Whatever you call it, it's supposed to implement the ECMA-2621 standard ("ECMAScript", as if we really needed yet another annoying mixed case name for the same thing). Netscape's current version, 1.5, comes pretty close, with a few proprietary extensions (e.g. the const keyword). JScript is, um, "getting there", as they say. The current version as of January 2001 is 5.6, I believe. 5.5 "introduced" a number of member functions for the Array object2 that should've been there years ago, like push(), pop(), shift(), and unshift() (I'll have find some way to check this, but if I recall corectly the interpreter shipped with IE has been up to speed for a longer time). Since their broken earlier versions don't properly support the delete operator for Array, implementing pop() or shift() on your own involves monstrously inefficient and ugly code. At least you can do it, though, thanks to JS's weird object model. You just can't count on having those functions around with Microsoft's interpreter, is the thing. There are some other inconsistencies and missing features as well. No implementation is perfect, but really, this one has been a mess for most of its history. On the other hand, their associative array lookup is an order of magnitude or so faster than Netscape's (unless that's been fixed in NS JS 1.5; I haven't checked with that version). You pays your money and you takes your choice.
Nowadays (still January 2002) Microsoft's so-called "JScript" interpreter is implemented as a COM object which fits into the Windows Scripting Host. This is much less exciting than it sounds. Mostly it means that the teenage h4x0r d00dz of the world could be writing their so-called "email viruses" in a much more powerful and expressive language than VBScript, if only they weren't teenage h4x0r d00dz. Hurrah.