During Beta1 and Beta2 pre-releases of IE8, we’ve blogged about the performance optimizations done in the Script engine and the addition of new language features such as native JSON support. We also provided details about the JScript Debugger and the JScript Profiler –that shipped as part IE8 Developer Tools
One of the big pieces of feedback during our beta cycles was for compatibility, which, for JScript, meant focusing on how we version some of the language features we’re adding. As a result, version 5.8 of the JScript engine (shipped as a part of IE8) introduced an opt-in versioning mechanism for all the new and breaking language features. The JScript engine now exposes new language features only when the layout (document) mode is set to “IE8 Standards Mode.” Versioned JScript language features available in this release include native JSON support and accessor methods for enabling Document Object Model prototypes.
In addition to addressing bugs that were reported during the betas and our internal testing, we have added some user requested changes in the developer tools. A more seamless debugging experience, profiling multi-frame web pages and searching the profile reports using the search bar are some of the cool features we have added. We think developer tools will make life a whole lot easier for you web developers out there – we would love to hear your feedback!
As outlined in the article I wrote in the Code Focus Magazine during beta2, we have made more targeted performance improvements in the JScript engine and the overall AJAX browser stack to improve end-to-end performance of real world applications. On typical AJAX applications such as GMail, commonly used operations have improved over 40% over IE7.
I would like to thank all of you for your support and feedback through the IE8 product development cycle and hope that you like the JScript enhancements done in IE8.
Product Unit Manager, JScript
Update 3/25/09 - updating JScript Debugger link