[robertlagrant]

And positive indices not as possible keys, but as array elements. It's crazily inconsistent.

  > l = [1,2,3]
  (3) [1, 2, 3]
  > l[3] = 4
  4
  > l
  (4) [1, 2, 3, 4]
  > l[-1] = "eek"
  "eek"
  > l
  (4) [1, 2, 3, 4, -1: "eek"]

So adding a 4 as a "key" is a proper array element; adding -1 is an object key. I'm struggling to see how anyone could think this is a good idea. Saying that they're objects is not reasonable. Objects don't need to do this (in most languages lists are objects, and they don't also do key-value pairs at the same time); they just decided to make them do this.

[qsort]

It's extremely consistent. I'd say that's the only consistent way to do it, if you want to preserve the semantics of JS objects. Lua does pretty much the same.

> Saying that they're objects is not reasonable.

This is a fair point, in the alternate universe where JS objects have different semantics.

> in most languages lists are objects, and they don't also do key-value pairs at the same time

This is a dynamic language we're talking about. You are very welcome to think dynamic languages are inferior choices, an opinion that I personally would share, but that's a discussion for another time. In the context of JS, that behavior is neither unreasonable nor inconsistent.

[robertlagrant]

Python is also dynamic. That has nothing to do with this.

  >>> l = []
  >>> l.x = "hello"
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: 'list' object has no attribute 'x'

Always adding arbitrary properties to an object is not a dynamic thing, it's just a bit silly. At best you could say it's a weak typing thing, but even then, it doesn't need to be.