This is exactly correct. Backwards compatability is a bitch. Additionally, you need to account for Microsoft scale approaches. There are two Javascript engines

This is exactly correct. Backwards compatability is a bitch. Additionally, you need to account for Microsoft scale approaches. There are two Javascript engines (that I know of) at Microsoft: IE's implementation and Managed JScript [1] which is built on the DLR [2]. The DLR is a set of extensions to the in-development .NET CLR 4.0 to support Managed JScript, IronPython, and IronRuby. The DLR is the place for all these fancy pants modern optimizations that Google, Mozilla, and Apple are implementing. In fact, the DLR already has many of them. I believe that Silverlight 2.0 (now released) has some version of the DLR in it and .NET 4.0 is coming sooner or later. Who knows if or when IE will ever use Managed JScript or the DLR. Remember: Microsoft is full of plenty of smart, non-evil engineers. You can try to call it ignorance, but really it is bloat, process, and scope. It's almost never malice ;-) [1] http://blogs.msdn.com/jscript/archive/2007/05/04/managed-jscript-announced.aspx http://blogs.msdn.com/jscript/archive/2007/05/04/managed-jsc... [2] http://en.wikipedia.org/wiki/Dynamic_Language_Runtime http://en.wikipedia.org/wiki/Dynamic_Language_Runtime