatibility issues with older websites, and the browser's still lagging performance compared to competitors have left it open to criticism.
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IE8 Improvements over IE7
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According to Microsoft engineers, JavaScript engine performance was the number one concern with the original beta release of IE8. String concatenation, the process of combining small bits of text, is "many times faster" in IE8. IE8s Jscript implementation is closer to the industry standard (ECMAScript 3 or ES3) than previous versions were. In addition, Microsoft claims to have closed a number of memory leaks which can, over time, degrade the performance of the Web browser during a browsing session.
Compatibility Mode
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To achieve these performance gains, the JavaScript engine changed radically enough between IE7 and IE8 that Microsoft had to introduce a "compatibility mode" in its new browser for Web pages which, while perfectly functional in IE7, refused to work in IE8. With compatibility mode on, pages load as they would in IE7: more slowly but without breaking older sites.
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Alternatives (Chrome, FireFox)
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According to most benchmarks, IE8s JavaScript engine speed, despite improvements over IE8 is still much slower than rival Web browsers Chrome version 8 and FireFox 3.1. Both of these browsers radically altered the way that JavaScript was interpreted within the browser and as a result achieved significant performance gains.
IE9 and IE10
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The most recent editions of Internet Explorer have great performance gains and most benchmarking studies of the major browsers acknowledge that IE9 is a serious competitor when it comes to JavaScript engine performance. Some commentators have suggested that JavaScript engine performance has reached a level of optimization where differences among the top performing browsers are so small that they hardly matter anymore. IE10, currently available to developers in a preview edition, finally outperforms FireFox and Chrome in a number of benchmarks, and more importantly is the most HTML5-compatible browser around.
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References
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