undefined | Better HN

0 pointsqsort5y ago0 comments

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.

0 comments

robertlagrant5y ago

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

_v7gu5y ago

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 ]

robertlagrant5y ago

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.

1 more reply

Silhouette5y ago

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

tsimionescu5y ago

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.

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0 comments

robertlagrant5y ago

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

_v7gu5y ago

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 ]

robertlagrant5y ago

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"]

1 more reply

Silhouette5y ago

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

tsimionescu5y ago

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