Sparse arrays are treated like arrays of undefined

[ninevra]

concordance treats empty slots in arrays like undefined, for both formatting and comparison.

format(new Array(5)) gives

[
  undefined,
  undefined,
  undefined,
  undefined,
  undefined,
]

whereas util.inspect(new Array(5)) gives [ <5 empty items> ]

compare(new Array(5), Array.from({length: 5}) returns true, although these values behave rather differently.

[Mark Wubben]

Nice find! If we could detect the gap, we could encode a custom "empty primitive"?

[ninevra]

That would make sense to me. Detecting the gap is simple enough, just add if (current in this.value) {} around line 122 in

concordance/lib/complexValues/object.js

Lines 110 to 123 in b30e7c8

 createListRecursor () { 
   if (!this.isList) return recursorUtils.NOOP_RECURSOR 
  
   const size = this.value.length 
   if (size === 0) return recursorUtils.NOOP_RECURSOR 
  
   let index = 0 
   const next = () => { 
     if (index === size) return null 
  
     const current = index 
     index++ 
     return this.describeItem(current, this.describeAny(this.value[current])) 
   } 

I think any change here would probably(?) be breaking, though, since it would mean introducing a new descriptor tag, meaning old versions couldn't decode new serializations, and existing snapshots of sparse arrays would compare inequal to new snapshots of the same values.

There's also a test case asserting the current behavior at

concordance/test/lodash-isequal-comparison.js

Line 135 in b30e7c8

 t.true(isEqual(array, [undefined])) 

I reviewed a few other libraries; it looks like chai, jest, and lodash all consider empty array items to equal undefined, while node's assert builtin module does not.

[Mark Wubben]

I think any change here would probably(?) be breaking, though, since it would mean introducing a new descriptor tag, meaning old versions couldn't decode new serializations, and existing snapshots of sparse arrays would compare inequal to new snapshots of the same values.

Yea. Let's see if we can get that into AVA 4 — and otherwise we can work out some compatibility between the different snapshot versions.

I reviewed a few other libraries; it looks like chai, jest, and lodash all consider empty array items to equal undefined, while node's assert builtin module does not.

I'm happy to follow assert, sparse arrays do behave differently so it seems useful to reflect that in snapshots and assertions.