Sparse arrays case

[medikoo]

Should it ignore sparse array characteristics?

e.g. if I read current spec, it'll work as:

var arr = new Array(3);
arr[0] = 'foo';
arr[1] = 'bar';
arr.last(); // undefined not 'bar';

Similarly with:

var arr = [0, 1, 3];
arr.length = 100;
arr.last(); // undefined

Is that expected?

[ljharb]

Very good question.

Most ES5 methods, like forEach etc, seem to respect sparsensss, whole most ES6+ methods seem to ignore it. Concptually, while one might expect a sparse array’s “last” item to be the last item it actually contains, i think “length minus one” is probably a simpler and more consistent meaning for it.

[jridgewell]

Which ES6 features? All I can think of is destructuring.

[ljharb]

All the ES6+ array iteration methods will interact with every index, present or not, eg:

• Array.from
• find
• findIndex
• includes
• fill

Also flatten and flatMap will do the same.

[zloirock]

@jridgewell Array.from, Array#{find, findIndex, fill, includes, keys, values, entries, @@iterator}, destructuring.

[jridgewell]

Huh, I had no idea those were different (minus iterator).

[medikoo]

I take that seeing it as ES2015+ feature, it should stay as it is, and it was officially stated that spareness should be ignored as a design side effect and not a real feature (hence not supported in all new methods).

I rely on util which respects sparseness, but it was implemented that way only to confirm well to ES5 state of things. I don't remember single use case where I would rely on that behavior