$.extend is primarily intended as a cross-browser method for our own internal use in copying properties from *plain objects*, although we do expose it for external code that might find it useful.

We have no plans to add features that we do not need internally, since that often leads to performance and size penalties. Feel free to create and release a plugin that implements the features you need.

But the documentation says that it "merge the contents", and contents can be understood as description too.

Another example is that when doing this:

$.extend(myVar, []);

myVar won't get the length property from the array, because it is not enumerable.

Just to clarify, your references are slightly inaccurate; JavsScript 1.8.5 is an irrelevant version concept that refers only to Mozilla's JavaScript implementation of EcmaScript. Object.defineProperty was introduced in ES5, but jQuery supports browsers that are barely ES3 compatible - introducing many of the great ES5 and 5.1 features is not realistic for jQuery from a maintenance and sanity perspective. Creating two code paths to allow for two different behaviours that are platform dependent would be absolute chaos.

All of that aside, there is no standard by which extend behaviour is explicitly defined and jQuery has _never_ claimed that it intends to be in parity with ES5