[qsort]

That behavior does make sense in the context of Javascript.

    > {a: 1, b: 2}['c']
    undefined

    > {1: 1, 3: 2}[2]
    undefined

If you're arguing that JS objects shouldn't be like that, you do have a point. But taking JS basic semantics as a given, I don't see what other behavior you would expect.

[robertlagrant]

The point is in the context of lists, not objects. It's in the article.

[_v7gu]

But lists are objects, with negative indices as possible keys:

    > const l = [1,2,3];
    undefined
    > l.a = l;
    [ 1, 2, 3, a: [Circular] ]
    > l === l.a
    true
    > l[-1] = 0;
    0
    > l
    [ 1, 2, 3, a: [Circular], '-1': 0 ]

[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.

[Silhouette]

Even without that argument, the behaviour is still consistent with other cases for arrays in JS anyway.

    > const a = [1, 2];
    > a[0]
    1
    > a[2]
    undefined
    > a[-1]
    undefined

[tsimionescu]

In JS semantics, lists (by which I assume you actually mean arrays) are just objects with numeric keys. Nothing special about them, except that there are certain functions which assume the objects they take in have only numeric keys starting from 0 and always increasing (they usually ignore non-numeric keys that those objects might have).

This is similar to LISP, where you can build many kinds of structures out of cons cells, and you have certain families of functions that assume that the cons cells you pass in have certain kinds of structures, defining lists or trees or association lists etc.